Influence of activation temperature and cooling rate in the performance of recycled cement

A major challenge today is reconciling the increasing global demand for concrete and cementitious materials with the need to minimize environmental impacts, particularly concerning natural resource depletion and CO 2 emissions from the cement industry. Additionally, the generation of construction and demolition waste associated with this sector further exacerbates these impacts. To address this, several solutions have been proposed, including the use of recycled concrete and mortar waste. In this case, after undergoing grinding and heat treatment, this waste can partially or even fully replace Portland cement. Grinding increases the reaction kinetics while heat treatment dehydrates the paste. Moreover, for a given waste composition and particle size, the degree of paste dehydration is influenced by temperature and treatment time. This process can potentially result in a reactive material known as Recycled Cement. This study investigates the influence of activation temperature and cooling rate on the mechanical properties of mortars prepared with treated cement pastes. First, cement pastes were prepared in the laboratory using a water-to-cement ratio of 0.5 and CPV-ARI cement. After 28 days of curing, the hardened pastes were comminuted thorough crushing and grinding. The resulting powder was then heated to 500°C, 600°C, and 700°C at a rate of 10°C/min, held at these temperatures for 2 hours, and subsequently cooled either in the air or inside the switched-off furnace. The thermomechanically treated material was characterized in terms of particle size distribution, chemical composition, thermal decomposition, and phase composition. Next, mortars with a w/c of 1.0 were prepared, each using 100% of one of the six Recycled Cements as the sole binder. After 28 days of curing, compressive strength tests were conducted to evaluate mechanical strength. Results indicate rehydration of the Recycled Cement under all conditions studied. The material heat-treated at 700°C and air-cooled exhibited the best mechanical performance.