Effect Of Doping SrO And SrSO₄ On Synthesis And Performance Of Alite-rich Cement

In 2010, the cement output reached 1.87 billion tons in China, which leads to enormous consumption of energy and natural resources as well as severe environmental pollution. Therefore, the main task is how to achieve the sustainable development of cement industry. The key to producing high-performance cement is the preparation of cement clinker with high cementitious. The effects of doping SrO and SrSO₄ on the preparation, structure and properties of the alite-rich cement clinker are studied. Alite-rich cement clinker modified by calcium strontium suphoaluminate is prepared by introducing calcium strontium suphoaluminate mineral with rapid hardening and early strength performances into alite-rich cement clinker. The optimum composition and sintering condition of the clinkers are determined by the orthogonal experimental method. The crystal structures and properties of C₃S doped SrO and SrSO₄ are also researched. The test methods, such as XRD, SEM-EDS, IR, heat of hydration and lithofacies analysis, are used to analyze the composition, structure and performance of clinker and mineral. The main conclusions are as follows:Calcined at 1450℃, the appropriate modulus value of alite-rich cement clinker is as follows: KH=0.92, SM=2.5, IM=1.5, and the calculation content of mineral composition C₃S is up to 65.1%; Under this conditions, the 1d, 3d and 28d compressive strength of alite-rich cement are 23.4MPa, 53.7MPa and 129.1MPa respectively, showing excellent mechanical properties. Overvalue KH of cement clinker doesn't contribute to obvious increase of alite; instead, it results in the difficulties for the sintering of clinker.Cement clinker doped SrO and SrSO₄ can reduce the contents of free calcium oxide, promote the formation of alite minerals, and be benefit to the improvement of cement strength. The optimum conditions for alite-rich clinker doped SrO are as follows: sintering temperature is 1450℃, sintering time is 60 min, and the appropriate content of SrO is 1.5% (by the mass, the same as following). The early strength of cement increases significantly, and the 3d strength can be improved by 15.5%. The optimum conditions for alite-rich clinker doped SrSO₄ are as follows: sintering temperature is 1450℃, sintering time is 60 min, and the appropriate content of SrSO₄ is 0.5%. The later strength of cement increases significantly, and the 28d strength can be improved by 9.9%.The introduction of calcium strontium suphoaluminate mineral can not only reduce the sintering temperature of alite-rich cement clinker to about 100℃, but also decrease the contents of free calcium oxide and promote the formation of alite minerals in clinker, which can play important role in the energy saving of cement industry. Under the experimental conditions, the optimum conditions of alite-rich cement clinker modified by calcium strontium suphoaluminate are as follows: sintering temperature is 1350℃, sintering time is 60 min, and the content of calcium strontium suphoaluminate is 2%. Under the optimum preparation conditions, the 1d, 3d and 28d compressive strength of cement are 28.5MPa, 66.7MPa and 128.6MPa respectively and excellent mechanical properties. The development degree of alite and calcium strontium suphoaluminate minerals in the clinker is incomplete, and the size of mineral particles is small. The size of calcium strontium suphoaluminate minerals is about 0.1μm, existing in the gaps and borders of silicate minerals.The effect of SrO and SrSO₄ on the crystal structure and properties of C₃S is also researched. The content of free calcium oxide increases with the increase of SrO, when the content of SrO is less than 0.075mol, the content of free calcium oxide is lower than that of the pure C₃S. When the SrO content is less than 0.025 mol, the diffraction peaks of tricalcium silicate solution show slight change but its crystal type is still triclinic; when the doping content of SrO is more than 0.025 mol, the tricalcium silicate solution is the monoclinic. As the increase of replacement level, the sharp extent of its absorption band decreases. After the doping of SrO, the absorption band of C₃S at 815cm-1 become to gradual disappearance from the very sharp spectrum, resulting in greater hydrate tendency of tricalcium silicate solutions. When the doping content of SrSO₄ is small, the content of free calcium oxide can be reduced in sample and consequently the formation of C₃S can be promoted. However, with the increase of the SrSO₄ content, the content of free calcium oxide in samples increases gradually. The characteristic of the R-C₃S solution sample mixed with 1.5% SrSO₄ is that the absorption band of C₃S at 815cm-1 gradually disappears from the very sharp spectrum.