Damage characteristics and constitutive model of phosphogypsum/fly ash/slag recycled aggregate concrete under uniaxial compression

The recycling of construction and industrial waste is essential for environmental protection and carbon emissions reduction. A phosphogypsum/fly ash/slag (PFS) composite was therefore employed in this study to fully replace ordinary Portland cement, producing PFS recycled aggregate concrete (PFSRAC). The mechanical properties of various PFSRAC mixes were first investigated using a series of uniaxial compression tests, showing the combination of PFS and recycled aggregate has an obvious effect on the compressive strength of PFSRAC. Next, the change in the mean and variance of grayscale values in cross-sectional images obtained by in-situ X-ray computed tomography were found to effectively reflect the damage characteristics of PFSRAC under uniaxial compression. A digital volume correlation analysis of these images indicated no deformation localization area in the interfacial transition zone (ITZ) between the PFS matrix and recycled aggregate, demonstrating effective densification of the ITZ that increased the compressive strength of the PFSRAC. Finally, the constitutive model for PFSRAC was established using a strain statistics analysis that was shown to effectively reflect the observed stress–strain relationship.