Research And Application Of Magnesium Oxysulfate Lightweight Wall Material

Magnesium oxysulfate cement is a type of air-hardening material with environmental protection and low-energy consumption,which has the advantages of low alkalinity,excellent corrosion resistance,good fire and heat insulation performance,good decorative effect and so on.However,the mechanical properties,physical properties and durability of magnesium oxysulfate cement are poor due to its insufficient hydration and the complexity and unstable composition of hydration products,which limits its large-scale application.In this paper,the mix proportion of raw materials,the modifiers and dual mineral admixture of magnesium oxysulfate matrix materials were investiagted.The physical foaming of modified magnesium oxysulfate cement was carried out,and the influence of bulk density and polypropylene fiber on the performance of magnesium oxysulfate foaming cement was studied.Additionally,advanced testing methods,including scanning electron microscope,X-ray diffraction,X-ray computed tomography experiment and mercury intrusion porosimetry,were employed for characterizing the phase composition and microstructure of the samples.A sort of magnesium oxysulfate cement with light weight,high strength,good water resistance and excellent volume stability was finally prepared,which is expected to provide experimental and theoretical basis for the development and application of magnesium oxysulfate lightweight wall materials.The basic mix proportion of magnesium oxysulfate cement,including the molar ratio of magnesium oxide to magnesium sulfate（M）and the molar ratio of water to magnesium sulfate（H）was first designed.Higher molar ratio of magnesium oxide to magnesium sulfate and lower molar ratio of water to magnesium sulfate were found to effectively improve the compressive strength of cement specimens.Through measuring the mechanical strength of the specimens,the more appropriate basic mix proportion was determined,that is,M was 8,and H was 20.After adding citric acid,calcium dihydrogen phosphate,commercial modifier S（Shandong Jiemei）and Meijiatu MJ,the mechanical strength of magnesium oxysulfate cement specimens was improved significantly.By comprehensively comparing the influence of modifier type and dosage on the performance of the cement specimens,it was found that the better modifier was calcium dihydrogen phosphate,and its optimal dosage was 1.5wt%.The compressive strength,water resistance and volume stability of magnesium oxysulfate cement could be improved by adding fly ash and mineral powder.The compressive strength of magnesium oxysulfate cement increased from 58.3 MPa to 71.8 MPa,and the softening coefficient increased from 0.89 to 0.94.Combined with the porosity analysis results of specimens containing mineral admixtures,fly ash and mineral powder was found to can effectively fill pores in the hardened matrix of magnesium oxysulfate cement,and reduced the porosity,as well as enhanced the water resistance and the volume stability.The best mix ratio was obtained in terms of the experimental results of the flexural strength,compressive strength,volume stability,water resistance,water absorption and setting time of the foamed cement,as well as the analytical results of the microstructure.In conclusion,the optimal mix proportion for Mg O:Mg SO₄:H₂O was 8:1:20.The modifier was calcium dihydrogen phosphate,the content of which was 1.5wt%.The total mineral admixture content was 45%,in which the ratio of fly ash to mineral powder was 2:1.The physical foaming method was adopted.Sodium dodecyl sulfonate was employed as the foaming agent,and the foam stabilizer and the water reducing agent were calcium stearate and calcium stearate,respectively.The content of the water reducing agent and the polypropylene fiber were 0.5wt%and 1%.The specimence of magnesium oxysulfate foamed cement with the dry density of 614.3kg/m³was finaly prepared using the above optimal mixing ratio.Its flexural and compressive strengths were respectively 1.7 MPa and 5.4 Mpa,and the water absorption rate was 47%.Through analyzing the hydration products and microstructure of magnesium oxysulfate foamed cement,it was found that the type of hydration products in foamed cement mixed with foam,stabilizer,water reducing agent and polypropylene fiber had not changed,whereas the relative content of each phase changed.The relative content of the 5·1·7 phase decreased,and the relative content of magnesium hydroxide increased,thereby resulting in higher porosity of the matrix.However,the distribution of microporous structure in the sample was uniform,the specimens,therefore,exhibited good performance.