Alkali Silica Reactivity of Blended Class C and Class F Fly Ash Systems
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This study presents an assessment of the reactivity and performance of blended systems containing Class C and Class F fly ashes, including fly ashes produced from blended coal sources (bituminous and lignite). Seven fly ashes comprising of three Class F and four Class C with varied calcium and alumina content were obtained from Texas and used in this study. The three Class F fly ashes were designated as F1, F2, and F3, while the four Class C fly ashes were designated as C1, C2, C3, and C4. The Class F fly ash F2 was produced from two different coal sources (bituminous and lignite). These fly ashes were tested to ascertain: the heat of hydration when used as partial replacement for cement, ability to improve mechanical properties and mitigate alkali-silica reaction (ASR). Mortar and concrete mixtures were produced. These mixtures involved binary blends of Class C and Class F fly ashes at 20 and 30% cement replacements, and ternary blends at 10/20%, 20/10% and 15/15% cement replacements for Class C and Class F fly ashes respectively. The following test were performend: compressive strength, splitting tensile strength, elastic modulus, drying shrinkage, heat of hydration using isothermal calorimetry, ASR mitigation using the accelerated mortar bar test (ASTM C1260/C1567) and the concrete prism test (ASTM C1293). The long-term durability was also investigated with the use of large concrete exposure blocks. The results of the mechanical property tests showed that the binary and ternary mixtures involving the Class C fly ashes (C2 and C3) and the Class F fly ash (F2) outperformed other fly ashes in improving mechanical properties especially at early age (Day 7). The heat of hydration test results showed that the binary and ternary mixtures of the Class C fly ash C2 and the Class F fly ash F2 generated more heat than that of Class F fly ash F1. Finally, the results of the ASR tests showed that the Class F fly ashes performed better than the Class C fly ashes in mitigating ASR. However, the Class F fly ash F1 was the most effective followed by the Class F Fly ashes F2 and F3. The effect of blending was also observed as the ternary mixtures of the Class C and Class F fly ashes were able to keep expansion within the 0.04% limit at year one in the concrete prism test (ASTM C1293) and large concrete exposure blocks.