Novel cementless ultra-high performance concrete using calcium carbide residue as activator by the aid of combined curing

In this study, a novel idea of using calcium carbide residue (CCR) as an activator to prepare cementless ultra-high performance concrete (UHPC) was proposed. Ground granulated blast furnace slag (GGBFS) and silica fume (SF) were used as single or composite precursors, activated by CCR, to prepare a series of mortars, which were cured under standard curing, hot water curing and combined curing, respectively. Workability and compressive strength of mortars were tested and the mix proportion of mortar suitable for preparing UHPC was selected. With addition of the steel fiber at a volume dosage of 2%, cementless UHPC was successfully prepared, which had high slump spread (660–701 mm) at fresh state and ultra-high compressive strength (120–175.4 MPa) and splitting tensile strength (10.6–20.4 MPa) after combined curing. Furthermore, the microstructure of cementless UHPC matrix was detected by a variety of microscopic testing methods. The results show that the pozzolanic reaction between Ca(OH)₂ and SiO₂ was significantly accelerated by thermal curing, especially with the addition of SF, which helped produce more C-(A)-S–H gels and a small amount of C-(A)-S–H crystals such as tobermorite and xonotlite, resulting in the refinement of pore structure and significant improvement of compressive strength of the cementless UHPC matrix. In addition, the cost and CO₂ emissions of CCR activated cementless UHPC were considerably lower than those of cement-based UHPC and other cementless UHPC reported in literature.