Improving the reactivity of industrial recycled concrete fines: Exploring mechanical and hydrothermal activation

Concrete waste from demolition sites is commonly crushed and sieved for aggregate reuse, yielding fine fractions. However, their low reactivity presents challenges for reuse in new cement-based mixtures. This study explores hydrothermal and mechanical activation as a method to increase the chemical reactivity of recycled concrete fines (RCf), facilitating their potential to be used as a supplementary cementitious material. A slurry of RCf was autoclaved for 8 hours and mechanically activated for 166 minutes. Different calcium hydroxide additions (0, 10, 20, and 30 wt%) were used in the slurry mix to evaluate the influence of Ca/Si ratios in forming C-S-H phases during hydrothermal treatment. Samples were assessed by particle size distribution (PSD), Rietveld quantitative phase analysis (QXRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and R³ (“rapid, reproducible, and reliable”) pozzolanic reactivity test using isothermal calorimetry. C-S-H phases (11 Å tobermorite and xonotlite) and amorphous phases were formed during autoclaving, and all samples exhibited considerable amorphization upon mechanical activation. On one hand, hydrothermal treatment, without adjusting the Ca/Si ratio with Ca(OH)₂, is ineffective in enhancing solely the reactivity of RCf. On the other hand, the sample RCf-A3–166, with 30 %wt Ca(OH)₂ and Ca/Si ratio of 0.47, presented the highest amorphous content of 73.6 wt% after the treatments proposed. Hence, this research provides valuable insights into using hydrothermal and mechanical activation as a promising route to improve RCf properties, offering innovative solutions for managing one of the largest industrial waste streams.