Partial replacement of OPC with CSA cements – effects on hydration, fresh and hardened properties

The effects of replacing OPC with three types of calcium sulfoaluminate (CSA) cements (40 wt%, 20 wt%) were studied. The results were broadly consistent with previously published data, with a few notable exceptions. Because of the production of ettringite, setting times were decreased. The maximal hydration temperature increased in concretes containing 40 wt% CSA but dropped in concretes with 20 wt% replacement. The drop was caused by a lack of accessible sulfates, which limited the synthesis of ettringite. In comparison to OPC concretes, the inclusion of additional anhydrite and calcium oxide was related with a delayed formation of the second temperature peak. Their excess slowed the calcium aluminate and belite reactions and reactivated the synthesis of ettringite, C–S–H, and portlandite. The effects of aluminum hydroxide were also indicated as potentially significant, although this was not proven experimentally in this study. The slightly lower compressive strength reported for mixes containing 40 wt% CSA was associated with better-formed ettringite. The same was identified as the key to lowering total shrinkage in mixtures containing 40 wt% CSA and increasing for lower CSA replacement levels. In that case, the insufficient amount of ettringite generated resulted in expansion that was insufficient to alleviate or compensate for the developed shrinkage.