Study On Modification Of Phosphogypsum-fly Ash Binder

Phosphogypsum is the byproduct of manufacturing phosphate acid by wet process in phosphate fertilizer industry. With the rapid development of phosphate fertilizer industry, the emission of phosphogypsum continues to increase. At present, China's annual emission of phosphogypsum has more than 50 million tons, but the utilization rate of the phosphogypsum is only 20%. A huge storage of phosphogypsum not only takes up a lot of land resources, but also make a serious threat to the security of groundwater, surface water and atmospheric environment near the phosphogypsum yard. The utilization of the phosphogypsum has been related to the harmony, sustainable development among the phosphate fertilizer industry, social and environment.The paper regards the phosphogypsum dried at low temperature (<60℃) and fly ash as raw materials.The influence of the temperatures, squeeze pressures and the content of lime on the drying performance of phosphogypsum was studied in the paper. Besides the effect of the foundation mixture ratio, grinding time, grinding method, different content and types of chemical additives on the properties of the phosphogypsum-fly ash binder and the modification mechanism were also studied in the paper.The results show that:(1) The major loss of phosphogypsum was its physical adsorbed water below 50℃; however, phosphogypsum would gradually take off the crystal water and generate semi-hydrated gypsum above 50℃. The addition of lime reduced the content of the physically adsorbed water of the phosphogypsum, and improved the temperature of the phosphogypsum taking off its crystal water. When the content of the physically adsorbed water was high, the mechanical compression method would be available for wiping off the physically adsorbed water. When the pressure reached 33KN, the content of the physically adsorbed water decreased from 22.3% to 12.3%,44.8% of physical adsorbed water removed.(2) Ettringite, CSH gel and calcium hydroxide were the main hydration products of the dihydrate gypsum-fly ash binder. Reducing the content of phosphogypsum, adopting the right ratio of fly ash and lime and increasing the content of cement would increase the amount of hydration products, reduce the porosity of hardened, improve the strength of the area between the hydration products and phosphogypsum and improve the strength of the the dihydrate phosphogypsum-fly ash binder. The foundation mixture should be adopted based on the raw materials used and the actual necessary in the actual production.(3) Grinding could reduce the water requirement of normal consistency of the dihydrate phosphogypsum-fly ash binder. The water requirement were reduced from 35% to 29% after the dihydrate phosphogypsum-fly ash binder were ground 25min. Increasing grinding time from Omin to 25min, the 28d wet compressive strength of samples were increased by nearly 253%, 28d dry compressive strength were increased by nearly 176%; the 28d wet bending strength were increased from 1.60MPa to 2.83MPa. Separately grinding the fly ash and the mixture of the gypsum and lime were better for its 28d strength than the mixed grinding, but the mixed grinding of the composite cementitious material were were better for its 28d strength than only grinding the fly ash. The 28d strength of the composite cementitious materials only grinding mixture of phosphogypsum and lime were the lowest.(4) During the four chemical additives in their range of the test, the 28d wet compressive strength of samples adding NaOH were the highest, followed by the Fe2 (SO4) 3 and Al2 (SO4) 3, CaCl2. The 28d wet compressive strength were separately 16.36MPa,15.88 MPa,15.12 MPa and 14.33MPa.(5) Replacing the dihydrate phosphogypsum with the hemihydrate phosphogypsum, the hemihydrate phosphogypsum could quickly generate hydrated gypsum crystals and form the network structure of the gypsum crystal. Replacing the dihydrate phosphogypsum with the hemihydrate phosphogypsum would shorten the setting time of complex binder, but also improved the system's strength. With the replacement of the hemihydrate phosphogypsum for the dihydrate phosphogypsum from 0% to 30%, the 28d wet compressive strength, wet flexural strength and dry compressive strength of the composite cementitious material increased from 12.67MPa, 2.30MPa and 28.98MPa to 16.94 MPa,3.13MPa and 35.63MPa.