Research On The Composition And Properties Of Phosphogypsum Based Cement

In this study, energy saving and emission reduction of cement industry were combined with the utilization of phosphogypsum (PG). Phosphogypsum based cement (PBC) had been developed by using PG as the main raw material and adding appropriate amount of slag, limestone and a small amount of Portland cement clinker. And through a large number of experiments, the performance of PBC was studied. The results showed that, clinker was added as alkaline activator in PBC, the optimal content of clinker was 4%. A small amount of PG was mainly used as sulfate activator, most of the unreacted PG was enclosed by the hydration products to act as aggregates. The slag was the major bonding component, it was hydrated in the activation of clinker and PG. As a result, the microstructure of hardened paste became increasingly denser, and the strength of PBC developed continuously.By pretreating PG with 2% steel slag and using ultrafine grinded clinker as alkaline activator, the early performance of PBC was significantly improved.through the optimization of components, the content of PG could be 45%, and the 7d compressive strength of PBC could be more than 10 MPa and the 28d compressive strength could reach to 49 MPa.The durability properties of PBC were also studied. The results showed that, compared with ordinary Portland cement, the long term strength, water resistance, volume stability, sulfate resistance, frost resistance and heat resistance are good. However, due to its low alkalinity, the PBC lacked the neutralizing ability of CO2, so that the carbonation resistance of PBC was significantly inferior to that of Portland cement.By changing the conservation system and adding some materials which can be carbonized such as MgO and Mg(OH)2, the carbonation resistance of PBC could be improved.In the conservation of steel slag mud and saturated lime water, the carbonation resistance of PBC could be improved with varying degrees. This was due to the increasing of the alkalinity in the conservation environment, and the conservation environment was rich in Ca2+, so that the hydration of surface slag in PBC sample was accelerated, and the density of the sample surface was increased, therefore the diffusion rate of CO2 was slowed down.By adding an appropriate amount of MgO in PBC, the strength of PBC after carbonation could be substantially increased. But the 28d strength of PBC was significantly reduced. By adding an appropriate amount of Mg(OH)2 in PBC, the carbonation resistance of PBC could be significantly improved, and Mg(OH)2 had little effect influence to the other properties of PBC.The reason of carbonation resistance improved by MgO and Mg(OH)2 was that, In the carbonization process, Mg(OH)2 in the hardened cement could be reacted with CO2, and the MgC03 was generated. MgCO3 blocked the pores in the hardened cement, so that the density of cement was increased, and the diffusion rate of CO2 was reduced, this results in the improvement of carbonation resistance.