Water desorption characteristics of saturated lightweight fine aggregate in ultra-high performance concrete

In this study, the water desorption behavior of saturated lightweight fine aggregate (LWA) in ultra-high performance concrete (UHPC) was systemmically investigated using isothermal calorimetry, relative humidity (RH), mercury intrusion porosimetry, X-ray microtomography methods, etc. The LWA with high porosity and coarse pore structure exhibited high absorption and easy desorption at high RH. The results indicate that a large amount of water in LWA was released before setting, resulting in an increase of water to binder ratio, which had an adverse effect on the performance. However, the water absorbed in LWA released fast after 6 h, inducing an internal curing effect. Mechanism on a “four-stage desorption” driven by capillary pressure and RH gradient was proposed. The water desorption that is beneficial for the internal curing is calculated. The results indicated that the internal curing efficiency could be enhanced by increasing LWA content and reducing its pore size.