Effect of retarder on hydration properties of light-burned magnesia

Light-burned magnesia (LBM) is a common expansive agent to compensate for the thermal shrinkage of mass concrete, and the expansive property of MgO mass concrete is affected by the hydration behavior of LBM. Besides, retarder is usually added to extend the setting time of mass concrete in hot climate construction. Retarder affects hydration of cement and LBM at the same time. However, little is known about the effect of retarder on the hydration of LBM expansive agent. This paper investigated the effects of four retarders (citric acid, sodium gluconate, tartaric acid and calcium sulfate) on the hydration behavior of LBM expansive agent. Setting time, hydration heat, hydration degree and compressive strength were employed to characterize the hydration process of LBM with various retarders. Moreover, the composition of the product was quantitatively and qualitatively analyzed by XRD and TG, and the morphology of the product was analyzed by SEM. The results showed that retarders mainly affected the hydration behavior in the early age of LBM. Citric acid inhibited the hydration of LBM, but improved the compressive strength by forming flower-like Mg(OH)(2) crystal. Tartaric acid inhibited the hydration of LBM, and produced products with poor crystallinity. Furthermore, sodium gluconate significantly prolonged the setting time of LBM and obtained complete brucite products. Besides, calcium sulfate accelerated the hydration of LBM at 1.0 water/LBM ratio, but inhibited the hydration at 0.45 water/LBM ratio. Finding from this study may provide new insights into the design and comparison of experimental studies on the expansive property of MgO mass concrete affected by retarders. (C) 2020 Elsevier Ltd. All rights reserved.