Investigation on the equivalent strength of Portland cement and PC-CSA blends: Experimental findings and thermodynamic modelling

Abstract

Mixing calcium sulfoaluminate (CSA) cement and Portland cement (PC) can provide several benefits for certain applications, especially reducing the carbon footprint of PC. This study investigated the heat of hydration, hydration products, mechanical properties, autogenous shrinkage and microstructure of blended cement, and phase composition calculations were also made with thermodynamic modelling. The results show that the autogenous shrinkage of the cement paste is effectively mitigated due to the swelling produced by the rapid hydration of ye'elimite before 1 d. However, when the CSA admixture is 15 wt%, many large holes and obvious cracks can be found by three-dimensional rendition of X-ray computed tomography (X-CT), this may be attributed to the adverse effects of phase transformation, specifically the conversion of ettringite into monosulfate. Combined with the thermodynamic modelling results, the hydration of PC and CSA in the blended cement is a competitive relationship, and the presence of AH3 is not found in the PC-rich blended systems. In addition, the compressive strength of the blended mortars with addition of 5 wt% CSA at 28 d reached 61.5 MPa which is equivalent to that of plain PC. However, the strength of 15 wt% CSA addition is about 30 MPa lower than that of 5 wt% CSA. This is significantly correlated with AFt/AFm, and once AFt/AFm exceeds a certain limit, the strength of PC-CSA blends is significantly lower than that of plain PC.