Study On The Modification Of Magnesium Oxysulfate Cement

Magnesium oxysulfate cement is the gas hardness cementitious material, which is formed by active magnesium oxide and magnesium sulfate solution. China has the advantages of developing magnesium oxysulfate cement, especially in Liaoning area. But due to low early strength, poor water resistance and other technical defects, magnesium oxysulfate cement is limited to be used in many areas, which is necessary to do further research to improve magnesium oxysulfate cementFirstly choosing the best basic ratio for magnesium oxysulfate cement was by adjusting the concentration of magnesium sulfate and water binder ratio at room temperature.And based on this ratio, five kinds of additives(phosphoric acid, sodium dihydrogen phosphate, Organic Acid Salt A, Organic Acid Salt B and compound additive) were selected to modify the cement. Experimental results showed that all the additives could improve the performances of magnesium oxysulfate cement.But compared with other additives, compound additive could effectively improve the strength and water resistance of magnesium oxysulfate cement.In this experiment, the phase analysis of magnesium oxysulfate cement by means of SEM and XRD method was carried out. The results showed that the additives could improve the degree of cross-linking of the crystals, and there was a new phase in the cement with Organic Acid Salt A, Organic Acid Salt B and compound additive. The new phase was the characteristic hydration product of basic magnesium sulfate cement(5Mg(OH)2·MgSO4·7H2O,Phase 5·1·7 for short). This indicated that the three additives could transform the magnesium oxysulfate cement system into a new cementitious material system, which was named basic magnesium sulfate cement.The influence of curing condition on the strength of magnesium oxysulfate cement was discussed. The experimental results showed that the curing temperature should be controlled in the range of 20 ℃ to 25℃, when the humidity was not more than70±2%RH. And the 28 d compressive strength and flexural strength of magnesium oxysulfate cement with a maximum at relative humidity of 60% to 70%, was 36.0MPa and 3.8MPa.In order to further improve the performance of cement, this paper was studied about the effect of admixtures on the performances of basic magnesium sulfate cement with compound additive.In the experiment, fly ash and slag were chosen. The results showed that the dosage of fly ash was not more than 40%, and the dosage of fly ash was not more than 30%. With the increase of the content of fly ash and slag, the strength of basic magnesium sulfate cement was decreased, and its water resistance was increased gradually.