Preperation And Modification Of The Magnesium Oxysulfate Cementitious Material

Magnesium oxysulfate?MOS?cement has many potential advantages such as fire resistance,lightweight and low corrosion for reinforcing steel,therefore it can be used as fireproof materials,reinforcement additives and building materials.However,the industrial application of MOS cement was significantly limited by its low mechanical strength,which was mainly due to the low hydration degree.This problem can be solved by adjusting the prospects of material ratio,production process and additives.Effect of material ratio on MOS cement's properties was studied with the evaluation indexes of flexural strength,softening coefficient,setting time and deformation rate.MOS cement prepared at the molar ratio of MgO:MgSO₄:H₂O was 11:1:24.9 showed excellent properties.It's flexural strength was 13.99MPa,softening coefficient was 0.2 to0.3,setting time was 360min,density was 2.03g/cm³,deformation rate was 1.7%.Effect of preparation temperature,stirring rate,stirring time and curing way,curing time,curing temperature were studied with the evaluation indexes of flexural strength and softening coefficient.The optimal preparation and curing conditions were listed as follow:The preparation temperature was 35?,stirring rate was 250-300r/min,stirring time was 20min.The curing time was 28d,curing temperature was 30-35?.The flexural strength of MOS cement can be increased to 14.09MPa,and the softening coefficient was0.35 prepared in the conditions described above.Effect of active filler,saturated fatty acid and hydroxyl polybasic acid were studied with the evaluation indexes of flexural strength,softening coefficient,setting time and pH value.When the addition of fly ash was 7.5wt%,flexural strength of MOS cement was increased to 16.74MPa,softening coefficient was increased to 0.78,setting time was decreased to 143min.When the addition of acid A was 1.25wt%,the setting time of MOS cement can be extended for 145min.The addtion of hydroxyl polybasic acid could increase the flexural strength of MOS cement to more than 20MPa,increase the softening coefficient to more than 0.85,and extend the setting time of it for 135-245min.The mechanism of MOS cement modified with hydroxyl polybasic acid was also discussed in this study.After modification,5Mg?OH?₂·MgSO₄7H₂O magnesium oxysulfate crystal whiskers were detected in the MOS cement system,which was resulted from the process as follow:At the early stage of hydration,[Mg?OH??H₂O?x]⁺was chelated with malate,and formed a more stable chelate to cover the surface of MgO particles,so that the hydration rate could be reduced effectively.Besides,the formation rate of Mg?OH?₂ was reduced,too.Then the 517 phase was formed through the self-assembling process of chelate molecule,OH^-,SO₄^2-and water molecule in the cement system.