Preparation And Properties Of Basic Magnesium Sulfate Cement

Basic magnesium sulfate cement(BMSC)is a new magnesium cementing material prepared by magnesium oxide,magnesium sulfate,water and admixture.It has the advantages of light weight,high strength,early strength,wear resistance,no moisture absorption and halogen return,and is not easy to rust steel bars,and can meet the requirements of green environmental protection and building energy saving.It is expected to become the core eco-friendly cement in the future.This text set about from the raw material,through the system test of the basic composition,types and dosage of mineral admixtures and curing conditions on the BMSC microstructure,mechanical properties and determine the light burned magnesia in Active magnesium oxide(?-MgO)content,the basic composition of molar ratios,curing condition and the dosage of mineral admixtures reasonable range,The main conclusions are as follows:?-MgO content has a significant effect on the properties of BMSC.At the same ratio,with the increase of?-MgO content,the strength of each phase of the test block decreases to different degrees.When the molar ratio of?-MgO/MgSO₄/H₂O=9/1/18,and the citric acid content is 1%of the mass of?-MgO,The strength of the cement test block prepared with45.9%?-MgO content is the highest,and the strength can reach 94.5 MPa on 28 days,which is 11.2%and 20.5%higher than that of the test block prepared with 47.0%and 60.4%?-MgO content under the same conditions.The strength of BMSC increases first and then decreases with the increase of?-MgO/MgSO₄molar ratio,and the optimal range is 8?9.With the increase of H₂O/MgSO₄molar ratio,the strength of BMSC increases first and then decreases with the increase of H₂O/MgSO₄molar ratio,and the optimal range is 17.5?18.5.The addition of citric acid can greatly improve its strength,but with the increase of dosage will prolong the setting time.The optimal range is 0.5%to 1.5%.The optimum ratio of?-MgO/MgSO₄/H₂O=8.5/1/17.7 and the citric acid content is 1%were optimized by orthogonal test.The influence order of each component on the compressive strength of BMSC is H₂O/MgSO₄>?-MgO/MgSO₄>the citric acid content.The incorporation of fly ash and mineral powder has little influence on the early strength of BMSC,but will reduce the strength of 28 days.The incorporation of silica fume can effectively improve the strength of BMSC,when the incorporation amount is 10%,the strength of 28 days is increased by 6.1%.The addition of mineral admixtures produced hydrated magnesium silicate gel(M-S-H),which adhered to the surface of5Mg(OH)₂·MgSO₄·7H₂O(5·1·7)phase.In terms of enhancing water resistance,the optimal dosage of fly ash,mineral powder and silica fume was 30%,20%and 10%,respectively.The softening coefficients on 28 days were 96.3%,85.6%and 88.0%,respectively.The optimum dosage of fly ash,mineral powder and silica fume is 30%,10%and 10%,respectively,and the softening coefficients of 28 days can reach 103%,95%and 80%,respectively.The strength of BMSC increases first and then decreases with the increase of curing temperature and curing humidity.The optimal curing temperature and humidity are 40?and60±2%RH,respectively.The strength of 7 days can reach the strength of 28 days at room temperature.After 3 days of pre-curing in air,MgSO₄solution curing was carried out.When the solution concentration reached 10%,the strength of 28 days exceeded that of air curing,and when the solution concentration reached 15%,the strength increased by 5.5%compared with that of air curing for 28 days.