The soundness of steel slag with different free CaO and MgO contents

In this study, five types of steel slags with different free CaO (f-CaO) and MgO contents were used as mineral admixtures to prepare concretes with W/B ratios of 0.5 and 0.35. The concretes were cured for 4 years. Compressive strength and permeability to chloride ion were tested to reveal the influence of f-CaO and MgO on concrete soundness over 4 years. An accelerated autoclave test was also adopted to investigate the influence of MgO on concrete soundness after 4 years. The results show that the primary component that leads to the bad soundness of steel slag is f-CaO. Ca(OH)2 primarily with a bulk structure is produced by the hydration of f-CaO, which is different from the Ca(OH)2 produced by cement hydration. Steel slag with a 4.96% content of CaO presents bad soundness and leads to the soundness-induced failure of concretes at both high and low water-to-binder (W/B) ratios. When the content of f-CaO is within 2.09%, the steel slag presents satisfactory soundness. The results of the autoclave test show that when the content of MgO is within 7.68%, steel slag has no negative influences on the compressive strength of concrete.