Potential Use of the Prewetting of Recycled and Lightweight Aggregates to Improve Cement Pastes for Residue Solidification/Stabilization Systems with Chromium and Zinc

Abstract

Internal curing agents are materials that supply additional water for cement hydration, the water being supplied by internal reservoirs (porous and absorbent aggregates). In this study, the potential for a recycled concrete aggregate (RCA) to act as an internal curing agent and chemical sorbent for heavy metal retention was investigated. Mortars were designed with a water/cement ratio of 0.5 and a mix ratio of 1∶3 (cement:fine aggregate; by mass), where 25% of the natural aggregate (NA) was replaced with a lightweight aggregate (LWA) or RCA, under prewetted or dry conditions. Mortars were also prepared with the addition of Cr6+ and Zn2+ in order to verify the chemical sorbent potential of the aggregate and the retention capacity of mortars designed with internal curing. Results show that the nature and prewetting of the aggregate affect the physical and mechanical properties of the mortars. The prewetted aggregate differs in terms of the period and duration of water release from the period and duration of water release from the pores of the material, modifying the paste microstructure at early ages. The results of the flexural strength clearly demonstrate the efficiency of the internal curing, because cracks in the interfacial transition zone strongly influence flexural strength. Regarding the results of the leaching test, it can be concluded that mortars prepared with RCA without prewetting show greater retention of added heavy metals, retaining 100% of the chromium and more than 91% of the zinc. In relation to mortars with LWA, there was 100% retention for zinc and 94% for chromium. Lastly, it was noted that RCA can be used as an internal curing agent without adversely affecting the final cement material, increasing the potential application of RCA, and can also be used as a chemical sorbent to aid in the solidification/stabilization process of heavy metals.