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.

This paper assesses the effect of direct and indirect factors on the shear performance of ultra-high performance fibre reinforced concrete beam containing coarse aggregate (UHPFRC-CA) using four point loading arrangement. The obtained results were used to categorize UHPFRC-CA beam's failure mode, establish the influence of key factors on shear performance, and develop UHPFRC-CA beam's compression zone resistance and first shear cracking load equations whose results were compared with those from this research, other researchers and existing equation. Findings revealed that UHPFRC-CA beam fails in cable stayed, shear partial compression, cable stayed-diagonal tension and cable stayed-partial shear tension. Shear span-depth ratio (a/d) has the most influence on the beams' failure mode. Higher percentage volume of steel fibre improves ultimate load capacity and midspan displacement resistance at failure load. Hooked-end steel fibre improves deformation (crack width and midspan displacement at failure load) resistance. Higher a/d is more beneficial to midspan displacement resistance at failure load than load capacity; while lower stirrup spacing leads to higher midspan displacement at failure load. Finally, the developed first shear cracking load equation can adequately capture the true first shear cracking load of UHPFRC-CA beam; and the developed compression zone resistance equation can be conveniently used to represent the joint contribution of compressive strength and fibre factor to UHPFRC-CA beam's shear resistance.