Comparative Study on the Utilization of Calcined Clay, Reclaimed Fly Ash, and Slag as Supplementary Cementitious Materials

Supplementary cementitious materials (SCMs) are key for the durability of transportation infrastructure. Fly ash (FA), the most used SCM in the U.S.A., is becoming scarce, creating issues for state departments of transportation. To address the FA shortage, reclaimed FA products are becoming available; however, this is only a temporary solution. A promising long-term solution is the use of calcined clay (CC) either in the form of limestone calcined clay cement (LC3) or LC3-like systems, known as LC2, where Portland limestone cement (PLC) is simply blended with CC. This study examines the particle size, chemical composition, reactivity, strength activity, and effects on workability of two CCs (CC-1 and CC-2) from U.S. sources, and compares these to reclaimed FA and slag cement (SC). CC-1 was chosen for further studies in mortars because of its commercial availability in powder form. Mortar specimens with six binder systems (LC3, LC2-30% CC, LC2-50% CC, PLC, PLC-30% FA, and PLC-50% SC) and two water-to-binder ratios (0.4 and 0.45) were produced and systematically compared. Early- and long-term strength development, heat of hydration, bound water, calcium hydroxide content, and environmental impacts were evaluated. Results suggest that LC2 and LC3 systems are promising alternatives to alleviate the FA shortage, particularly because of their comparable or superior performance in key parameters such as strength development and environmental impact compared to the PLC system.