Effect and mechanism of composite early-strength agents on sulfoaluminate cement-based UHPC

Some major construction projects require materials that can quickly and easily obtain high mechanical properties. Nowadays sulfoaluminate cement (SAC) has attracted attention for such projects due to its fast hardening rate, high early strength, and low alkalinity. Added early strength agents are commonly used to improve the early strength of Ultra-high performance concrete (UHPC). Most early-strength agent research has focused on Portland cement systems. Due to the different components of SAC and Portland cement, the compatibility between different early strength agents and SAC needs to be further explored.This paper selected three inorganic early-strength agents, calcium formate (Ca(HCOO)2), aluminum sulfate (Al2(SO4)3), and lithium carbonate (Li2CO3), and an organic early-strength agent, triethanolamine (TEA), which are commonly used. Experimental studies were carried out for single and compound early-strength agent systems in a SAC-based Ultra-high performance concrete (UHPC) system with a low water-cement ratio of 0.23. The results showed three inorganic early-strength agents reduced the setting time and improved the strength of the UHPC. The 4 h compressive strength of UHPC with 0.4% Ca(HCOO)2 increased from 22.1 MPa to 41.2 MPa due to the improved microstructure and denser matrix.The composite mixture of Ca(HCOO)2 and Li2CO3 had a good synergistic effect in the early and middle stages, the compressive strength can reach 88.7 when curing 3 days. Different from Portland cement system, TEA had an adverse effect in the both single and compound systems.