Effect of Fly Ash and Silica
Effect of Fly Ash and Silica
Effect of Fly Ash and Silica
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continue to refine the properties of the hardened concrete as it matures[5, 9, 17, 18, 19]. Also, their ternary combination would result in reduced dosage of water reducing admixtures [20]. Sorptivity is a standard test for measuring the rate of absorption of water by hydraulic cement concretes and it determines the susceptibility of an unsaturated concrete to water penetration by absorption when no head of water exist. Minimising sorptivity is important in order to reduce the ingress of chloride or sulphate into concrete[21]. Sorptivity would increase with increasing content of fly ash[22]. Compared with CEM I and fly ash binary cement concretes, the addition of silica fume as binary and ternary cement component would reduce sorptivity[23]. Cement combinations, by virtue of their delayed strength development at early ages, would be more suitable for mass concreting and concrete work in hot climate than CEM I. Due to their long-term pozzolanic reaction with curing age, cement combinations would also be good for under-water concrete structures. While BS EN 197- 1 permits the use of silica fume and fly ash of up to 10% and 55% respectively, data from the European Ready Mixed Concrete Organisation show that the cement addition content (majorly GGBS and fly ash) was less than 20% of the total cement consumption in ready-mixed concrete. At equivalent strengths, the use of cement combinations could result in better performance[24, 25]. However, while concrete in practice is prescribed on the basis of strength, most researches in literature were conducted at equal water/cement ratio. Hence this paper, within the standard limits, examined the effect of cement combinations on the sorptivity of concrete at equal water/cement ratios and strengths. 2. Experimental Materials and Mix Proportions The cements used were Portland cement (CEM I, 42.5 type) conforming to BS EN 197- 1, siliceous or Class F fly ash (FA) conforming to BS EN 450- 1 and silica fume (SF) in a slurry form (50:50 solid/water ratio by weight) conforming to BS EN 13263- 1. The physical and chemical properties of the cements are presented in Table 1. The aggregates consisted of 0/4mm fine aggregates and 4/10mm and 10/20mm coarse aggregates. The coarse aggregates were uncrushed and they come in varied shapes. The 4/10mm aggregates have rough texture and the 10/20mm aggregates were smooth. The physical properties of the aggregates are presented in Table 2.
Table 1: Physical and chemical properties of cements
CEMENTS PROPERTY CEM I Blaine fineness, m2/kg Loss on ignition, % a) Particle density, g/cm3 % retained by 45m sieve b) FA SF
* 2.7 2.17 -
Particle size distribution, cumulative % passing by mass c) 100 125 m 98.2 100 m 75 m 93.2 81.8 45 m 25 m 57.1 30.1 10 m 13.5 5 m 2 m 5.6 2.9 1 m 0.7 m 1.3 0.2 0.5 m * Fineness for SF = 15,000-30,000 m2/kg[26] b) In accordance with EN 450- 1
100 99.2 96.5 87.0 66.2 40.6 24.1 10.9 4.8 1.9 0.3
100 100 100 100 98.8 93.8 87.5 85.5 78.7 50.7 10.5
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4/10 mm
10/20 mm
Shape, visual Surface texture, visual Particle density 2) Water absorption, % 3) % passing 600 m sieve
1) Aggregates were obtained from Wormit Quarry. 2) In accordance with BS EN 1097- 6 3) In accordance with BS EN 1097- 6, Laboratory-dry condition
Potable water, conforming to BS EN 1008, was used for mixing, curing and testing the concrete specimens. In order to provide reasonably workable concretes and a uniform basis for comparing concrete performance at low water/cement ratios and a fixed water content, a superplasticiser based on carboxylic ether polymer conforming to EN 934-2 was applied during mixing to achieve a consistence level of S2 defined by a nominal slump of 50100mm in BS EN 206- 1. The yield corrected concrete mix proportions, to the nearest 5 kg/m3, based on the BRE Design Guide[27], a free water content of 165 kg/m3 (to avoid an excessively sticky mix) and saturated surface-dry (SSD) aggregates are presented in Table 3 for 0.35, 0.50 and 0.65 water/cement ratios.
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MIX
PROPORTION, kg/m3
MIX COMBINATION
w/c
CEMENTS
CEM I
FA
SF a)
0/4 mm
4/10 mm
10/20 mm
100% CEM I
0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65
475 330 255 375 260 200 375 260 200 305 210 165 300 210 165 300 210 165 205 145 110 205 145 110 450 315 240 425 295 230
95 65 50 70 50 40 165 115 90 140 100 75 115 80 65 255 180 135 205 145 110 -
25 15 15 25 15 15 45 35 25 45 30 25 25 15 15 45 35 25
650 740 820 640 735 815 640 735 815 635 730 815 635 730 815 635 730 815 625 725 810 625 725 810 645 740 820 645 740 820
375 385 380 370 385 375 370 380 375 365 380 375 365 380 375 365 380 375 360 375 370 360 375 370 375 385 380 370 385 380
755 770 765 745 765 760 745 765 760 740 760 755 740 760 755 740 760 755 730 755 750 730 755 750 750 770 760 750 770 760
165 165 165 165 165 165 165 165 165 165 165 165 165 165 165 165 165 165 160 160 160 160 160 160 165 165 165 165 165 165
0.41 0.33 0.25 0.37 0.30 0.23 0.40 0.31 0.24 0.33 0.27 0.20 0.38 0.29 0.23 0.40 0.35 0.26 0.31 0.26 0.19 0.36 0.31 0.24 0.43 0.35 0.26 0.46 0.38 0.28
80%CEM I+20%FA
80%CEM I+15%FA+5%SF
65%CEM I+35%FA
65%CEM I+30%FA+5%SF
65%CEM I+25%FA+10%SF
45%CEM I+55%FA
45%CEM I+45%FA+10%SF
95%CEM I+5%SF
90%CEM I+10%SF
a) Dry powder content. b) % Superplasticiser (SP) required for consistence class 2 (BS EN 206-1) is related to the total cement content.
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3. Experimental Methods Concrete was prepared to BS EN 12390- 2 and the specimens were cast, cured under a layer of damp hessian covered with polythene for 20-24 hours, demoulded and cured in water tanks maintained at about 20oC until the tests dates. Tests were carried out on hardened concrete specimens to determine their cube compressive strength and sorptivity at equal water/cement ratios. The cube compressive strengths were obtained in accordance with BS EN 12390- 3 using 100mm cubes at the curing age of 28 days at the water/cement ratios of 0.35, 0.50 and 0.65. Since absorption into concrete is a function of the drying temperature and immersion duration[28], sorptivity was determined with specimens oven-dried to constant mass at 1055oC to ensure a uniform basis for the comparison and repeatability of the results. This is because it is generally believed that at this temperature the pozzolanic reactions of the cement additions would be stopped, the plastic shrinkage cracking associated with reduced bleeding that normally characterise the use of fine materials would be avoided and the microstructure of the test specimens would not be adversely affected to prevent the repeatability of the results. Sorptivity was carried out in accordance with ASTM C1585- 05 using concrete specimens 100mm in diameter and about 50mm thick. After being cooled to room temperature in a dessicator, the oven-dried specimens were waxed on the side and covered on one end with a loose plastic sheet attached with masking tape to allow the entrapped air to escape from the concrete pores while at the same time preventing water loss by evaporation. After obtaining the initial mass, the test surface (i.e. uncovered end) of each sample was placed on two lines of roller support placed in water maintained at 3-5mm level above the top of the support throughout the duration of the test (Figure 1).
The sorptivity test was conducted over six hours and the cumulative change in mass at specific intervals was determined. For each mass determination, the test specimen was removed from water and the surface was cleaned with a dampened paper towel to remove water droplets. The mass of the sample was then measured and the sample was replaced to continue the test. The cumulative change in mass at one minute, five minutes, ten minutes, 20 minutes, 30 minutes, one hour, two hours, three hours, four hours, five hours and six hours were used to obtain the respective cumulative absorption values (i) expressed by i =
m A
(1)
where m = cumulative change in mass due to water absorption, A = cross-sectional area of test specimen, mm2 and = density of water.
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It has been shown by Hall [29] that there exists a relation of the form i = St0.5 (Darcys Law) (2)
where S = sorptivity in mm/min (1mm/min = 1.29x10-0.4 m/s) and t = time in minutes Hence the cumulative absorption values were plotted against the square root of the times and sorptivity (the initial rate of water absorption) was obtained as the slope of the line that best fits the plot. 4. Analysis and Discussion of Results Table 4 shows that the cube compressive strength of concrete reduced with increasing water/cement ratio and that while the addition of fly ash would reduce strength with increasing content, the addition of silica fume as binary and ternary cement component resulted in improved strength at 28 days. Also, the sorptivity of fly ash binary cement concretes which were slightly higher than that of CEM I concrete at 28 days reduced progressively such that at 180 days they became lower. This is because fly ash would require a higher level of alkalinity which increased progressively with the release of Ca(OH)2 by the hydration reaction of CEM I to improve the resistance of its concretes to sorption. Since the reductions at these ages increased with increasing content of fly ash, sorptivity would reduce at equal water/cement ratio with increasing content of fly ash. The Table also shows that the addition of silica fume as binary and ternary cement component reduced the sorptivity of concrete at the test ages. The resistance of the ternary cement concretes to sorptivity also increased with increase in the total content of the cement additions. The higher fineness of silica fume (Table 2) must have resulted in better packing between the cements and at the interface zones between the cement paste and the aggregates. Also, more nucleation sites would be provided by the finer silica fume for Ca(OH)2 to improve the pozzolanic reactions and reduce the sorptivity values of concrete. Table 4 shows that sorptivity of concrete reduced with increasing water/cement ratio and that equivalent sorptivity values of the concretes at equal water/cement ratio would be achieved at different ages and therefore at different compressive strengths. Concrete is specified in practice on the basis of strength and since at 28 days a substantial quantity of hydration would have taken place, the sorptivity of these concretes has been investigated at equivalent strength at 28 days. The cube compressive strength and sorptivity of the concrete at the curing age of 28 days at the water/cement ratios of 0.35, 0.50 and 0.65 (Table 4) were used, by interpolation, to obtain the sorptivity of the concretes at the equivalent strengths of 30, 40 and 50 N/mm2 at 28 days (Table 5) which happen to be the range of strengths that would commonly be used in practice. These strengths also satisfy most of the strength requirements in BS EN 206-1 and BS 8500. Table 5 shows that sorptivity of concrete reduced with increasing strength and compared with CEM I, the incorporation of cement additions at equivalent strengths would reduce the sorptivity of concrete. This is because all the cement combination concretes now have lower sorptivity values than CEM I concrete at the equivalent strengths. The fly ash and silica fume binary cement concretes performed better than CEM I and their sorptivity values reduced with increasing content of the cement additions. At a total replacement level of 3555%, the addition of silica fume, as a ternary cement component, resulted in concretes with sorptivity values lower than that of their respective fly ash binary cement concretes. However, at a total replacement level less than 35%, silica fume as a ternary cement component only resulted in further reduction in sorptivity at lower strengths as the situation reverted as the strength increased.
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MIX COMBINATION
w/c
28 Days
90 Days
180 Days
100% CEM I
0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65 0.35 0.50 0.65
80.0 54.0 38.5 72.0 46.5 30.0 83.0 55.0 36.0 60.0 35.0 20.0 65.0 43.0 26.0 77.0 49.5 32.0 42.0 24.0 12.0 57.0 36.0 22.0 78.0 58.0 41.0 82.0 59.0 45.0
200 260 335 205 265 345 190 245 315 210 270 360 170 225 300 150 190 255 210 275 375 95 140 250 120 205 300 115 185 270
160 215 295 155 220 300 150 200 260 155 220 295 130 170 230 120 155 205 150 210 295 80 120 170 100 160 235 95 145 210
135 190 260 120 185 255 125 165 220 120 180 250 100 135 180 95 125 165 110 155 250 70 95 125 85 120 175 80 115 170
80%CEM I+20%FA
80%CEM I+15%FA+5%SF
65%CEM I+35%FA
65%CEM I+30%FA+5%SF
65%CEM I+25%FA+10%SF
45%CEM I+55%FA
45%CEM I+45%FA+10%SF
95%CEM I+5%SF
90%CEM I+10%SF
5. Conclusion At equal water/cement ratio, fly ash binary cement concretes have poor resistance against sorption and the resistance reduced with increasing content of fly ash. However, the resistance increased with increasing age such that at 180 days they became better than that of CEM I concrete. Silica fume binary and ternary cement concretes have better resistance to sorption than CEM I and fly ash binary cement concretes at both early and later ages and their resistance increased with increasing content of silica fume. The sorptivity of concrete is influenced by strength. Compared with CEM I concrete, the sorptivity of the cement combination concretes were lower at the equivalent strengths and they reduced with increasing total content of the cement additions.
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100% CEM I
380
330
275
345 340
295 300
255 260
250 175
215 125
180 105
370 360
305 290
250 235
References
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