Drying shrinkage of ultra-high-performance concrete incorporating fine porous aggregate

The autogenous shrinkage of ultra-high-performance concrete (UHPC) can be mitigated using internal curing agents. However, owing to the low water/cement ratio typically used in UHPC, long-term drying shrinkage is significantly less pronounced than early-age autogenous shrinkage and has therefore received limited research attention. The impact of an internal curing agent (pre-wetted fine porous aggregate (FPA)) on the long-term drying shrinkage of UHPC was investigated in this work. The research provides new insights into the effects of FPA on rheological properties, multi-scale pore structures and hydration products. The results showed that, over 500 days, drying shrinkage accounted for 37–66% of the total shrinkage in UHPC with pre-wetted FPA. The FPA reduced non-connected pores but increased the volume and size of connected pores, with early drying shrinkage at 14 days strongly associated with coarser pore structures. Additionally, the water released from the FPA promoted the formation of C-S-H (II) gels with higher interlayer water content. This water was gradually released during extended curing, up to 500 days, and this contributed partially to the increased drying shrinkage observed at later stages. These findings emphasise that the incorporation of FPA contributes to higher drying shrinkage, underscoring the importance of considering this factor in the design of low-shrinkage UHPC.