Influence of thermally activated industrial concrete fines of different origin on mortar strength development

Thermal activation of recycled concrete fines (RCFs) has gained traction as a promising method for mineral waste recycling and the substitution of Portland cement, thereby reducing CO 2 emissions. However, as the composition of concrete fines varies, their performance as supplementary cementitious materials (SCMs) also changes. The present work compares 25 industrial concrete fines from various sources and compositions concerning their thermal decomposition and their influence on hydration heat and mortar compressive strength development. RCFs were processed at 100 °C, 400 °C, and 600 °C, and tested for their hydration heat development and compressive strength. Findings show that blended cements exhibit fast rehydration, which results in high early strength. Compressive strength of mortar containing processed RCFs increases with activation temperature. However, the mass loss from the disintegration of hydraulic hardened cement paste phases is a more significant parameter for assessing the strength contribution. Based on these findings, the study presents a new model for predicting compressive strength alongside current provisions for established SCMs. While compressive strength contribution of RCFs processed at 100 °C on average assumes about 10 % of CEM I reactivity and may be even negative, the same RCFs show an in increase to 41 % for 600 °C, with some RCFs even exceeding 60 %. This allows for the substitution of increasingly scarce fly ash, which exhibits strength contribution around 40 %-50 %. • 25 different concrete fines were thermally activated and used as SCMs. • Thermally activated fines exhibit increased initial hydration heat development. • Compressive strength contribution increases with higher activation temperature. • The mass loss from cement paste dehydration correlates with strength contribution. • Compressive strength contribution of activated concrete fines may exceed fly ash.