Effect of calcium sulphoaluminate cement on mechanical strength and waterproof properties of beta-hemihydrate phosphogypsum

Beta-hemihydrate phosphogypsum (beta-HPG) as a cementitious material obtained from the dehydration of phosphogypsum (PG) can be widely used for the preparation of building materials. The main problems for its utilization are its low mechanical strength and poor waterproof properties. In this study, calcium sulphoaluminate cement (SAC) was used to improve the properties of beta-HPG. The compressive strength, flexural strength, ratio of compressive to flexural strength, water absorption rate and softening coefficient were measured. The volume stability, pore structure and microstructure were characterized to evaluate the effect of SAC. The results show that SAC can improve the mechanical strength and waterproof properties of beta-HPG significantly. With the addition of 20% SAC, the increase of drying shrinkage is relatively small, and the compressive strength and softening coefficient were 21.5 MPa and 0.66, increasing by 60.85% and 96.04% respectively compared to the mixture without SAC. Specifically, ettringite and aluminate hydroxide are formed and interconnected with calcium sulfate dihydrate to improve the waterproof properties of beta-HPG under the effect of SAC. The crystal morphology of calcium sulfate dihydrate changes from long rods to short columns, and the pore size and total porosity of the mixture decrease, resulting in the improvement of the mechanical strength of beta-HPG. The purpose of this work is to optimize the properties of beta-HPG and provide a high performance cementitious material for the effective utilization of PG. (C) 2020 Elsevier Ltd. All rights reserved.