Enhancing the microstructure and surface texture of recycled concrete fine aggregate via magnesium-modified carbonation

Investigations on the carbonation of recycled concrete aggregates have been extensively carried out. In this work, recycled concrete fine aggregates (RCFs) were firstly treated with magnesium nitrate solutions of various concentrations and then carbonated. Extensive techniques were employed to characterize the changes of physical properties, pore structures, surface morphologies and phase assemblages of the RCFs after carbonation. Besides, the influences of carbonated RCFs on the hydration, compressive strength and phase assemblages of new cement systems were also evaluated. The results suggested that the magnesium-modified carbonation was more effective than pure carbonation in enhancing the microstructures of RCFs considering the reduction of total pore volume and water absorption. The refinement of microstructure was attributed to the formation of magnesium-calcite, hydromagnesite, etc. as a result of four sequential steps: adsorption of Mg²⁺ into RCF matrices, precipitation of magnesium hydroxide, enhanced dissolution of Ca²⁺ and the carbonation of magnesium hydroxide and calcium hydroxide mixes. In addition, the magnesium-modified carbonation was also found to significantly change the morphologies of carbonation products by generating spear-like magnesium-calcite, rosette-like hydromagnesite, plate-like hydrotalcite/mononitrate and spheroidal vaterite. The certain morphological changes of reaction products could promote the formation of an intersected 3D surface texture for RCFs. This particular surface texture could establish a well-connected interface with enhanced interlocking effects when the RCFs were used in new mortars, contributing to the bonding strength and compressive strength enhancement, and the best enhancement was achieved at low Mg²⁺ dosages (e.g., 0.05 M and 0.1 M).