Effect Of MgO And P₂ O₅ On Synthesis And Properties Of Alite-Calcium Strontium Sulphoaluminate Cement

Alite-calcium strontium sulphoaluminate cement with excellent performance was prepared by introducing calcium strontium sulphoaluminate mineral into the traditional Portland cement clinker. There are a lot of high-magnesium limestones and phosphorus slag in our country, therefore, studying the effect of MgO and P₂O₅ on the composition, structure and properties of alite-calcium strontium sulphoaluminate cement is significant to make more efficient use of low-grade raw materials and industrial refuse as well as serve the industrial production of cement. In this paper, the work extended towards the effect of MgO and P₂O₅ on the composition, structure and properties of alite-calcium strontium sulphoaluminate cement. The calcination technology of the cement and the amount of SO₃ and CaF₂ added in the raw material were also researched. The microstructure of clinker and hardened pastes, the composition and properties of cement were observed by means of X-ray diffraction, SEM–EDS and light analysis. The main conclusions are as follows:The MgO and P₂O₅ have major impacts on the formation of the calcium strontium sulphoaluminate mineral. At 1300℃for 150min, a suitable amount of MgO can refine grain and promote the hydration activity of minerals, as a result, the acceleration period of the cement appeares earlier and the compressive strength in the early stage increases obviously. The optimal addition of MgO is 0.5%²%, and when the content of MgO exceeds 2%, the compressive strength decreases gradually. An appropriate amount of P₂O₅ can be benefit to the formation of the calcium strontium sulphoaluminate mineral, while excessive P₂O₅ is adverse to the formation and the properties of mineral. When the addition of P₂O₅ is 0.3%, the compressive strengths of cement can reach 65.5, 83.6 and 96.7MPa at 3, 7 and 28 days curing age. P₂O₅ can not accelerate the hydration rate of the calcium strontium sulphoaluminate mineral. At 1150¹300℃,the kinetics of calcium strontium sulphoaluminate mineral formation obey the rate law equation f(α)=1-(1-α)^1/3.At 1380℃, the early strength of cement increase significantly when the content of MgO in the clinker ranges from 1.0% to 5.0%. The optimal strength is obtained when the addition of MgO is 2%, and the compressive strengths of cement can reach 29.5, 64.3 and 103.6 MPa at 1, 3 and 28 days curing age. Excessive SO₃ can not only increase the solid solution capability of intermediate phase to MgO, but also can be benefit to the formation of C_1.5Sr_2.5A₃(S|—) . When the excessive content of SO₃ is higher than 70%, the formation of C₃S can be hindered. The MgO can decrease the addition of CaF₂ in the clinker, and the formation and coexistence of C₃S and C_1.5Sr_2.5A₃(S|—) can be further improved due to the addition of MgO and CaF₂. The foundation for the expanded applications of low-quality high-magnesium limestone can be established because a higher amount of MgO can be permitted in the alite-calcium strontium sulphoaluminate cement than the Portland cement.When the addition of P₂O₅ is 0.3% and the calcination temperature is 1350℃, the optimal strength is obtained, achieving values of 27.0, 59.1 and 110.9MPa at 1, 3 and 28d days curing age. P₂O₅ mainly exists in the belite and a suitable amount of P₂O₅ can promote the formation of C_1.5Sr_2.5A₃(S|—) and alite. P₂O₅ can enhance the hydration heat evolution rate in the acceleration period and the hydrate heat of cement contained P₂O₅ increase slightly. At 1350℃, P₂O₅ can not decrease the addition of CaF₂ obviously and the optical content of CaF₂ is 0.6%.