Effect of Superabsorbent Polymers and Presoaked Coarse Recycled Shale Lightweight Aggregates on Relative Humidity Development in Early-Age Concrete

Self-desiccation-induced shrinkage may result in cracking at an early age, which is averse to the durability of concrete. Internal curing (IC) agents, such as superabsorbent polymers (SAP), are normally used for moisture regulation and shrinkage reduction. In addition, the make-up of recycled shale lightweight aggregate (RSLA) results in a good absorbing capacity, which makes it a potential candidate for IC. In this paper, the synergistic effect of SAP and RSLA on the relative humidity (RH) variation in early-age concrete under sealed conditions is investigated experimentally in terms of the setting time, relative humidity, and autogenous shrinkage. The results indicate that adding SAP and presoaked RSLA can significantly postpone the initial and final setting times. The initial setting time of RSLA30 and SAP06 is delayed by 127 and 171 min, respectively, compared to the benchmark mixture. In addition, increasing the amounts of SAP and presoaked RSLA can effectively extend the duration of the vapour-saturated stage, reducing the decrease in RH and autogenous shrinkage at 28 days. When the RSLA dosage increases from 0 to 10%, 20%, and 30%, the duration of the vapour-saturated stage is extended by 2, 9.4, and 26 days, respectively. Moreover, due to different water desorption behaviours, more IC water released by RSLA during the initial stage can slow the water release of SAP and lead to a higher RH level at 28 days.