Mechanical, freeze-thaw resistance and heavy metals leaching properties of alkali-activated recycled concrete powder solidified sludge

Solidified sludge was an efficient method to consume the large amount of sludge, which can be used as the subgrade filler in road engineering. However, the traditional solidified materials, such as cement and lime, consumed a large amount of natural resources and increased carbon emissions. This study used the alkaliactivated recycled concrete powder (ARCP) to solidify sludge, which could be used as a potential subgrade stabilization filler. The optimal factors of ARCP were firstly determined by orthogonal test according to the compressive strength. Then the influences of different ARCP dosage, sludge moisture content and curing method on the mechanical properties, freeze-thaw resistance and heavy metals immobilization of the ARCP solidified sludge (ARCPS) were investigated. At last, microstructures of ARCPS were analyzed by SEM, XRD and TG-DTG tests. The experimental results demonstrated that the optimal mixture for ARCP: the silicate modulus of 1, liquidsolid ratio of 0.48 and calcium hydroxide content of 3 %. The solidified process by ARCP improved the mechanical properties and freeze-thaw resistance of ARCPS greatly. The 7 d unconfined compressive strength of ARCPS could reach 1305-2033 kPa when the sludge moisture content was 15 %, which meet the requirement of 500 kPa from the Chinese standard. The frost heaving rates of ARCPS were between non-frost heaving and mild frost heaving. The sludge moisture content was the most important factor affecting the freeze-thaw resistance of ARCPS. The properties of ARCPS were further improved after carbonation curing. The leaching mass concentration of heavy metals decreased significantly after the solidification of sludge. After 5 freeze-thaw cycles, the heavy metal leaching concentration increased but was still met the requirement of standards. SEM results showed that the surface of carbonated ARCPS was covered by dense scale-like carbonation products, which further enhanced the strength and freeze-thaw resistance of ARCPS. The results can be used to guide the application of sludge and ARCP in construction.