Long-term volume stability of steel slag sand mortar and concrete

The stability of steel slag sand as an aggregate in mortar and concrete is currently a focal point of concern. The evaluation of steel slag stability typically employs short-term rapid methods, and research on the long-term stability of steel slag sand under natural curing conditions is relatively scarce. In this study, the rapid mortar bar method and alkaline-aggregate reaction method were used to compare the volumetric and apparent morphological changes in steel slag mortar and concrete under different curing conditions, with a focus on the effects of the steel slag sand aging time, substitution rate, particle size and curing conditions on the stability of steel slag sand. A combination of scanning electron microscopy and energy-dispersive X-ray spectroscopy was used to comprehensively investigate the mechanism of the deterioration of the stability of steel slag sand. The results show that the rapid mortar bar method exacerbates the degree of hazard of steel slag sand, whereas the long-term alkali-aggregate reaction test provides a more realistic assessment of the stability of steel slag sand. The results from the long-term alkali-aggregate reaction tests conducted over 0−156 weeks indicated that the replacement ratio of steel slag sand has a minimal impact on the volume of mortar and concrete. The degree of pinpoint surface damage to the samples is significantly correlated with the uneven distribution of free calcium oxide (f-CaO) within the steel slag sand, and the influence of f-CaO on surface damage showed a stochastic nature. When the particle size of steel slag is larger than 1.18 mm, the mortar sample is prone to point damage and fracture. The concrete samples made with single-particle-size steel slag sand simultaneously exhibit significant dry shrinkage and suppression of the hazards caused by the slow hydration of f-CaO. Overall, this paper evaluates the damage forms and degree of damage to the steel slag sand in mortar and concrete and provides a scientific basis for assessing steel slag sand stability. • The rapid mortar bar method exaggerates the expansion hazards of steel slag sand. • Expansion rates of new & aged steel slag mortar similar under 80°C hydrothermal conditions. • Steel slag sand replacement rate has minimal impact on concrete volume expansion at 38°C. • Steel slag sand larger than 1.18 mm is prone to localized damage and fracture of the steel slag mortar.