Improvement in pozzolanic reactivity of coarse fly ash by mechano-chemical method

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1 Improvement in pozzolanic reactivity of coarse fly ash by mechano-chemical method B.Singh 1, Simmi Tyagi 2, M.Gupta 3 and S.K. Bhattacharyya 4 1 Scientist, CSIR-Central Building Research Institute, Roorkee (INDIA) 2 Research Scholar, CSIR-Central Building Research Institute, Roorkee (INDIA) 3 Scientist, CSIR-Central Building Research Institute, Roorkee (INDIA) 4 Director, CSIR - Central Building Research Institute, Roorkee (INDIA) ABSTRACT: The coarse fly ash was ground to > 80% passing through a 45 µm sieve and activated with NaOH, NaNO 3, Ca(OH) 2 and their binary mixtures. The resulting ash was characterized for the specific surface area, particle size distribution, surface morphology and microanalysis. It is found that the activated fly ashes are finer, higher total pore volume and lower mean particle size than the non-activated coarse ash. FE-SEM shows irregular shape of the particles containing pre-zeolitic and C-S-H phases. EDAX mapping indicates that Si/Al ratio decreases and Al/Fe ratio increases in the activated ash when compared with the coarse ash. The coarse ash exhibited 0.5% lime removal capability whereas the activated ash removed 4-32% lime depending on the efficiency of single and binary mixture of activators as observed in lime solubility curve. The strength of lime mortars consisted of the various activated fly ash are in the range of MPa as compared to 2.2 MPa of coarse ash containing mortar. The compressive strength of cement-activated ash mortar retained 82-95% of the plain cement mortar after 28 days age while the strength of cement-coarse ash mortar was 68.27% only. Based on these results, it is observed that the binary mixture of NaOH and NaNO 3 gave best pozzolanic reactivity with the maximum lime removal efficiency at 1 N solution supportive of its suitability for use in cement concrete. 1 INTRODUCTION The use of fly ash as a pozzolan is a well established worldwide practice in the manufacture of blended cements /concrete in the drive for developing sustainable materials (Roy (1988), Malhotra and Ramezanianpour (1994)). This is due to its reactivity with calcium hydroxide produced during the hydration of Portland cement and can be converted into the C-S-H gel. As a result, the microstructure of hardened pastes is significantly improved. This lime consuming process and the associated pore refinement process lead to increase strength, impermeability and durability of the blended cements. However, there is a limitation in the fly ash because of the pozzolanic rate of reaction is very slow at room temperature causing initial low strength and fast neutralization (Shi and Day (1995), Arjunan et al (2001), Qian et al (2001)). This explains that fly ash tends to act as a preferential sink for initial calcium adsorption causing retardation. The fly ashes which satisfy the specification well are homogenized by a selection process whereas ashes which are outside the specification, processes to satisfy them provided the carbon content is low and fineness, strength activity index, water requirement and soundness are restrictive in classifying the quality of ashes. The rate of interaction with the ordinary Portland cement depends on chemical composition, physical-chemical state, morphology and particle size of the fly ash. As for the effect of coarse ash addition to Portland cement is concerned, the previous results indicated a weak pozzolanic behaviour (Poon et al (2003)). Therefore, it is necessary to propose a formulation and process to activate coarse fly ash for use it as a supplementary cementitious material.

2 In the present study, a systematic R & D programme is initiated at the Institute on use of processed fly ash and municipal solid waste incineration ash as pozzolanic materials in cement concrete. The mechano-chemical treatment was applied to the coarse ash to enhance its pozzolanicity through formation of active centres on the surface. It is known that the coarse ash gave a very low compressive strength when used as part of cement replacement. Its large particle size contained crystalline phase which are inert to pozzolanic reactions (Donatello et al (2010)). In earlier studies (Shi and Day (1995), Arjunan et al (2001), Shi and Day (2001), Poon et al (2003), Paul et al (2007), Polettini et al (2009), Donatello et al (2010)), efforts were made to activate the coarse ash/reject fly ash for use in blended cements/concrete to enhance its low hydraulic reactivity for obtaining desired set time and strength in the products. The reported results revealed that adding small quantity of single or binary activators significantly accelerated hydration reactions and consequently, the strength development. In the present paper, we report efficacy of mechanochemical activation on the pozzolanic reactivity of coarse fly ash in terms of its lime removal efficiency, lime reactivity and compressive strength. The activated coarse ash was assessed according to the requirements of standard specification for use in cement mortar and concrete. 2 EXPERIMENTAL 2.1 Materials The coarse fly ash used in the present study was collected from coal fired Thermal Power station, Dadri, India (retention on 45µm sieve, 45%; Blaine specific surface area, 160 m 2 /kg; loss on ignition - initial unburnt carbon, 1.8%). Its chemical composition is: SiO %; Al 2 O %; Fe 2 O %; MgO 0.29%; CaO 3.68%; sulphur trioxide 0.12%. Laboratory grade chemicals such as sodium hydroxide (97.5% purity), sodium nitrate (99.5% purity) and calcium oxide (90% purity) were procured from the local market. Hydrated lime powder standard (CRM No : CaO 96%; insoluble residue 1%; sp.gr 2.3) was obtained from the National Council for Cement and Building Materials, Ballabgarh, India. 2.2 Activation of coarse fly ash The coarse ash was ground in a jar mill for 4 hrs using stainless steel balls in a ratio of 1:2 at 88 rpm. The retention of ground ash on a 45 µm sieve was recorded as 5%. The ash was then treated with different activators such as NaOH, NaNO 3, Ca(OH) 2 and their binary mixtures as per procedure described by Arjunan et al (2001). In an experiment, 40 g of the ground fly ash was mixed with 60 ml of deionised water in a beaker. The activator was then added to the slurry. The slurry was kept on a magnetic stirrer for 2-3 hrs at 90 º C and allowed to cool at room temperature. The resulting slurry was dried in an air circulating oven and ground in an agate mortar for further tests. 2.3 Methods Characterization of fly ash particles The average particle size of the fly ashes was measured by a laser based Particle size analyzer (WCIS-100 ANKERSMID) and also recorded their particle size distribution. The specific surface area and pore volume of ashes were determined by nitrogen adsorption using BET method (Chemisorb 2720 Micrometrics). The samples were degassed at 400 º C before testing. Blaine permeability method and Particle size analysis were also used for the measurement of specific surface area of ashes.

3 The coarse and activated ashes were coated with gold/palladium to render them conductive. The surface morphology of these ashes was evaluated by Field emission scanning electron microscope (FESEM-QUANTA 200 F, FEI Netherlands). The microanalysis of ash samples was carried out by an EDAX attached to FESEM Pozzolanicity Tests Frattini Test: The pozzolanicity of the ash particles was assessed as per the procedure described in BS EN 196 (British Standard (2005)). 20 g of test sample consisting of 80% ordinary Portland cement (43 grade) and 20% fly ash was mixed with 100 ml of distilled water and was then kept in an oven at 40 ⁰ C in the sealed plastic bottle. After 8 days, the sample was vacuum filtered through Whatman filter paper No The filtrate was analyzed for hydroxyl ion by a titration against dilute HCl with methyl orange as an indicator and for calcium ion, adjusting ph of filtrate to 12.5, followed by the titration with 0.03 mol/l EDTA solution using a Patton and Readers indicator. The solubility curve of calcium oxide in the solution was plotted as a function of hydroxyl ion concentration to assess pozzolanicity of the ash. Lime reactivity of the coarse and activated fly ashes was determined as per IS: 1727 (Indian Standards (1999)). The hydrated lime, fly ash and standard sand were blended in the proportion of 1: 2: 9 by weight with a consistency of 70 ± 5%. The specimens of 50 mm 3 were cast in a layer of about 25 mm thickness with tamping and covered with a smooth and grease lined glass plate for 48 hrs under wet gunny bag. Subsequently, the specimens were removed from the mold and then cured at 90% relative humidity for a period of 8 days at 50 º C in the curing chamber. After curing, the samples were tested for their compressive strength and averaged the value of three test results. The compressive strength of cement-fly ash mortar samples was tested as per IS 1727 (Indian Standards (1999)). The test mixture was prepared by blending the fly ash, cement and standard sand in proportion of 0.2: 0.8: 3 by weight at 105 ± 5% flow. The cement mortars of different mixes were molded into 50 mm 3 cubes and kept for 24 hrs at 90% relative humidity. The de-molded samples were cured in water for different periods till testing. The average of the three samples was reported for the compressive strength. 3 RESULTS AND DISCUSSION 3.1 Morphology of activated fly ash Figure 1 shows volume cumulative particle size distribution of the coarse and activated fly ashes. It can be seen that the coarse ash contains particle size ranges between 3.2 to 300 µm and its mean particle size (D50) was µm. Upon mechano-chemical activation, the ash exhibited particle size distribution ranging between 0.1 and 200 µm with the mean particle size between 0.5 and 16 µm. The percentage of larger size particles (D90) was also less in the activated ash. It is noted that the coarse ash treated with a binary mixture of NaOH and NaNO 3 imparted smaller particle size than the ash treated with other activators. When compared with cement (particle size µm; D 50 = µm), the activated ash was finer with a smaller mean particle size. Table 1 displays the specific surface area of the fly ash measured by different methods. Upon activation, the BET specific surface area has increased in the range of 8.3 to m 2 /g from 7.29 m 2 /g for the coarse ash. The total pore volume of the particles also increased ( m 2 /kg)

4 NaOH + NaNO 3 Ca (OH) 2 Cement Percentage CV Ground NaOH + Ca(OH) 2 NaOH Coarse NaNO 3 Size in Microns Figure 1. Particle size distribution of coarse and activated fly ashes Table 1. Specific surface area of coarse and activated fly ashes Fly Ash Blaine (m 2 /g) Particle Size Analysis (m 2 /g) BET (m 2 /g) Total Pore volume (m 2 /kg) Coarse Ground NaNO NaOH Ca(OH) NaOH+NaNO NaOH+Ca(OH) Pure Cement Condition: 0.8 N solution; 5% of the fly ash with respect to the coarse ash (3.7 m 2 /kg). The higher surface area of the activated ashes would expect to increase their pozzolanic activity. These values were in the range to that of the ordinary Portland cement (12.64 m 2 /g; pore volume: 6.4 m 2 /kg). The Blaine fineness and specific surface area of the activated ashes obtained by particle size analysis were also higher than the coarse ash. The difference in the specific surface area measured by BET method and other methods is due to the fact that BET technique measures the totality of voids in the surface of particles (Malhotra and Ramezanianpour (1994)). The particle shape was also responsible for such difference. It is noted that the coarse ash treated with a binary mixture of NaOH and NaNO 3 showed higher specific surface area than those of other activators. FE-SEM images of the coarse and activated fly ashes are shown in Figure 2. The ash particles are mostly smooth and spherical in shape having varying particle sizes. [Fig. 2 (a)] When the ash was

5 (a) (b) (c) (d) (e) (f) Figure 2. FE-SEM images of fly ashes: (a) Coarse (b) NaNO 3 (c) NaOH (d) Ca (OH) 2 (e) NaOH + NaNO 3 (f) NaOH + Ca(OH) 2 subjected to mechano-chemical treatment, its spherical structure was destroyed. The particles are smaller in size, irregular in shape and spongy in the appearance. The presence of zeolitic precursor phase in the ash treated with NaOH and NaOH + NaNO 3 and C-S-H phase developed on the ash surface upon Ca(OH) 2 treatment supportive of their reactivity when used in the cementitious binders [Fig. 2 (b-f)]. The smaller particles viewed in the FE-SEM after treating with a binary mixture of NaOH and NaNO 3 as also observed in particle size analysis are expected to provide their high reactivity over the other treatments. This is related to the reaction between reactive silica and alumina of the ash particles and hydroxyl ions of the activators. The minimal activation effect was noted in the case of NaNO 3 because of low destruction of ash surface in activator solution (low ph) as viewed in SEM image [Fig. 2 (b)]. The compositional changes in the activated ash were also seen during EDAX mapping (Table 2). The Si/Al ratio in the coarse ash increased from 1.63 to while Al/Fe ratio decreased from 3.74 to after mechano-chemical treatment except in the case of NaOH. After processing, the sodium and magnesium contents in the coarse ash decreased from 1.31 to wt % and 1.43 to wt % respectively. The coarse ash treated with the binary mixture of NaOH and NaNO 3 activator exhibited low Si and Al contents compared with the other treated ashes. This indicates that glass surface no longer keeps its original stable state; the incomplete coordinated Si 4+ is likely to be exposed on the particle surface, so that the surface free energy of ash is increased which makes it more reactive (Arjunan et al (2001)). The activated ash in cement concrete is to react with excess calcium hydroxide to form cementitious calcium aluminosilicates, thereby improves the strength of cement mortar/concrete over the longer periods, shrink pores and makes calcium hydroxide unavailable for deleterious reactions involving sulphate, chloride and carbonate ions.

6 3.2 Pozzolanicity Figure 3 shows lime solubility curves of the coarse and activated ashes plotted as per EN 196 (British Standard (2005)) between CaO concentration and hydroxyl ion concentration at 40º C. It is found that values for the coarse and NaOH treated ashes produced points directly on the line of saturation curve showing their zero pozzolanic activity. These values are in the similar line as observed in the lime reactivity test. It is believed that the optimum concentration of NaOH is necessary to break the glassy layer of ash particles. On the other hand, other activators showed points below the line of curve indicating a removal of calcium ions from the solution attributable to their pozzalanic activity. The CaO reduction obtained from the difference between the theoretical and experimental values was 32% for the binary mixture of NaOH and NaNO 3, % for the Ca(OH) 2, 24.93% for the ground and 4-5% for the other activators respectively. This may be attributed to the more reactive silicate and aluminate surfaces produced during binary treatment of NaOH and NaNO 3. After assessing the effectiveness, the optimum concentration of the binary activator was finalized in terms of its lime removal. The values below the line of lime solubility curve are an indicative of their pozzolanicity. The maximum lime removal (73.46%) of the activated ash was obtained at 1N solution. EDAX analysis indicates that Si/Al ratio in the activated ash reduced to 1.95 compared with the 2.44 for the ground ash. The surface area calculated by particle size analyzer and BET method was 7.69 m 2 /g and m 2 /g with pore volume 8.9 m 2 /kg respectively. However, it is noted that higher concentration (2-3 N) of the binary activation results in more specific surface area in BET method contrary to the results of particle size analysis. FE-SEM indicates that low concentration of binary activator gives amorphous structure with zeolite precursor phase. Increasing the concentration, the microstructure of ash became coarser with fine crystalline zeolite on the surface of ash particles. 3.3 Lime reactivity and compressive strength The pozzolanic activity evaluation of the coarse and activated ashes is given in Table 3. Lime reactivity of the coarse ash was 2.2 MPa which is outside the specification requirement (4.5 MPa). When the coarse ash was mechano-chemically activated, its strength reached in the range of 4.7 to 9.1 MPa. The improvement in the specific surface area, particle size distribution and soluble fraction is responsible to create reactive centres for the lime absorption. The reactive silica and alumina existed in the activated ash may react in the lime mortar system to produce major phases such as C-S-H, C 4 AH 13 and C 2 ASH 8 (Shi and Day (1995)). This can be well supported with the SEM images [Fig 2 (b-f)]. It is observed that the use of binary activators produced higher lime (a) (b)( 2 (b) [CaO] mmol/l Coarse NaNO 3 NaOH Ground NaOH+NaNO NaOH+Ca(OH) 2 3 Ca(OH) 2 [CaO] mmol/l [OH] mmol/l [OH] mmol/l Figure 3. Lime saturation curve (a) activated ash (b) NaOH + NaNO 3 treated ash

7 Table 2. EDAX analysis of coarse and activated ashes Fly Ash Si Al Fe Si/Al Al/Fe Coarse Ground NaNO NaOH Ca(OH) NaOH+NaNO NaOH+Ca(OH) Table 3. Reactivity of activated fly ash in lime and cement mortars Fly Ash Particle Size Distribution D 10 D 50 D 90 Specific Surface area (BET) ( m 2 /g ) % Compressive strength of Lime plain cement mortar (days) Reactivity (Mpa) Coarse Ground NaNO NaOH Ca(OH) NaOH+NaNO NaOH+(CaOH) strength than the single activator by creating active surface for reaction at high ph. Among the activators used, the binary mixture of NaOH and NaNO 3 gave maximum lime strength. It is also noted that as the fineness of activated ash increases, the water requirement in the mix increases during casting. The use of ash treated with Ca(OH) 2 in the lime mortar requires lower water demand than the coarse ash due to the existence of C-S-H on its surface at the time of treatment whereas other treatments showed higher water demand for hydration due to its amorphous structure (presence of zeolitic precursor phase). Without using activator, the lime reactivity strength is basically manifested out of calcium silicate hydrates. Though calcium aluminate hydrates formed in the lime- fly ash reactions, they do not contribute for strength. The compressive strength of cement-ash mortars is given in Table 3. It is observed that compressive strength of the cement mortars containing coarse and activated ashes increases with increasing curing age. The compressive strengths of cement-coarse ash mortar were 64, and 77.87% only of the plain cement mortar at 7, 28 and 90 days respectively which are lower than the minimum of 80% as specified in IS: 3812 (Indian Standard (2003): Pulverized Fuel Ash - Specification, Part 1: for use as Pozzolana in Cement, Cement Mortar and Concrete)). Contrary to this, the cement-activated ash mortars exhibited their compressive strength ranging between % of the plain cement mortar at different ages. The ash treated with Ca(OH) 2 attained more strength due to the presence of dominant C-S-H phase than the other treatments at different ages except the binary mixture of NaOH + NaNO 3 at 90 days curing. This is contrary to the results obtained in the lime reactivity test.

8 4 CONCLUSIONS Results indicate that mechano-chemical activation of the coarse ash is an effective mean over the mechanical treatment to enhance its hydraulic reactivity. The surface morphology of coarse ash has been changed significantly from inert to the reactive surfaces. Specific surface area of ash increased and mean particle size decreased. Si/Al ratio also decreased after processing. As a result, lime removal characteristics, compressive strength of the cement mortar and lime reactivity increased substantially. The binary mixture of NaOH and NaNO 3 activator gives superior pozzolanic reactivity than the single activator. It is concluded that processed coarse ash meets the specification in terms of fineness and lime reactivity and can be suitably used in cement concrete. ACKNOWLEDGEMENT This paper forms part of a Supra Institutional Project of CSIR R & D program (Govt. of India) and is published with the permission of CSIR-Central Building Research Institute, Roorkee. REFERENCES 1. Arjunan, P., Silsbee, M.R. and Roy, D.M. (2001) Chemical Activation of Low Calcium Fly Ash, Part 1: Identification of Suitable Activators and their Dosage, Intl Ash Utilization Symposium, Centre for Applied Energy Research, University of Kentucky, Paper # British Standard (BS EN) 196 (2005), Methods of Testing Cement. Part 5: Pozzolanicity Test for Pozzolanic Cements. 3. Donatello, S., Freeman-Pask, A., Tyrer, M., Cheeseman, C. R. (2010) Effect of Milling and Acid Washing on the Pozzolanic Activity of Incinerator Sewage Sludge Ash, Cement and Concrete Composites, 32: Donatello, S., Tyrer, M. and Cheeseman, C.R. (2010) Comparison of Test Methods to Assess Pozzolanic Activity, Cement and Concrete Composites, 32: IS: 1727 (1999), Indian Standard, Methods of Tests for Pozzolanic Materials, Bureau of Indian Standards, New Delhi, India. 6. Malhotra, V.M. and Ramezanianpour, A.A. (1994) Fly Ash in Concrete, CANMET, Natural Resources Canada, Ottawa, Canada. 7. Paul, K.T., Satpathy, S.K., Manna, I., Chakraborty, K.K. and Nando, G.B. (2007) Preparation and Characterization of Nano Structured Materials from Fly Ash: A Waste from Thermal Power Stations, by High Energy Ball Milling, Nanoscale Research Letter, 2: Polettini, A., Pomi, R. and Fortuna, E. (2009), Chemical Activation in View of MSWI Bottom Ash Recycling in Cement-Based Systems, Journal of Hazardous Materials, 162: Poon, C.S., Qiao, X.C. and Lin, Z. S. (2003) Pozzolanic Properties of Reject Fly Ash in Blended Cement Pastes, Cement and Concrete Research, 33: Qian, J., Shi, C. and Wang, Z. (2001) Activation of Blended Cements Containing Fly Ash, Cement and Concrete Research, 31(8): Roy, D. M.(1988), Fly ash and Silica Fume Chemistry and Hydration, International Workshop on the use of Fly ash, Slag, Silica Fume and other Siliceous Materials in Concrete (W. G. Ryan eds), July, 4-6, 1-15, Sydney, Australia. 12. Shi, C. and Day, R.L. (1995) Acceleration of the Reactivity of Fly Ash by Chemical Activation, Cement and Concrete Research, 25: Shi, C. and Day, R.L. (2001) Comparison of Different Methods for Enhancing Reactivity of Pozzolanas, Cement and Concrete Research, 31: