Research On The New Technology Of Wall-bricks From FGD Gypsum

The industrial by-product FGD gypsum is generated from limestone wet flue gas desulphurization system.The large quantities of FGD gypsum would not only wastes the resources,but also occupied a great of land resources and caused environmental pollution.It is meaningful to use wet FGD gypsum to make wall materials for low energy consumption and low cost.It is an important way to realize resource conservation to a large extent and protect the environment.In this paper,mix up theα-Hemihydrate Gypsum with FGD gypsum firstly and then autoclaved,its broken block could be used asα-Hemihydrate Gypsum repeatedly.The influence of the drying temperature and the autoclave temperature on the compressive strength ofα-hemihydrate gypsum were discussed.The result indicated that the compressive strength of theα-Hemihydrate Gypsum with the condition of autoclaved 5h at 135℃and dried at 100℃was 9.8MPa.Furthermore,the effects of different water-solid ratio and curing regimes on blocks was analysed based on the above study.The optimum technological conditions was water-solid ratio of 0.5,nature curing 7 days.The results proved that the flexural strength was 2.4 Mpa and the compressive strength was 5.9 MPa.A large amount of data showed that the technology was feasible.By comparing it data of scanning electron microscopy（SEM）and quantitative X-ray diffraction（XRD）ofβ-Hemihydrate Gypsum,the formation mechanism of hemihydrate gypsum was studied.The result indicated thatα-Hemihydrate Gypsum formed by dissolution-crystalization,whileβ-Hemihydrate Gypsum dehydrated directly.A film of water was on the surface of gypsum in hot and humid conditions.Gypsum partially dissolves in the film of water.The nuclear particle would precipitate out when calcium sulfate was in a super-saturated state.α-Hemihydrate Gypsum precipitated out on the condition of saturated steam and high temperature.The Crystals structure of beam-like formed intact after alternative occurrence of dissolution and crystallization.β-Hemihydrate Gypsum formed by the water molecules escape by heat-treatment.The hydration mechanism ofα-Hemihydrate Gypsum hydration were introduced by observing its hydration process with 3D microscope.The water molecules in solution entered water channel of gypsum,which lead to the structure of gypsum damaged and splited into tiny crystals.Then the free calcium ions and sulfate radicals promote nucleation to grow rapidly and contact with each other.Finally,crystals formed the dense structure.At last,the paper presented the relationship between the crystal size and time by the method of curve fitting.The results proved that the curve fitted exponential equation and the reduced data had a high fitting degree,up to 0.98.The influence of crystal modifier to strength of blocks preparation from different gypsum was discussed by t-Verify method.The compound crystal modifier of aluminum sulfate and sodium citrate might be most appropriate.Al³⁺adsorped in 110and carboxyl adsorped in 111.Al³⁺and carboxyl controlled crystal growth and crystal size.The strength had improved significantly while crystal face was smooth and edge length was shorter.But after adding crystal modifier into the phosphorus gypsum,which contain iron,aluminum and other metal oxides or metal phosphide impurities,the growth of the crystal was restrained by excessive levels of cation and carboxyl.