Effect of calcinated CaO on expansion of cement paste

The application of steel slag in engineering was limited due to the presence of free- CaO (f-CaO). The aim of this study is to investigate the effect of different grades of CaO to soundness of cement paste and the mechanism that lead to volumetric expansion. Moreover, to find out its influence to the test results of f-CaO content, thereby to discuss if the test of f-CaO content is a reliable index to reveal the volumetric expansion of cementitious materials. The results in this research found that increasing the calcination temperature of CaO, indicating a larger particle size, larger crystal grain size and a smaller specific surface area. The citric acid reactivity and the hydration test was a rapid screening method to determine the grades of CaO. For higher grades of CaO, both experiments show a decrease in reactivity and the reaction rate could further be compared in hydration test. The test results of f-CaO measured by ASTM C114-15 with different grades of CaO were completely opposite in revealing the soundness of cementitious materials.The lightly-burnt CaO measured with a higher f-CaO content indicated a greater potential of unsoundness in cementitious materials, while the dead-burnt CaO had the least potential. However, in autoclave expansion test, only the dead-burnt CaO will lead to unsoundness of cementitious materials. The f-CaO content test will misjudge the soundness of cementitious materials. The mechanism of dead-burnt CaO in cement paste that lead to unsoundness was estimated due to the delay hydration to Ca(OH)₂ and ettringite-like crystals which will damage the internal strength of cement paste.