Potential use of lightweight aggregate (LWA) produced from bottom coal ash for internal curing of concrete systems

This study evaluates the potential use of a novel lightweight aggregate (LWA), spherical porous reactive aggregate (SPoRA), produced from waste coal bottom ash, for internal curing of concrete. The engineering properties of SPoRA required for concrete internal curing were assessed including specific gravity, porosity, sphericity, water absorption, and water desorption. SPoRA showed a low oven dry specific gravity, ranging from 0.83 to 1.43, accompanied by high porosity which makes it capable of storing the amount of water needed for concrete internal curing. Through X-ray computed tomography (XCT), the high porosity of SPoRA (39.6%-57.8%, by volume) was measured and confirmed, and its sphericity, which influences the workability of concrete, was evaluated. XCT evaluation also showed that the pore structure of SPoRA is well connected, allowing stored water to move through the pore structure to the outer surface during the self-desiccation of concrete. SPoRA's 72 h water absorption not only passed the requirements of ASTM C1761 for internal curing (water absorption > 5% and minimum of 85% water desorption at 94% relative humidity, RH), but also showed superior performance in comparison to LWAs available in the market, which were also evaluated in this study. Moreover, the SPoRA desorption isotherms showed its capability for releasing a favorable amount of absorbed water as the internal RH decreases during concrete self-desiccation, thereby providing promising desorption behavior for concrete internal curing and can be a promising LWA for internal curing of concrete.