Synergistic enhancement of recycled aggregates via acetic acid dissolution and silica fume filling: A two-step mechanistic study

This study investigates a two-step treatment method for recycled aggregates (RA) obtained from crushed concrete waste, aiming to enhance the RA properties. In the first step, RA are immersed in an acetic acid solution to dissolve the weak calcium-based hydration products attached to the RA. In the second step, silica fume (SF) is incorporated into the same soaking suspension at different dosage levels to react with the released Ca 2+ ions, forming additional Calcium-(Aluminum)-Silicate-Hydrate (C-(A)-S-H) gels that effectively fill the pores and refine the interfacial transition zones (ITZs). Experimental results demonstrate that, compared with untreated RA, acetic acid treatment alone reduces water absorption by 9.4 % and the crushing value by 3.0 %, while the subsequent addition of SF further decreases water absorption and crushing values by up to 16.9 % and 14.8 %, respectively. Mortar specimens prepared with these treated RA show up to a 22.1 % increase in compressive strength at 28 d. Microstructural analyses, including mercury intrusion porosimetry, scanning electron microscopy, X-ray diffraction, and 29 Si nuclear magnetic resonance, support that the synergistic action of acetic acid and SF leads to enhanced dissolution of calcium-based phases followed by the formation of dense hydration products, which improve RA packing and ITZs quality.