Hydration mechanism and orthogonal optimisation of mix proportion for steel slag–slag-based clinker-free prefabricated concrete

Steel slag and blast furnace slag-based clinker-free concrete has very low early strength. This paper focuses on the effect of different curing temperatures on the compressive strength of clinker-free concrete and on optimising the mix proportion by orthogonal tests based on the perspective of prefabricated concrete, and then discusses the hydration procedures. The results showed that the early strength of concrete could be improved by 13.43 or 22.15 MPa when cured at 30 °C or 45 °C, respectively. The proportion of steel slag could be increased to 29.3% under the optimised conditions of an SS:GBFS ratio of 3:6, desulfurisation gypsum composition of 12%, sand ratio of 0.43, and water consumption of 140 kg/m3. The compressive strength of the concrete prepared with the optimised mix proportion and cured at 45 °C meets the requirement for C40 concrete. During the hydration, the generation of calcium hydroxide through the reaction of tricalcium silicate in the steel slag could promote the breaking of Si–O–Al and O–Si–O bands. The dissolving alumina tetrahedra could react with SO42− and Ca2+ ions to form ettringite, and the dissolving silica tetrahedra could participate in the generation of calcium silicate hydrate gels. The network formed by aciculate ettringite crystals was the main source of early strength. With increasing duration of curing, the growth of ettringite crystals, accompanied by the calcium silicate hydrate gels filling their pore spaces, could be observed by field-emission scanning electron microscopy; these products were responsible for the improvement in strength of the concrete.