Preparation, Constituent, Structure And Performance Of Alite-Barium Calcium Sulphoaluminate Cement

Traditional Portland cement has been widely used, and it is still one of the most important building materials in the 21st century. But it has some shortcomings, such as lower early strength, high sintering temperature and the bulk shrinkage of hardening paste etc., which can't suit for the need of high-performance cementing material for modern construction. Therefore, how to improve the performance of cement, especially its early mechanics performance, bulk stabilization and endurance, saving energy sources and resource, protecting environment, are the main researching direction of science and technology of cement. It will take on important significance for country economy and social development. The technology of composite minerals is an availability approach for synthesizing a new high performance cement.In recent years, a mineral named barium calcium sulphoaluminate (3CaO3Al2O3-BaSO4) has been synthesized in laboratory successfully and many scientists are interested in the mineral performance. The barium calcium sulphoaluminate , whose composition is 2.75CaO3Al2O3-1.25BaSO4 (C₂.75B₁.25A₃S ), has the optimal performance of rapid hardening and early strength. Its early mechanics performance is better than that of calcium sulphoaluminate(C4A3 s) which is the main mineral of sulphoaluminate cement . Furthermore, the sintering temperature of the mineral is lower and has tiny bulk expansion in the hydration process that can improve the early performance and endurance of Portland cement. In this paper, the mineral of barium calcium sulphoaluminate and the mineral system of Portland cement clinker were compound. The high cementing alite-barium calcium sulphoaluminate cement was obtained that has the main excellences both of alite and barium calcium sulphoaluminate minerals. The technology preparation, constituent, structure and performance of the cement were investigated carefully.First, the synthesis dynamics, decomposition and hydration behavior of C2.75B1.25A3 S were studied. The results show that the mineral start to be synthesized at about 1000℃, the producing rate is rapid at 1300-1350℃. When temperature is higher than 1370℃, C2.75B1.25A3S begins to be decomposed veryslowly. The hydrating rate of C2.75B1.25A3 5" is fast and it has excellent fast-hardening and early-strength performance. The kinetics of C2.75B1.25A3 S formation obey Glinstling formula F(a)=l-2a/3-(l-a)2/3=K{TC)t from 1150"C to 1300 °C, the diffusing activation energy of the mineral is 280.39kJmol'1. At 1350 *C, the kinetics of C2.75B1.25A3 S formation not only obeys Glinstling formula but also the equation of interface reaction F(a)=l-(l-a)1/3=K(TC)t. The presence of suitable amount of CaF2 can accelerate the formation of C2.75B1.25A3S when the sintering temperature is lower than 1300"C, but when the sintering temperature is higher man 1300"C, due to the formation of CnA7CaF2, the formation rate of C2.75B1.25A35 is retarded.Clinker constituent and burning technology were picked out and optimized by using orthogonal test method. The optimal mineral constituent of clinker is confirmed on the basis of mechanics performance, new dry process technology and theory count of material thermodynamics. At the base of the optimizing composition, excessive amount of SO3 and BaO was mixed into the clinker system to compensate SO3 and BaO volatilization or solid phase dissolution. The suitable addition of SO3, BaO and CaF2 in the clinker system was chosen out according to mechanics performance of cement. The results show that the alite and barium calcium sulphoaluminate minerals can compound and coexist in a clinker. It established the basic of alite - barium calcium sulphoaluminate cement. When the content of barium calcium sulphoaluminate is 6% and that of Portland clinker system with silica modulus 2.5, iron modulus 1.5 and KH 0.92 is 94%, the composite clinker has low Gibbs free energy and high reaction trend. In addition, mixed CaF2 can improve the reaction trend. Excessive SO3 and BaO in the clinker is benefit to the mechanics performance of cement, and the optimal excessive content of SO3 and BaO is 50% The suitable addtion of CaF2 is 0.6-1% and the sintering temperature is about 1380TI:. In the conditions of the optimal composition and preparing technology, the compressive strength of the alite-barium calcium sulphoaluminate cement is 27, 70 and 119MPa at Id, 3d and 28d curing age respectively, indicating excellent early strength and steady long-term strength.The influence of water/cement ratio, aluminium/sulfur ratio on the hydration rate, hydration degree, composition, microcosmic structure and porosity of hardened cement paste were researched by means of XRD, SEM-EDS, ESEM,DTA-TG and light microscopy, mercury intrusion porosimetry and hydration heat analysis. The results show that the hydration rate and early strength of the cement can be obviously improved with increasing gypsum and the optimal aluminium/sulfur ratio is 1/0.9. The hardened paste has dense structure and the most size of pores is smaller than lOnm.The hardened cement paste contains much more mineral AFt than that of Portland cement, which don't make harmful pores decreasing with the increase of gel/space. It may be one of the reasons that the relation between the mechanics performance and the gel/space of the cement discord with f = Axn.Based on the work of laboratory, the product experiment obtained good effect. The results show that the compressive strength of alite-barium calcium sulphoaluminate cement is up to 28.1, 38.6 and 60.7MPa at Id, 3d and 28d respectively. The cement exhibits early-high strength, lower sintering temperature, about 1350°C and good grindability that can save energy sources. In addition, making the cement can use barium industrial residue that can save resource and protect environment.