Properties evolution of phosphogypsum-slag based composite cementitious materials: mechanical strength and durability

This paper mainly studied the properties of phosphogypsum-slag based composite cementitious materials (PSCM) by measurements of flexural and compressive strengths, drying shrinkage, carbonation resistance, sulfate corrosion resistance and exploring the impact mechanism by multiple techniques of XRD, TG-DTG, SEM-EDS and MIP. Results showed that the addition of NaOH and triethanolamine decreased the flexural and compressive strengths and increased the drying shrinkage, but it increased the fluidity, shortened the setting time and improved the early carbonization resistance of PSCM. The results showed that the 90d compressive strengths of RS, RSQ and RSF reached 68.1, 64.2 and 56.6 MPa, respectively, indicating that the PSCM system maintained relatively high long-term strength. The incorporation of NaOH and TEA improved slurry fluidity and shortened the setting time, but reduced the 28d compressive strength and increased drying shrinkage. After 28d of accelerated carbonation, the compressive strengths of RS, RSQ and RSF decreased by 20.9%, 17.3% and 15.1%, respectively, compared with the uncarbonated samples. Microscopic test results indicated that carbonization decreased the content of ettringite and gel products of PSCM resulting in a coarsening pore structure and lower compressive strength. While PSCM exhibits good resistance to sulfate attack. Sulfate soaking can increase the content of reaction products and optimize the pore structure of PSCM resulting in a higher compressive strength in comparison to the un-soaked samples.