Investigation into Improving the Water Resistance and Mechanical Properties of Calcined Gypsum from Phosphogypsum Composites

This study aimed to improve the mechanical properties and water resistance of calcined gypsum from phosphogypsum (CGP) by incorporating organic additives and inorganic admixtures. The effects of the dosage of these additives-including kaolin, nano-SiO₂, polycarboxylic acid superplasticizer, and sodium methyl silicate-on the properties (flexural strength, compressive strength, water absorption, and softening coefficient) of CGP composites (CGPCs) were investigated. A high water resistance of the CGPCs was achieved using nano-SiO₂ and sodium methyl silicate modification, superplasticizer addition, and the partial replacement of gypsum with mineral admixtures. The results showed that the flexural and compressive strength of the composites hit 4.61 MPa and 19.54 MPa, respectively, while the softening coefficient was 0.70 and the water absorption rate was 19.85%. Microstructural investigation confirmed that the combination of nano-SiO₂ and kaolin led to the formation of calcium silicate hydrate. Additionally, the superplasticizer played a crucial role in reducing the water-to-cement ratio, while unhydrated mineral particles had a filling effect, thereby enhancing the density of the hardened paste. The sodium methyl silicate formed a hydrophobic film on the surface of the hardened paste, increasing the contact angle to 109.01° and improving the water resistance of the CGPCs.