Study On Autoclaved Bricks With Phosphogypsum Pretreated By Steam And Its Mechanism Of Strength

Recycling and re-utilization of phosphogypsum has been investigated in this thesis. The phosphogypsum pretreated by steam was made autoclaved bricks.The phosphogypsum contains impurities, which is hardly utilized without pretreatment. The thesis proposed a process pretreated using steam for phosphogypsum. The results indicated the phosphogypsum pretreated by low pressure steam (0.12 MPa, temperature 120℃, time 900 min) was fitter to make autoclaved bricks more than that pretreated by high pressure steam (0.80 MPa, temperature 180℃, time 480 min). The strength was not influenced by storage time of pretreated phosphogypsum in the open-air. The experiment presented two optimum mix proportions of autoclaved bricks: 1-consisted of 40%-42% phosphogypsum pretreated by low pressure steam, 27%-32% detritus, 15%-22% fly ash, 10%-15% slaked lime and 1%-3% multi-additive; 2-consisted of 55% phosphogypsum pretreated by low pressure steam, 0%-5% sand, 25%-27% fly ash, 10%-15% slaked lime, 3%-5% multi-additive. The slaked lime must have been slaked completely and passed 0.5 mm sieve. The proportion of slaked lime influenced the strength dramatically.The mechanism of strength was investigated by analyzing the phosphogypsum material and the samples of autoclaved bricks. The strength mainly contributed by the Tobermorite formed in pozzuolana reaction between Ca(OH)2 and active SiO2 in fly-ash and surface of detritus, which wrapped the phosphogypsum and detritus to form the compact unit under high pressure steam. Silicate gel formed under low pressure steam contributed much to strength.In the pilot-scale experiment, the flexural strength and the compressive strength of the autoclaved bricks reached 3.86 MPa and 17.51 MPa respectively, also the resistance to frost-melting was good, the strength was meet the need for main building material. The project about autoclaved brick made by phosphogypsum can utilize the industrial waste, which is benefit to environment and the public, also have good profit. The technology is reliable and should be encouraged to industrialize.