| Magnesium oxysulfide cement is a kind of air hardening cementitious material,which is prepared from light burned magnesium oxide and a certain concentration of magnesium sulfate solution.It has excellent properties such as light weight,good volume stability and so on.However,compared with magnesium oxychloride cement,magnesium oxysulfide cement has lower early strength and higher production cost.Therefore,based on the basic proportion of magnesium oxychloride cement,selective addition of additives and industrial waste,and by the synergistic effect of different materials,a kind of magnesium oxychloride cement with high mechanical strength,good water resistance and low production cost was obtained.First,with the dosage of Mg O being constant,the influence of the concentration of Mg SO4 solution,water-solid ratio and other factors on the mechanical strength and water resistance of the product were investigated,and the basic ratio was determined:water-solid ratio is 0.7,Baume degree of Mg SO4 solution is 28°Bé.On this basis,the effect of different additives on the modification of magnesium oxysulfide cement was studied.The results show that:citric acid and phosphoric acid have a significant improvement on the mechanical strength of magnesium oxysulfide cement.When the content of citric acid is 1.5%,the compressive strength of the test block is 2.3 times of the blank.When the content of phosphoric acid is 0.8%,the compressive strength of the test block is 80.4%higher than the blank,and the cost is lower.Fly ash has great effect on the water resistance of magnesium oxysulfide cement.When the content of fly ash is 25%,the softening coefficient of cementitious material is increased by 66%.Silicon ash,zeolite and gypsum products with different crystal water also have certain effect on the performance improvement of magnesium oxysulfide cement.Secondly,the synergistic effect of different materials mixed in the modification process of magnesium oxysulfide cement was studied,and two kinds of suitable composite modifier were obtained:citric acid/fly ash system and citric acid/CS residue system.The results show that when citric acid is 0.3%and fly ash is 35%,the compressive strength of the cement with the former reaches 52.4 MPa,and the softening coefficient reaches 1.10;when citric acid is 0.3%and CS waste residue is 2%,the compressive strength of the cement with the former reaches 68.3 MPa,and the softening coefficient reaches 1.07.Desulfurization wastewater was used to replace magnesium sulfate heptahydrate,and various industrial wastes were added to form magnesium oxysulfide composite building materials,so as to reduce the cost and realize the utilization of magnesium desulfurization wastewater.The results show that when water-solid ratio is 0.7 and the wastewater 20°Bé,the suitable dosages（based on mass of magnesium oxide）of additives are:phosphoric acid 0.8%,waste polyurethane foam 1%,waste glass fiber reinforced plastics 1.5%,desulphurization ash 15%.The mechanical strength and water resistance of the material can be further improved by improving the Baume degree of desulfurized wastewater.When the desulfurization wastewater is of 28°Bé,the compressive strength of the material is increased to 48.8 MPa,the softening coefficient is up to 0.95,and the thermal conductivity of the material is 0.19 w/（m·K）.XRD and SEM were used to analyze the microstructure and hydration products of the modified materials.Before modification,the sample is a loose flake structure containing Mg（OH）2.After modification,it becomes a dense structure full of needle like crystals which is 5Mg（OH）2·Mg SO4·7H2O（5-phase）.The main reason for the poor water resistance of magnesium oxychloride cement is that the existence of a large amount of Mg（OH）2.It is found that the crystallinity of the modified magnesium oxysulfide material is significantly improved after hydration,and large amount of 5-phase crystals still exist after soaking in water,which shows that the addition of modifier has a protective effect on5-phase crystals,so as to ensure the good water resistance of the magnesium oxysulfide material.It is also showed in XRD spectrum that the increase in Baume degree of desulfurized wastewater in the process of substituting desulfurized wastewater for magnesium sulfate heptahydrate to produce magnesium oxysulfide,resulted in no changes in the type of hydration products,but increase in the content of 5-phase crystals,which improved the compressive strength and water resistance of the system. |
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