Title of Invention	A PROCESS FOR MANUFACTURING ACTIVATED FLY ASH WITH IMPROVED MECHANICAL AND CORROSION-RESISTANCE PROPERTIES
Abstract	In the present invention there is provided a process for manufacturing activated fly ash from pond ash / bottom ash by physical, thermal and chemical activation, resulting in activated fly ash with improved mechanical and corrosion resistance properties. The novel process of the present invention involves physical activation, thermal activation and chemical activation of as-received un-activated pond fly ash / bottom fly ash. The performance characteristics were tested by aggressive macro cell corrosion technique. The activated fly ash system showed lowest corrosion rate, highest durability factor and lowest macro cell current under accelerated macro cell corrosion conditions. The process of the present invention particularly provides activated fly ash which is useful for making blended cement concrete with improved mechanical and corrosion resistance properties for structural applications.

Title of Invention

A PROCESS FOR MANUFACTURING ACTIVATED FLY ASH WITH IMPROVED MECHANICAL AND CORROSION-RESISTANCE PROPERTIES

Abstract

In the present invention there is provided a process for manufacturing activated fly ash from pond ash / bottom ash by physical, thermal and chemical activation, resulting in activated fly ash with improved mechanical and corrosion resistance properties. The novel process of the present invention involves physical activation, thermal activation and chemical activation of as-received un-activated pond fly ash / bottom fly ash. The performance characteristics were tested by aggressive macro cell corrosion technique. The activated fly ash system showed lowest corrosion rate, highest durability factor and lowest macro cell current under accelerated macro cell corrosion conditions. The process of the present invention particularly provides activated fly ash which is useful for making blended cement concrete with improved mechanical and corrosion resistance properties for structural applications.

Full Text	The present invention relates to a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties. This invention particularly relates to a process for manufacturing activated fly ash from pond ash / Dottom ash by physical, thermal and chemical activation, resulting in activated fly ash with improved mechanical and corrosion resistance properties. The activated fly ash with improved mechanical and corrosion resistance properties, of the present invention, is particularly useful in making blended cement having improved mechanical and corrosion resistance properties. In our co-pending patent application no. NF - 276 / 03 we have described and claimed an activated fly ash blended cement composition having improved mechanical and corrosion resistance properties. Fly ashes produced by different thermal power stations are classified into different categories namely ESP fly ash (fly ashes collected by using electrostatic precipitators), bottom ash and pond ash. The FSP fly ashes are of superior quality when compared to the properties of pond ash / bottom ash, which are found to be inferior. The production of ESP fly ashes is very less and the cement manufacturers directly utilize the same. The bottom ash / pond ash are generated as a by-product in large quantities and pose an environmental threat and cause pollution to the environment such as air, water, land. Hence, utilization of pond ash / bottom ash is a critical factor as-such in the existing condition. The importance of the problem of environmental protection and conservation of resources and the technological impact thereof is addressed by the process of the present invention which provides activated fly ash with improved mechanical and corrosion resistance properties, which is useful for making blended cement having improved mechanical and corrosion resistance properties for applications such as structural concrete. The use of fly ash in concrete technology has been popular throughout the world, mainly due to the conservation of cost savings, environmental protection and conservation of resources. In India, at present there are more than 72 thermal power stations, producing 60 -100 miiiion tones of fly ash per year. Only 2% of the produced fly ash is currently being utilized and there is vast scope for effective fly ash utilization. The use of fly ash in concrete technology is not new in India as elsewhere but its full potential does not seem to have been fully exploited by the construction industry for various reasons. The manufacture of ordinary Portland cement (OPC) is not only energy intensive but also releases green house gases and hence has a negative societal impact. The production of every tonne of Portland cement contributes about one tonne of C02 into the atmosphere. Minor amounts of N02 and CH4 are also released into the atmosphere. The production of fly ash by thermal power stations may likely to exceed 175 million tones by the end of 2007 and therefore will become a serious problem as far as availability of land and pollution hazards are concerned. Thus safe, environment friendly disposal of the fly ash is of major concern. Moreover attempts should be made to utilize fly ash rather than dumping the same thereby losing thousand of hectares of land. A possible use of fly ash is in producing blended cements. Generally, the word "blending" is often used in connection with a product with some superior property over the original constituents. In construction industry similar is the case with blended cements. Ordinary Portland cement (OPC) is blended with some mineral admixtures like fly ash in fairly large proportions. The final blend produces a hydraulic cementitious product characteristically similar to ordinary Portland cement but with improved properties. These supplementary cementitious materials possess properties, which impart certain desirable characteristics to the concrete mix. Of course, a number of tests and a lot of processing of these materials have to be carried out before they can be suitably blended with cement. Fly ash is a siliceous material obtained from thermal power stations and is now being considered as a cementitious ingredient of concrete. The use of fly ash in concrete in partial replacement of Portland cement is growing fast not only as blended cements but also to minimize the pollution of the environment. Reference may be made to P.S.Mangat and B.T.Molly "Influence of PFA, Slag and micro silica on chloride induced corrosion of reinforcement in concrete" published in the journal "Cement and Concrete Research", 1991, Vol.31, page numbers 819 - 834, wherein it is reported that fly ash accelerates reinforcement corrosion due to the presence of un-burnt carbon and sulfur and hence the usage of fly ash in reinforcements is still questionable. In order to produce fly ash with stable properties and adequate quality, many power plants have implemented its own sophisticated quality control measures. Reference may be made to J.P.Behera, B.Sarangi, B.D.Nayak and H.S.Ray investigations on the development of blended cement using activated fly ash", published in The Indian Concrete Journal, 2000, Vol.74, Page Numbers 260-263, wherein it has been reported that the physical and mechanical properties matched that of ordinary Portland cement. In this the corrosion resistance properties have not been studied. Reference may be made to F.Yueming, Y.Suhong, W.Zhiyun and Z.Jingyu, "Activation of fly ash and its effects on cement properties" published in the journal Cement and Concrete Research, Vol.29, 1999, PP.467-472, wherein the importance of activation of fly ash has been reported and concluded that activated fly ash can accelerate early hydration of cement and promote setting and hardening. In this also the corrosion resistance properties have not been studied. Reference may be made to patent number US 5,435,843 dated 25-07-1995 and US 5,565,028 dated 15-10-1996 by Roy Amitava; Schilling Paul J; Eaton Harvill C; regarding "Alkali activated class C fly ash cement", wherein a quick setting cement was developed using class-C fly ash activated with strong alkali. This cement will have uses in a number of areas including fixation of hazardous wastes such as radioactive wastes, applications where rapid setting is desired and formation of concrete in hot environments where ordinary Portland cement (OPC) may tend to crack due to their heat of hydration. They have used class C fly ash, which contains total sum of silica, alumina and ferric oxide contents less than 70% but more than 50%. Class C fly ash typically is high in calcium, and is normally produced as a by-product of the combustion of lignite or sub-bituminous coal. The starting material in this case is class C fly ash which is typically is high in calcium and will not give the desired results with pond ash / bottom ash, which contains calcium of the order of 2.5%. Reference may be made to patent number US 4,992,102 dated 12-02-1991 by Barbour; Ronald. I.; wherein a synthetic class-C fly ash results from a substantially homogeneous biend of about 40-60% by weight of ciass-F fiy ash and about 60 - 40% by weight of cement kiln dust (CKD). This new fly ash can replace 25 - 50% by weight of Portland cement in conventional formulations with coarse and fine aggregate for making general purpose concrete and particularly ready-mix concrete with comparable compressive strength properties. This invention relates synthetic class C fly ash which is a mixture of cement kiln dust and class F fly ash and will not give the desired results with pond ash / bottom ash, which contains calcium of the order of 2.5%. Further, there is no activation of fly ash and there is no mention of improvement of corrosion resistance properties. In US patent numbers 5,714,002 and 5,714,003 dated 03-02-1998 by Styron; Robert William; discloses a process for making a blended hydraulic cement based on fly ash, retarding agent, citric acid and potassium carbonate. The drawbacks are use of citric acid, which is relatively expensive. There is no express mention on activation of existing pond and or bottom ashes. In all the above noted hitherto known prior art, the improvement is only in physical and mechanical properties of the cement. There is no mention of improvement of corrosion resistance properties of the fly ash or the corrosion resistant properties of the cement developed. Further, the starting material is not pond ash / bottom ash, which contains less calcium (2.5%). At present the utilization of fly ash in cement and concrete in India is very minimal. Therefore, enhancing the activity of fly ash is the critical key to large scale applications. From the hitherto available prior art it is clearly seen that the focus is on the improvement of physical and mechanical properties of fly ash and fly ash blended cement concrete. Systematic and detailed studies on the corrosion performance of activated fly ash as such and activated fly ash blended cements are very scarce and there is a definite need for work emphasisng on both the physical and mechanical as well as corrosion resistant properties of the activated fly ash and activated fly ash blended cements for use in structural concrete The main object of the present invention is to provide a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties. Another object of the present invention is to provide a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond fly ash / bottom fly ash. Yet another object of the present invention is to provide a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties useful for making blended cement having improved mechanical and corrosion resistance properties, which obviates the drawbacks as detailed in the description of prior art. Still another object of the present invention is to provide a process for manufacturing activated fly ash by activation methods like physical, thermal and chemical activation. Still yet another object of the present invention is to provide a process for manufacturing activated fly ash, such that physical and mechanical properties of the fly ash improves. A further object of the present invention is to provide a process for manufacturing activated fly ash, such that the corrosion resistance properties of the fly ash improves. In the present invention there is provided a process for manufacturing activated fly ash from as-received un-activated pond ash / bottom ash by physical, thermal and chemical activation, resulting in activated fly ash with improved mechanical and corrosion resistance properties. The novel process of the present invention involves physical activation, thermal activation and chemical activation of as-received un-activated pond fly ash / bottom fly ash. The performance characteristics were tested by aggressive macro cell corrosion technique. The activated fly ash prepared by the process of the present invention showed lowest corrosion rate, highest durability factor and lowest macro cell current under accelerated macro cell corrosion conditions. The process of the present invention particularly provides activated fly ash which is useful for making blended cement concrete with improved mechanical and corrosion resistance properties for structural applications. Accordingly the present invention provides a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties, which comprises: (i) removing unwanted material and coarse particles from as-received fly ash by conventional methods; (ii) subjecting the cleaned fly ash to mechanical grinding, such as ball milling to obtain a fine powder; (iii) subjecting the ground fly ash obtained in step (ii) to thermal treatment at a temperature in the range of 900 to 1000°C; (iv) allowing the thermally treated fly ash to cool at room temperature; (v) subjecting the finely ground thermally treated fly ash obtained in step (iv) to chemical activation by treatment with 1 M sodium hydroxide solution to make a slurry; (vi) subjecting the slurry so obtained in step (v) to vigorous stirring, followed by filtering and drying by conventional methods to obtain activated fly ash. In an embodiment of the present invention, the as-received fly ash is un-activated pond fly ash / bottom fly ash. In another embodiment of the present invention, the as-received fly ash is ground to a fine powder having particle size distribution in the range of 40 and 90 urn. In yet another embodiment of the present invention, the thermal treatment at a temperature in the range of 900 to 1000°C is carried out for a period of about one hour. In still another embodiment of the present invention, the vigorous stirring is effected by mechanical stirring for a period of about 2 to 3 hours. The process of the present invention provides for manufacturing activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond ash / bottom ash by physical, thermal and chemical activation. The presence of un-burnt carbon, sulfur and other impurities in the unprocessed fly ash causes and may enhance the corrosion of reinforcements. Un-burnt carbon content is a undesirable constituent of fly ash for use in reinforced concrete constructions. Besides its various harmful effects, it increases the electrical conductivity of concrete. Due to the oxidizing atmosphere at power stations, the sulfur present in the fly ash is usually in the form of sulfates and has an effect similar to that of sulfates present in the normal Portland cement (OPC). Thermal activation affects both fly ash reactivity and kinetics of dissolution. The carbon, sulfur and other impurities present in the fly ash gets removed by thermal activation. Further, thermal activation induces the fly ash reactivity and kinetics of dissolution. In general significantly faster glass breakdown occurs at elevated temperatures. The need of chemical activation of fly ash mainly involves the breaking of bonds and dissolution of three dimensional network of glass. The surface layer of the fly ash is etched by chemical activation. When Ca(OH)2 is present the solubility of Si02 in fly ash markedly increases. The reasons for improved mechanical and corrosion resistance properties such as lower activity of the activated fly ash arise mainly due to two factors, which are: 1, Glassy surface layer of glass beads is highly dense and chemically stable and this layer protects the inside constituents, which were porous, spongy, and amorphous. 2. Silica-alumina glassy chain of high silica, alumina and low calcium is firm, the chain must be disintegrated if activity is to take place. With Ca(OH)2 addition and higher basicity, the densified outer layer is corroded and active core is exposed. In normal cement paste pore solution, the pH is relatively lower, and so the speed of corrosion on the glassy surface is quite slow. If the concentration of OH" is high enough, the silica-alumina glassy chain will be rapidly disintegrated and will produce a large number of active groups Obviously, the pH value of iiquid environment possesses a significant determinant for fly ash activation. The novelty of the present invention resides in providing a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond fly ash / bottom fly ash, which is the most inferior type of fly ash available in large quantities and is a potential hazard to the environment. The novelty of providing improved mechanical and corrosion resistance properties to the most inferior pond fly ash / bottom fly ash has been made possible by the non-obvious inventive steps of subjecting the inferior pond fly ash / bottom fly ash to step-wise treatment constituting a combination of physical activation, thermal activation and chemical activation to obtain activated fly ash with improved mechanical and corrosion resistance properties. The detailed process steps of the present invention are: Step-1: As-received pond fly ash / bottom fly ash is sieved to remove any coarse and foreign particles. Step-2: The cleaned fly ash is mechanically ground in a ball mill to a fine powder having particle size distribution between 40 and 90 um. Step-3: Fly ash obtained in Step-2 is thermally treated by keeping at a temperature of about 900 to 1000°C in a graphite pot for a period of about one hour. Step-4: The thermally treated fly ash is allowed to cool at room temperature, the finely ground fly ash obtained was used for further investigation. Step-5: Finely ground and thermally treated fly ash obtained from step-4 is subjected to chemical activation by treatment with 1 M sodium hydroxide solution to make a slurry. Step-6: The slurry obtained in step-5 is stirred vigorously for about 2 to 3 hours using mechanical stirrer, followed by filtering and drying by conventional methods to obtain activated fly ash. In our co-pending patent application no. NF-276 / 03 we have described and claimed an activated fly ash blended cement composition having improved mechanical and corrosion resistance properties, wherein the fly ash blended cement composition comprises: Ordinary Portland cement (OPC) : 70 to 90 wt.%; Activated fly ash : 10 to 30 wt.%; Sodium silicate 1 to 2 % by weight of OPC; Calcium oxide 2 to 5 % by weight of OPC. The ordinary Portland cement (OPC), sodium silicate and calcium oxide are of commercial grade. The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention. Example -1 As-received pond / bottom fly ash from Neyveli Lignite Corporation (NLC), Tamilnadu was sieved to remove coarse and foreign particles. The cleaned fly ash was mechanically ground in a ball mill to a fine powder having particle size distribution between 40 and 90 µm. The powdered fly ash was thermally treated at a temperature of 950 °C in a graphite pot for a period of one hour. This thermally treated fly ash was allowed to coo! at room temperature and then treated with 1 M sodium hydroxide solution to make a slurry. The slurry was stirred vigorously for 3 hours in a mechanical stirrer, followed by filtering and drying by conventional methods to obtain activated fly ash. The composition of the as receved ash was as given in table-1 below: Table -1 (Table Removed) In the following examples individual and the combined effects of the activated fly ash prepared by the process of the present invention have been studied for analyzing the mechanical and corrosion-resistant properties. Performance of the novel activated fly ash was evaluated by conducting the following tests: Mechanical property was assessed by assessing the compressive strength of concrete cubes at various curing periods. Corrosion-resistance properties were assessed by: a. Gravimetric weight loss measurements in mortar for 18 months exposure period under alternative wetting and drying conditions. b. Anodic polarisation of steel embedded in mortar in 3% NaCI solutions. c. Macrocell corrosion studies as per ASTM G-109-92 for 12 months exposure period. Example - 2 The mechanical property was assessed by assessing the compressive strength of concrete cubes at various curing periods. The composition of ordinary Portland cement (OPC) and as receved fly ash used was as given in table -2 below: Table -2 (Table Removed) Ordinary Portland cement (OPC) was used as control. Un-activated as-received fly ash (AFA) and physically (PFA), thermally (TFA) and chemically (CFA) activated fly ashes (AFA, PFA, TFA, CFA) were used as replacement of 10, 20, 30 and 40% of ordinary Portland cement (OPC). To the mixture of OPC and activated fly ashes was added 1.5% of sodium silicate and 2.5 % of calcium oxide (both by weight of cement).). Using Ordinary Portland cement (OPC) was used as control and the blended OPC, concrete mixture of the following composition was prepared: Mix design used 1 : 1.71 : 3.1 (OPC / blended OPC : Fine aggregates : Coarse Aggregates) OPC / blended OPC 415 kg/m3 Fine aggregates 710 kg m3 Coarse Aggregates 1287 kg/m3 Water cement ratio (w/c) 0.50 Using this OPC / blended OPC concrete mixture 100 mm X 100 mm X 100 mm concrete cubes were cast using the above 1:1.71:3.1 mix proportion with water cement ratio (w/c) ratio of 0.50. Compressive strength data was collected for ordinary Portland cement (OPC) as control, un-activated as-received fly ash, physically, thermally and chemically activated fly ashes (AFA, PFA, TFA, CFA) concretes for 7, 14, 28 & 90 days of curing. Average compressive strength (MPa) data for OPC and various activated fly ashes admixed concrete is given in Table- 3. Table -3 (Table Removed) It can be seen from the above table that OPC (control) concrete developed 46.5 MPa after 90 days of curing. Unactivated fly ash developed 33.5 MPa and 27.5 MPa at 30% and 40% replacement levels respectively. Chemically activated fly ash developed 44.6 MPa and 40.2 MPa respectively. These data indicated that unactivated fly ash showed lower compressive strength than activated fly ash at all the curing periods. The critical level of replacement was found to be 10 to 30%. It is observed that, activation of as-received fly ash is essential to improve the mechanical properties of concrete. Even among the activated systems (PFA, TFA and CFA), CFA system is found to be the top performer. This interesting observation is made due to the fact that in the case of chemically activated system, the surface layer is etched by a strong alkali to facilitate more cement particles to join together and also the addition of CaO which is further promoting the growth of CSH gel and Ca(OH)2 which is more advantageous to enhance the strength development. Moreover, PFA and ThA systems improve the properties only by filler effect. On the other hand, CFA system improves the properties both by filler and as well as buffer effects. As a result CFA showed greater improved properties when compared to the PFA and TFA systems. Example - 3 Corrosion-resistance properties of un-activated and activated fly ash blended concrete was assessed by: a. Gravimetric weight loss measurements in mortar for 18 months exposure period under alternative wetting and drying conditions. b. Anodic polarisation of steel embedded in mortar in 3% NaCI solutions. c. Macrocell corrosion studies as per ASTM G-109-92 for 12 months exposure period. (a). Gravimetric weight loss measurements: Mild steel rod of 6 mm diameter and 45 mm height was embedded centrally in cylindrical mortar (55 mm diameter and 60 mm height, 1:3 mix, w/c ratio 0.45) Systems were cast for OPC (control), un-activated and activated fly ashes. After 28 days of curing, mortar specimens were subjected to immersion in 3% NaCI solution. The specimens were maintained in the same condition for 15 days and then subjected to drying for another 15 days. One cycle consists of 15 days wetting with 3% NaCI solutions and 15 days drying at open atmosphere. All the systems were subjected to 18 complete cycles of test period. The corrosion rate obtained from gravimetric weight loss measurements for OPC (control), un-activated (AFA), physically (PFA), thermally (TFA) and chemically activated (CFA) are reported in Table-4. Table – 4 (Table Removed) It can be seen from the Table-4 above that, OPC showed a corrosion rate of 0.0739 mmpy. Unactivated system showed comparable corrosion rate only upto 10% replacement level. Beyond 10%, higher corrosion rates were observed. Activated systems showed comparable corrosion rate even upto 30% replacement level. Particularly chemically activated system showed much less corrosion rate of 0.0152 mmpy & 0.0494 mmpy at 10 & 30 % replacement level, respectively. These data are in agreement with compressive strength data which showed that activation of as-received fly ash is essential to improve the corrosion-resistance properties of concrete. Even among the activated systems studied (PFA, TFA and CFA), CFA system is found to be the top performer. (b). Anodic polarisation test: Another set of mortar specimens (as mentioned in gravimetric weight loss test) were subjected to anodic polarization in 3% NaCI medium using standard three-electrode system. The current flowing at a constant potential of +300 mV and +600 mV shift from OCP is measured as a parameter to assess the corrosion-resistance properties of OPC (control), un-activated and activated systems The current measured for control system is found to be 0.43 mA at +300 mV shift and 1.04 mA at +600 mV shift respectively. Un-activated system showed higher magnitude of current when compared to activated systems. For example at 30% replacement level, the current measured were found to be 0.34 mA and 0.15 mA (for +300 mV shift) respectively for un-activated fly ash and activated (chemically) fly ash. These data clearly indicated that activated system showed 50% reduction in current flow when compared to the un-activated system. This observation again confirmed that activation of as-received fly ash is essential to improve the corrosion-resistance properties of concrete. Even among the activated systems (PFA, TFA and CFA) studied, CFA system is found to be highly corrosion-resistance. (c) Macrocell corrosion test (ASTM G 109 -92): A rectangular concrete specimen (279 mm x 152 mm x 114 mm) using mix design 1:2.19: 3.73 w/c ratio 0.6 were cast as per ASTM procedures. A cold twisted deformed CTD rod (12 mm diameter, 300 mm length was used as cathodes, 220 mm length was used as anode). After 28 days of curing all the specimens were subjected to wetting cycle with 3% NaCI. The alternate wetting and drying cycle consists of 15 days wetting with 3% NaCI and 15 days drying. Experiments were conducted for 12 complete cycles. The macrocell current and the gravimetric corrosion rate were measured for ordinary Portland cement concrete, un-activated fly ash and activated fly ash systems. Corrosion rate obtained from weight loss and macro cell corrosion studies, are reported in Table-5, which is given below. Table – 5 (Table Removed) It can be seen that OPC (control system) showed 0.0075 mmpy. Un-activated fly ash showed higher corrosion rate even at 10% replacement level. On the other hand activated fly ash showed lower corrosion rate even at 30% replacement level. For example 0.0095, 0.0012 mmpy were recorded for un-activated and activated (chemically) systems, respectively. These data clearly indicate that activated system showed 8 times decrease in corrosion rate when compared to the un-activated system. These observations are in confirmation that even among the activated systems (PFA, TFA and CFA) studied, CFA system is found to be highly corrosion-resistance. The galvanic macrocell current flow between anode and cathodes were also measured for every cycle of exposure. Macrocell current values for different systems at the end of 12 months of exposure are reported in Table-6. Table – 6 (Table Removed) The macrocell current for OPC (control) concrete showed 25 uA after 12 cycles of exposure. Un-activated fly ash showed 36 uA at 30% replacement level. Activated (chemically) fly ash showed 12 uA at 30% replacement level. These data clearly indicated that activated system showed 3 times decrease in macrocell current flow when compared to the un-activated system. These data prove that activation of as-received fly ash is essential to improve the corrosion-resistance properties of concrete. Even among the activated system (PFA, TFA, CFA) studied, CFA system is found to be highly corrosion-resistance. Example - 4 Comparison of test results for OPC, un-activated and CFA systems at 30 % replacement level: From the above examples, the tolerable limit of replacement with better corrosion-resistance and mechanical properties was observed upto 30% replacement level. Among the activation systems investigated, the combined effect of activated fly ash system performed very well when compared to the individual activated systems. The comparison of consolidation of OPC, un-activated and chemically activated fly ash systems at 30% replacement level under various techniques are as follows: 1. Gravimetric corrosion rate of steel embedded in mortar and concrete. From weight loss From Macroceli Medium 3% NaCI 3 % NaCI Exposure period 18 months 12 months OPC 0.0739 mmpy 0.0075 mmpy Unactivated system 0.1955 mmpy 0.0095 mmpy CFA system 0.0494 mmpy 0.0012 mmpy 2. Anodic polarisation Test results: Anodic current measured for steel embedded in mortar (+300 mV & +600 mV shift from OPC) +300 mV shift +600mV shift Medium 3% NaCI 3 % NaCI Exposure period 12 hours 12 hours OPC 0.43 mA 1.04 mA Un-activated system 0.34 mA 0.99 mA CFA system 0.11 mA 0.15 mA 3. Macro cell corrosion current test results: Medium 3% NaCI Exposure period 12 months OPC 25 uA Un-activated system 36 uA CFA system 12 uA 4. Compressive strength test results : Curing period 90 days OPC 46.5 MPa Unactivated system 33 5 MPa (30%) 27.5 MPa (40%) CFA system 44.6 MPa (30%) 40.2 MPa (40%) Studies conducted on the mechanical properties of activated fly ash blended concrete revealed that compressive strength measurements showed that, up to 30 % replacement level, the combined activated fly ash systems improved the mechanical properties of concrete. The gain of strength has very close relation with number of connecting points among cement hydration particles. Activated fly ash particle is smaller than that of unactivated cement particles, which can increase the degree of connection and for inhomogeneous coagulation among cement particles. Studies conducted on the corrosion resistance by short-term and long term conditions revealed that accelerated test such as anodic polarisation test confirmed that upto a critical level of 30% replacement levels, the combined activated system improved the corrosion resistance properties of steel in concrete. Steel embedded in OPC and un-activated fly ash blended concrete suffered severe corrosion under macro cell conditions. But OPC containing fly ashes improved the corrosion resistance properties to a maximum of 23 times as observed from gravimetric measurements. Among all, the combined activated system showed superior performance when compared to individual (thermally and physically) activated systems. The main advantages of the present invention are: 1. Activated fly ash with improved mechanical and corrosion resistance properties. 2. Activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond fly ash / bottom fly ash. 3. Activated fly ash with improved mechanical and corrosion resistance properties useful for making blended cement having improved mechanical and corrosion resistance properties. 4. A very simple and cost effective activation process has been developed with commercially available materials. 5. The use of activated fly ash in mortar as well as in concrete in partial replacement of ordinary Portland cement will constitute a very satisfactory outlet for this industrial by product. We claim: 1. A process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties, which comprises: (i) removing unwanted material and coarse particles from as-received fly ash by conventional methods; (ii) subjecting the cleaned fly ash to mechanical grinding, such as ball milling to obtain a fine powder; (iii) subjecting the ground fly ash obtained in step (ii) to thermal treatment at a temperature in the range of 900 to 1000°C; (iv) allowing the thermally treated fly ash to cool at room temperature; (v) subjecting the finely ground thermally treated fly ash obtained in step (iv) to chemical activation by treatment with 1 M sodium hydroxide solution to make a slurry; (vi) subjecting the slurry so obtained in step (v) to vigorous stirring, followed by filtering and drying by conventional methods to obtain activated fly ash. 2. A process as claimed in claim 1, wherein the as-received fly ash is un-activated pond ash / bottom ash. 3. A process as claimed in claim 1-2, wherein the as-received fly ash is ground to a fine powder having particle size distribution in the range of 40 and 90 urn. 4. A process as claimed in claim 1-3, wherein the thermal treatment at a temperature in the range of 900 to 1000°C is carried out for a period of about one hour. 5. A process as claimed in claim 1-4, wherein the vigorous stirring is effected by mechanical stirring for a period of about 2 to 3 hours. 6. A process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties, substantially as herein described with reference to the examples.

Full Text

The present invention relates to a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties. This invention particularly relates to a process for manufacturing activated fly ash from pond ash / Dottom ash by physical, thermal and chemical activation, resulting in activated fly ash with improved mechanical and corrosion resistance properties.
The activated fly ash with improved mechanical and corrosion resistance properties, of the present invention, is particularly useful in making blended cement having improved mechanical and corrosion resistance properties. In our co-pending patent application no. NF - 276 / 03 we have described and claimed an activated fly ash blended cement composition having improved mechanical and corrosion resistance properties.
Fly ashes produced by different thermal power stations are classified into different categories namely ESP fly ash (fly ashes collected by using electrostatic precipitators), bottom ash and pond ash. The FSP fly ashes are of superior quality when compared to the properties of pond ash / bottom ash, which are found to be inferior. The production of ESP fly ashes is very less and the cement manufacturers directly utilize the same. The bottom ash / pond ash are generated as a by-product in large quantities and pose an environmental threat and cause pollution to the environment such as air, water, land. Hence, utilization of pond ash / bottom ash is a critical factor as-such in the existing condition.
The importance of the problem of environmental protection and conservation of resources and the technological impact thereof is addressed by the process of the present invention which provides activated fly ash with improved mechanical and corrosion resistance properties, which is useful for making blended cement having improved mechanical and corrosion resistance properties for applications such as structural concrete.
The use of fly ash in concrete technology has been popular throughout the world, mainly due to the conservation of cost savings, environmental protection and conservation of resources. In India, at present there are more than 72 thermal power stations, producing 60 -100 miiiion tones of fly ash per year. Only 2% of the produced fly ash is currently being utilized and there is vast scope for effective fly ash utilization. The use of fly ash in concrete technology is not new in India as elsewhere but its full potential does not seem to have been fully exploited by the construction industry for various reasons.
The manufacture of ordinary Portland cement (OPC) is not only energy intensive but also releases green house gases and hence has a negative societal impact. The production of every tonne of Portland cement contributes about one tonne of C02 into the atmosphere. Minor amounts of N02 and CH4 are also released into the atmosphere. The production of fly ash by thermal power stations may likely to exceed 175 million tones by the end of 2007 and therefore will become a serious problem as far as availability of land and pollution hazards are concerned. Thus safe, environment friendly disposal of the fly ash is of major concern. Moreover attempts should be made to utilize fly ash rather than dumping the same thereby losing thousand of hectares of land.
A possible use of fly ash is in producing blended cements. Generally, the word "blending" is often used in connection with a product with some superior property over the original constituents. In construction industry similar is the case with blended cements. Ordinary Portland cement (OPC) is blended with some mineral admixtures like fly ash in fairly large proportions. The final blend produces a hydraulic cementitious product characteristically similar to ordinary Portland cement but with improved properties. These supplementary cementitious materials possess properties, which impart certain desirable characteristics to the concrete mix. Of course, a number of tests and a lot of processing of these materials have to be carried out before they can be suitably blended with cement.
Fly ash is a siliceous material obtained from thermal power stations and is now being considered as a cementitious ingredient of concrete. The use of fly ash in concrete in partial replacement of Portland cement is growing fast not only as blended cements but also to minimize the pollution of the environment.
Reference may be made to P.S.Mangat and B.T.Molly "Influence of PFA, Slag and micro silica on chloride induced corrosion of reinforcement in concrete" published in the journal "Cement and Concrete Research", 1991, Vol.31, page numbers 819 - 834, wherein it is reported that fly ash accelerates reinforcement corrosion due to the presence of un-burnt carbon and sulfur and hence the usage of fly ash in reinforcements is still questionable. In order to produce fly ash with stable properties and adequate quality, many power plants have implemented its own sophisticated quality control measures.
Reference may be made to J.P.Behera, B.Sarangi, B.D.Nayak and H.S.Ray investigations on the development of blended cement using activated fly ash", published in The Indian Concrete Journal, 2000, Vol.74, Page Numbers 260-263, wherein it has been reported that the physical and mechanical properties matched that of ordinary Portland cement. In this the corrosion resistance properties have not been studied.
Reference may be made to F.Yueming, Y.Suhong, W.Zhiyun and Z.Jingyu, "Activation of fly ash and its effects on cement properties" published in the journal Cement and Concrete Research, Vol.29, 1999, PP.467-472, wherein the importance of activation of fly ash has been reported and concluded that activated fly ash can accelerate early hydration of cement and promote setting and hardening. In this also the corrosion resistance properties have not been studied.
Reference may be made to patent number US 5,435,843 dated 25-07-1995 and US 5,565,028 dated 15-10-1996 by Roy Amitava; Schilling Paul J; Eaton Harvill
C; regarding "Alkali activated class C fly ash cement", wherein a quick setting cement was developed using class-C fly ash activated with strong alkali. This cement will have uses in a number of areas including fixation of hazardous wastes such as radioactive wastes, applications where rapid setting is desired and formation of concrete in hot environments where ordinary Portland cement (OPC) may tend to crack due to their heat of hydration. They have used class C fly ash, which contains total sum of silica, alumina and ferric oxide contents less than 70% but more than 50%. Class C fly ash typically is high in calcium, and is normally produced as a by-product of the combustion of lignite or sub-bituminous coal. The starting material in this case is class C fly ash which is typically is high in calcium and will not give the desired results with pond ash / bottom ash, which contains calcium of the order of 2.5%.
Reference may be made to patent number US 4,992,102 dated 12-02-1991 by Barbour; Ronald. I.; wherein a synthetic class-C fly ash results from a substantially homogeneous biend of about 40-60% by weight of ciass-F fiy ash and about 60 - 40% by weight of cement kiln dust (CKD). This new fly ash can replace 25 - 50% by weight of Portland cement in conventional formulations with coarse and fine aggregate for making general purpose concrete and particularly ready-mix concrete with comparable compressive strength properties. This invention relates synthetic class C fly ash which is a mixture of cement kiln dust and class F fly ash and will not give the desired results with pond ash / bottom ash, which contains calcium of the order of 2.5%. Further, there is no activation of fly ash and there is no mention of improvement of corrosion resistance properties.
In US patent numbers 5,714,002 and 5,714,003 dated 03-02-1998 by Styron; Robert William; discloses a process for making a blended hydraulic cement based on fly ash, retarding agent, citric acid and potassium carbonate. The drawbacks are use of citric acid, which is relatively expensive. There is no express mention on activation of existing pond and or bottom ashes.
In all the above noted hitherto known prior art, the improvement is only in physical and mechanical properties of the cement. There is no mention of improvement of corrosion resistance properties of the fly ash or the corrosion resistant properties of the cement developed. Further, the starting material is not pond ash / bottom ash, which contains less calcium (2.5%).
At present the utilization of fly ash in cement and concrete in India is very minimal. Therefore, enhancing the activity of fly ash is the critical key to large scale applications. From the hitherto available prior art it is clearly seen that the focus is on the improvement of physical and mechanical properties of fly ash and fly ash blended cement concrete. Systematic and detailed studies on the corrosion performance of activated fly ash as such and activated fly ash blended cements are very scarce and there is a definite need for work emphasisng on both the physical and mechanical as well as corrosion resistant properties of the activated fly ash and activated fly ash blended cements for use in structural
concrete
The main object of the present invention is to provide a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties.
Another object of the present invention is to provide a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond fly ash / bottom fly ash.
Yet another object of the present invention is to provide a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties useful for making blended cement having improved mechanical and corrosion resistance properties, which obviates the drawbacks as detailed in the description of prior art.
Still another object of the present invention is to provide a process for manufacturing activated fly ash by activation methods like physical, thermal and chemical activation.
Still yet another object of the present invention is to provide a process for manufacturing activated fly ash, such that physical and mechanical properties of the fly ash improves.
A further object of the present invention is to provide a process for manufacturing activated fly ash, such that the corrosion resistance properties of the fly ash
improves.
In the present invention there is provided a process for manufacturing activated fly ash from as-received un-activated pond ash / bottom ash by physical, thermal and chemical activation, resulting in activated fly ash with improved mechanical and corrosion resistance properties. The novel process of the present invention involves physical activation, thermal activation and chemical activation of as-received un-activated pond fly ash / bottom fly ash. The performance characteristics were tested by aggressive macro cell corrosion technique. The activated fly ash prepared by the process of the present invention showed lowest corrosion rate, highest durability factor and lowest macro cell current under accelerated macro cell corrosion conditions. The process of the present invention particularly provides activated fly ash which is useful for making blended cement concrete with improved mechanical and corrosion resistance properties for structural applications.
Accordingly the present invention provides a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties, which comprises:
(i) removing unwanted material and coarse particles from as-received fly ash by conventional methods;
(ii) subjecting the cleaned fly ash to mechanical grinding, such as ball milling to obtain a fine powder;
(iii) subjecting the ground fly ash obtained in step (ii) to thermal treatment at a temperature in the range of 900 to 1000°C;
(iv) allowing the thermally treated fly ash to cool at room temperature;
(v) subjecting the finely ground thermally treated fly ash obtained in step (iv) to chemical activation by treatment with 1 M sodium hydroxide solution to make a slurry;
(vi) subjecting the slurry so obtained in step (v) to vigorous stirring, followed by filtering and drying by conventional methods to obtain activated fly ash.
In an embodiment of the present invention, the as-received fly ash is un-activated pond fly ash / bottom fly ash.
In another embodiment of the present invention, the as-received fly ash is ground to a fine powder having particle size distribution in the range of 40 and 90 urn.
In yet another embodiment of the present invention, the thermal treatment at a temperature in the range of 900 to 1000°C is carried out for a period of about one hour.
In still another embodiment of the present invention, the vigorous stirring is effected by mechanical stirring for a period of about 2 to 3 hours.
The process of the present invention provides for manufacturing activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond ash / bottom ash by physical, thermal and chemical activation. The presence of un-burnt carbon, sulfur and other impurities in the unprocessed fly ash causes and may enhance the corrosion of reinforcements. Un-burnt carbon content is a undesirable constituent of fly ash for use in reinforced concrete constructions. Besides its various harmful effects, it increases the electrical conductivity of concrete. Due to the oxidizing atmosphere at power stations, the sulfur present in the fly ash is usually in the form of sulfates and has an effect similar to that of sulfates present in the normal Portland cement (OPC). Thermal activation affects both fly ash reactivity and kinetics of dissolution. The carbon, sulfur and other impurities present in the fly ash gets removed by thermal activation. Further, thermal activation induces the fly ash reactivity and kinetics of dissolution. In general significantly faster glass breakdown occurs at elevated temperatures. The need of chemical activation of fly ash mainly involves the breaking of bonds and dissolution of three dimensional network of glass. The surface layer of the fly ash is etched by chemical activation. When Ca(OH)2 is present the solubility of Si02 in fly ash markedly increases.
The reasons for improved mechanical and corrosion resistance properties such as lower activity of the activated fly ash arise mainly due to two factors, which are:
1, Glassy surface layer of glass beads is highly dense and chemically stable and
this layer protects the inside constituents, which were porous, spongy, and
amorphous.
2. Silica-alumina glassy chain of high silica, alumina and low calcium is firm, the
chain must be disintegrated if activity is to take place. With Ca(OH)2 addition
and higher basicity, the densified outer layer is corroded and active core is
exposed. In normal cement paste pore solution, the pH is relatively lower, and
so the speed of corrosion on the glassy surface is quite slow. If the
concentration of OH" is high enough, the silica-alumina glassy chain will be
rapidly disintegrated and will produce a large number of active groups
Obviously, the pH value of iiquid environment possesses a significant
determinant for fly ash activation.
The novelty of the present invention resides in providing a process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond fly ash / bottom fly ash, which is the most inferior type of fly ash available in large quantities and is a potential hazard to the environment. The novelty of providing improved mechanical and corrosion resistance properties to the most inferior pond fly ash / bottom fly ash has been made possible by the non-obvious inventive steps of subjecting the inferior pond fly ash / bottom fly ash to step-wise treatment constituting a combination of physical activation, thermal activation and chemical activation to obtain activated fly ash with improved mechanical and corrosion resistance properties.
The detailed process steps of the present invention are:
Step-1: As-received pond fly ash / bottom fly ash is sieved to remove any coarse and foreign particles.
Step-2: The cleaned fly ash is mechanically ground in a ball mill to a fine powder having particle size distribution between 40 and 90 um.
Step-3: Fly ash obtained in Step-2 is thermally treated by keeping at a temperature of about 900 to 1000°C in a graphite pot for a period of about one hour.
Step-4: The thermally treated fly ash is allowed to cool at room temperature, the finely ground fly ash obtained was used for further investigation.
Step-5: Finely ground and thermally treated fly ash obtained from step-4 is subjected to chemical activation by treatment with 1 M sodium hydroxide solution to make a slurry.
Step-6: The slurry obtained in step-5 is stirred vigorously for about 2 to 3 hours using mechanical stirrer, followed by filtering and drying by conventional methods to obtain activated fly ash.
In our co-pending patent application no. NF-276 / 03 we have described and
claimed an activated fly ash blended cement composition having improved
mechanical and corrosion resistance properties, wherein the fly ash blended
cement composition comprises:
Ordinary Portland cement (OPC) : 70 to 90 wt.%;
Activated fly ash : 10 to 30 wt.%;
Sodium silicate 1 to 2 % by weight of OPC;
Calcium oxide 2 to 5 % by weight of OPC.
The ordinary Portland cement (OPC), sodium silicate and calcium oxide are of commercial grade.
The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention.
Example -1
As-received pond / bottom fly ash from Neyveli Lignite Corporation (NLC), Tamilnadu was sieved to remove coarse and foreign particles. The cleaned fly ash was mechanically ground in a ball mill to a fine powder having particle size distribution between 40 and 90 µm. The powdered fly ash was thermally treated at a temperature of 950 °C in a graphite pot for a period of one hour. This thermally treated fly ash was allowed to coo! at room temperature and then treated with 1 M sodium hydroxide solution to make a slurry. The slurry was stirred vigorously for 3 hours in a mechanical stirrer, followed by filtering and drying by conventional methods to obtain activated fly ash. The composition of the as receved ash was as given in table-1 below:
Table -1 (Table Removed)
In the following examples individual and the combined effects of the activated fly ash prepared by the process of the present invention have been studied for analyzing the mechanical and corrosion-resistant properties. Performance of the novel activated fly ash was evaluated by conducting the following tests:
Mechanical property was assessed by assessing the compressive strength of concrete cubes at various curing periods.
Corrosion-resistance properties were assessed by:
a. Gravimetric weight loss measurements in mortar for 18 months
exposure period under alternative wetting and drying conditions.
b. Anodic polarisation of steel embedded in mortar in 3% NaCI
solutions.
c. Macrocell corrosion studies as per ASTM G-109-92 for 12 months
exposure period.
Example - 2
The mechanical property was assessed by assessing the compressive strength of concrete cubes at various curing periods.
The composition of ordinary Portland cement (OPC) and as receved fly ash used was as given in table -2 below:
Table -2 (Table Removed)
Ordinary Portland cement (OPC) was used as control. Un-activated as-received
fly ash (AFA) and physically (PFA), thermally (TFA) and chemically (CFA)
activated fly ashes (AFA, PFA, TFA, CFA) were used as replacement of 10, 20,
30 and 40% of ordinary Portland cement (OPC). To the mixture of OPC and
activated fly ashes was added 1.5% of sodium silicate and 2.5 % of calcium
oxide (both by weight of cement).). Using Ordinary Portland cement (OPC) was
used as control and the blended OPC, concrete mixture of the following
composition was prepared:
Mix design used 1 : 1.71 : 3.1
(OPC / blended OPC : Fine aggregates : Coarse Aggregates)
OPC / blended OPC 415 kg/m3
Fine aggregates 710 kg m3
Coarse Aggregates 1287 kg/m3
Water cement ratio (w/c) 0.50
Using this OPC / blended OPC concrete mixture 100 mm X 100 mm X 100 mm concrete cubes were cast using the above 1:1.71:3.1 mix proportion with water cement ratio (w/c) ratio of 0.50. Compressive strength data was collected for ordinary Portland cement (OPC) as control, un-activated as-received fly ash, physically, thermally and chemically activated fly ashes (AFA, PFA, TFA, CFA) concretes for 7, 14, 28 & 90 days of curing. Average compressive strength (MPa) data for OPC and various activated fly ashes admixed concrete is given in Table- 3.
Table -3 (Table Removed)
It can be seen from the above table that OPC (control) concrete developed 46.5 MPa after 90 days of curing. Unactivated fly ash developed 33.5 MPa and 27.5 MPa at 30% and 40% replacement levels respectively. Chemically activated fly ash developed 44.6 MPa and 40.2 MPa respectively. These data indicated that unactivated fly ash showed lower compressive strength than activated fly ash at all the curing periods. The critical level of replacement was found to be 10 to 30%.
It is observed that, activation of as-received fly ash is essential to improve the mechanical properties of concrete. Even among the activated systems (PFA, TFA and CFA), CFA system is found to be the top performer. This interesting observation is made due to the fact that in the case of chemically activated system, the surface layer is etched by a strong alkali to facilitate more cement particles to join together and also the addition of CaO which is further promoting the growth of CSH gel and Ca(OH)2 which is more advantageous to enhance the strength development. Moreover, PFA and ThA systems improve the properties only by filler effect. On the other hand, CFA system improves the properties both by filler and as well as buffer effects. As a result CFA showed greater improved properties when compared to the PFA and TFA systems.
Example - 3
Corrosion-resistance properties of un-activated and activated fly ash blended concrete was assessed by:
a. Gravimetric weight loss measurements in mortar for 18 months exposure
period under alternative wetting and drying conditions.
b. Anodic polarisation of steel embedded in mortar in 3% NaCI solutions.
c. Macrocell corrosion studies as per ASTM G-109-92 for 12 months
exposure period.
(a). Gravimetric weight loss measurements:
Mild steel rod of 6 mm diameter and 45 mm height was embedded centrally in cylindrical mortar (55 mm diameter and 60 mm height, 1:3 mix, w/c ratio 0.45) Systems were cast for OPC (control), un-activated and activated fly ashes. After 28 days of curing, mortar specimens were subjected to immersion in 3% NaCI solution. The specimens were maintained in the same condition for 15 days and then subjected to drying for another 15 days. One cycle consists of 15 days wetting with 3% NaCI solutions and 15 days drying at open atmosphere. All the systems were subjected to 18 complete cycles of test period.
The corrosion rate obtained from gravimetric weight loss measurements for OPC (control), un-activated (AFA), physically (PFA), thermally (TFA) and chemically activated (CFA) are reported in Table-4.
Table – 4 (Table Removed)
It can be seen from the Table-4 above that, OPC showed a corrosion rate of 0.0739 mmpy. Unactivated system showed comparable corrosion rate only upto 10% replacement level. Beyond 10%, higher corrosion rates were observed. Activated systems showed comparable corrosion rate even upto 30% replacement level. Particularly chemically activated system showed much less
corrosion rate of 0.0152 mmpy & 0.0494 mmpy at 10 & 30 % replacement level, respectively.
These data are in agreement with compressive strength data which showed that activation of as-received fly ash is essential to improve the corrosion-resistance properties of concrete. Even among the activated systems studied (PFA, TFA and CFA), CFA system is found to be the top performer.
(b). Anodic polarisation test:
Another set of mortar specimens (as mentioned in gravimetric weight loss test) were subjected to anodic polarization in 3% NaCI medium using standard three-electrode system. The current flowing at a constant potential of +300 mV and +600 mV shift from OCP is measured as a parameter to assess the corrosion-resistance properties of OPC (control), un-activated and activated systems The current measured for control system is found to be 0.43 mA at +300 mV shift and 1.04 mA at +600 mV shift respectively. Un-activated system showed higher magnitude of current when compared to activated systems. For example at 30% replacement level, the current measured were found to be 0.34 mA and 0.15 mA (for +300 mV shift) respectively for un-activated fly ash and activated (chemically) fly ash. These data clearly indicated that activated system showed 50% reduction in current flow when compared to the un-activated system. This observation again confirmed that activation of as-received fly ash is essential to improve the corrosion-resistance properties of concrete. Even among the activated systems (PFA, TFA and CFA) studied, CFA system is found to be highly corrosion-resistance.
(c) Macrocell corrosion test (ASTM G 109 -92):
A rectangular concrete specimen (279 mm x 152 mm x 114 mm) using mix
design 1:2.19: 3.73 w/c ratio 0.6 were cast as per ASTM procedures. A cold
twisted deformed CTD rod (12 mm diameter, 300 mm length was used as
cathodes, 220 mm length was used as anode). After 28 days of curing all the
specimens were subjected to wetting cycle with 3% NaCI. The alternate wetting
and drying cycle consists of 15 days wetting with 3% NaCI and 15 days drying.
Experiments were conducted for 12 complete cycles. The macrocell current and
the gravimetric corrosion rate were measured for ordinary Portland cement
concrete, un-activated fly ash and activated fly ash systems. Corrosion rate
obtained from weight loss and macro cell
corrosion studies, are reported in Table-5, which is given below.
Table – 5 (Table Removed)
It can be seen that OPC (control system) showed 0.0075 mmpy. Un-activated fly ash showed higher corrosion rate even at 10% replacement level. On the other hand activated fly ash showed lower corrosion rate even at 30% replacement level. For example 0.0095, 0.0012 mmpy were recorded for un-activated and activated (chemically) systems, respectively. These data clearly indicate that activated system showed 8 times decrease in corrosion rate when compared to the un-activated system. These observations are in confirmation that even
among the activated systems (PFA, TFA and CFA) studied, CFA system is found to be highly corrosion-resistance.
The galvanic macrocell current flow between anode and cathodes were also measured for every cycle of exposure. Macrocell current values for different systems at the end of 12 months of exposure are reported in Table-6.
Table – 6 (Table Removed)
The macrocell current for OPC (control) concrete showed 25 uA after 12 cycles of exposure. Un-activated fly ash showed 36 uA at 30% replacement level. Activated (chemically) fly ash showed 12 uA at 30% replacement level. These data clearly indicated that activated system showed 3 times decrease in macrocell current flow when compared to the un-activated system. These data prove that activation of as-received fly ash is essential to improve the corrosion-resistance properties of concrete. Even among the activated system (PFA, TFA, CFA) studied, CFA system is found to be highly corrosion-resistance.
Example - 4
Comparison of test results for OPC, un-activated and CFA systems at 30 % replacement level:
From the above examples, the tolerable limit of replacement with better corrosion-resistance and mechanical properties was observed upto 30% replacement level. Among the activation systems investigated, the combined effect of activated fly ash system performed very well when compared to the individual activated systems. The comparison of consolidation of OPC, un-activated and chemically activated fly ash systems at 30% replacement level under various techniques are as follows:
1. Gravimetric corrosion rate of steel embedded in mortar and concrete.
From weight loss From Macroceli
Medium 3% NaCI 3 % NaCI
Exposure period 18 months 12 months
OPC 0.0739 mmpy 0.0075 mmpy
Unactivated system 0.1955 mmpy 0.0095 mmpy
CFA system 0.0494 mmpy 0.0012 mmpy
2. Anodic polarisation Test results:
Anodic current measured for steel embedded in mortar (+300 mV & +600 mV shift from OPC)
+300 mV shift +600mV shift
Medium 3% NaCI 3 % NaCI
Exposure period 12 hours 12 hours
OPC 0.43 mA 1.04 mA
Un-activated system 0.34 mA 0.99 mA
CFA system 0.11 mA 0.15 mA
3. Macro cell corrosion current test results:
Medium 3% NaCI
Exposure period 12 months
OPC 25 uA
Un-activated system 36 uA
CFA system 12 uA
4. Compressive strength test results :
Curing period 90 days
OPC 46.5 MPa
Unactivated system 33 5 MPa (30%)
27.5 MPa (40%)
CFA system 44.6 MPa (30%)
40.2 MPa (40%)
Studies conducted on the mechanical properties of activated fly ash blended concrete revealed that compressive strength measurements showed that, up to 30 % replacement level, the combined activated fly ash systems improved the mechanical properties of concrete. The gain of strength has very close relation with number of connecting points among cement hydration particles. Activated fly ash particle is smaller than that of unactivated cement particles, which can increase the degree of connection and for inhomogeneous coagulation among cement particles.
Studies conducted on the corrosion resistance by short-term and long term conditions revealed that accelerated test such as anodic polarisation test confirmed that upto a critical level of 30% replacement levels, the combined activated system improved the corrosion resistance properties of steel in concrete. Steel embedded in OPC and un-activated fly ash blended concrete suffered severe corrosion under macro cell conditions. But OPC containing fly ashes improved the corrosion resistance properties to a maximum of 23 times as observed from gravimetric measurements.
Among all, the combined activated system showed superior performance when compared to individual (thermally and physically) activated systems.
The main advantages of the present invention are:
1. Activated fly ash with improved mechanical and corrosion resistance properties.
2. Activated fly ash with improved mechanical and corrosion resistance properties from as-received un-activated pond fly ash / bottom fly ash.
3. Activated fly ash with improved mechanical and corrosion resistance properties useful for making blended cement having improved mechanical and corrosion resistance properties.
4. A very simple and cost effective activation process has been developed with commercially available materials.
5. The use of activated fly ash in mortar as well as in concrete in partial replacement of ordinary Portland cement will constitute a very satisfactory outlet for this industrial by product.

We claim:
1. A process for manufacturing activated fly ash with improved mechanical and
corrosion resistance properties, which comprises:
(i) removing unwanted material and coarse particles from as-received fly ash by conventional methods;
(ii) subjecting the cleaned fly ash to mechanical grinding, such as ball milling to obtain a fine powder;
(iii) subjecting the ground fly ash obtained in step (ii) to thermal treatment at a temperature in the range of 900 to 1000°C;
(iv) allowing the thermally treated fly ash to cool at room temperature;
(v) subjecting the finely ground thermally treated fly ash obtained in step (iv) to chemical activation by treatment with 1 M sodium hydroxide solution to make a slurry;
(vi) subjecting the slurry so obtained in step (v) to vigorous stirring, followed by filtering and drying by conventional methods to obtain activated fly ash.
2. A process as claimed in claim 1, wherein the as-received fly ash is un-activated pond ash / bottom ash.
3. A process as claimed in claim 1-2, wherein the as-received fly ash is ground to a fine powder having particle size distribution in the range of 40 and 90 urn.
4. A process as claimed in claim 1-3, wherein the thermal treatment at a temperature in the range of 900 to 1000°C is carried out for a period of about one hour.
5. A process as claimed in claim 1-4, wherein the vigorous stirring is effected by mechanical stirring for a period of about 2 to 3 hours.
6. A process for manufacturing activated fly ash with improved mechanical and corrosion resistance properties, substantially as herein described with reference to the examples.

Documents:

828-del-2003-abstract.pdf

828-del-2003-claims.pdf

828-del-2003-complete specification (granted).pdf

828-del-2003-correspondence-others.pdf

828-del-2003-correspondence-po.pdf

828-del-2003-description (complete).pdf

828-del-2003-form-1.pdf

828-del-2003-form-19.pdf

828-del-2003-form-2.pdf

828-del-2003-form-3.pdf

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Patent Number

199582

Indian Patent Application Number

828/DEL/2003

PG Journal Number

38/2008

Publication Date

19-Sep-2008

Grant Date

16-Mar-2007

Date of Filing

20-Jun-2003

Name of Patentee

COUNCIL OF SCIENTEFIC AND INDUSTRIAL RESEARCH,

Applicant Address

RAFI MARG, NEW DELHI 110 001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	MURALIDHARAN, SRINIVASAN	CENTRAL ELECTRO CHEMICAL, RESURCH INSTITUTE, KARAIKUDI 630006, TAMIL NADU, INDIA
2	THANGAVEL, KANAGAMANI	CENTRAL ELECTRO CHEMICAL, RESURCH INSTITUTE, KARAIKUDI 630006, TAMIL NADU, INDIA
3	SARASWATHY, VELU	CENTRAL ELECTRO CHEMICAL, RESEARCH INSTITUTE, KARAIKUDI 630006, TAMIL NADU, INDIA.
4	SRINIVASAN, SESHADHRI	CENTRAL ELECTRO CHEMICAL, RESURCH INSTITUTE, KARAIKUDI 630006, TAMIL NADU, INDIA
5	RAGHAVAN, MEENAKSHI SUNDARAM	CENTRAL ELECTRO CHEMICAL, RESEARCH INSTITUTE, KARAIKUDI 630006, TAMIL NADU, INDIA.

PCT International Classification Number

C04B 012/04

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA