Hydration of Binary and Tertiary Fly ash and Quicklime Blended Cement Paste System
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Abstract
The hydration kinetics in blended cementitious systems with fly ash are quite different from those in conventional cementitious systems. The pozzolanic nature of fly ash not only reduces the rate of hydration in cement-fly ash blends, but also reacts with hydrates from cement hydration to form additional calcium silicate/aluminate hydrates. Depending on the cement content and the reactivity level of the fly ash, the pozzolanic reaction could be a fairly slow process compared to straight Portland cementitious systems. This study investigates the hydration kinetics of cement-fly ash-quicklime blended paste at 3 distinct fly ash replacement levels of 20%, 40% and 95% to determine their suitability for engineering applications. Two sets of cement-fly ash blends were studied in this work, one set of cement-fly ash blended pastes with quicklime and the second set without quicklime. Since the siliceous minerals in fly ash react with the Ca(OH)2 produced during hydration of cement, the inclusion of quicklime was to provide this additional Ca(OH)2 to aid the fly ash pozzolanic hydration reactions. Additional water was also used in two sets of fly ash replacement of 20% and 95% to account for moisture consumed during hydration of quicklime (CaO to Ca(OH)2) reaction and to test the effect of quicklime slaking on hydration in cement-fly ash blended paste with moderate and high fly ash replacement levels. The thermogravimetric analysis (TGA) was used in assessing the degree of hydration for all paste mixes in this study at curing intervals of 3,7, 28 and 180 days. TGA results indicated an additional increase in hydration for all cement-fly ash blended pastes with quicklime compared to those without quicklime, at the end of the 180-day curing period with the effect of quicklime being most significant in mixes with high fly ash replacement levels. Although for the high-volume fly ash of 95% replacement cement blended paste, the effect of additional water increased the overall hydration by an additional 40% in the slaked mix, indicating the potential of quicklime slaking as a technique for increasing pozzolanic hydration reactions in applications where high fly ash contents are desired. However, TGA measurements also indicate that increased weight which correlates degree of hydration in fly ash cement blended paste could be as a result of the formation of AFm and AFt phases and not necessarily hydration products like calcium silicate/aluminate hydrates. Pearson ANOVA analysis showed a correlation of 0.718 between the calcium silicate/aluminate hydrates and the compressive strength for cement-fly ash blended paste with fly ash replacement of 0%, 20% and 40%. The ANOVA correlation of 0.8049 was established between compressive strength and DTG peak. The DTG peak values for most of the paste values were established at temperatures within 90 to 100 oC. Which were slightly less than the initial decomposition temperature of 105oC for calcium silicate and aluminate hydrates. It is possible that this correlation was established due to a portion of the calcium silicate and aluminate hydrate mineral phases decomposing at temperatures lower than 105oC.