Study On Water Resistance Modifier Of Magnesium Oxychloride Cement

Magnesium oxychloride cement is a typical cementing material,which is also known as the Sorel cement or magnesium cement.It is an air-hardening binding material prepared from a certain concentration of magnesium chloride and magnesium oxide.It has the advantages of fast setting hardening,high mechanical strength,good wear resistance,low alkali corrosion resistance,fire insulation etc.,It is mainly used in building panels,concrete,fire prevention materials,adsorption materials and other fields.However,its disadvantages of poor water resistance,easy deformation,moisture absorption and halogenations limited its uses.Therefore,the purpose of this work is to prepare water-resistant magnesium oxychloride cement through modification research.Firstly,the basic formula of magnesium oxychloride cement was obtained by adjusting the molar ratio of MgO/MgCl₂ and the concentration of MgCl₂.The results showed that the mechanical strength and water resistance of cementing material were both higher when the molar ratio of MgO/MgCl₂ was 8 and the concentration of MgCl₂ was 28%.Secondly,the study of modification of the magnesium oxychloride cement was carried out by single factor test.Through the modification with a variety of additives,it was found that phosphoric acid and phosphate can improved the water resistance of Sorel cement obviously,but the mechanical strength loss was larger.The use of citric acid and sodium citrate was obviously enhanced the water resistance of Sorel cement,and the loss of mechanical strength was small.In general,the effect of phosphoric acid,citric acid and sodium citrate was good when the dosage was around 1%.The effect of some mineral admixtures on magnesium oxychloride cement was investigated.With the increase of silica fume content,the mechanical strength and water resistance were both increased.When the dosage was 15%,the mechanical strength was 66.51MPa and the softening coefficient was 0.735.The influence of common cementing materials on magnesium oxychloride cement was studied.The proper content of hemihydrate gypsum was 15%with the softening coefficient 0.734.The ideal dosage of Portland cement was 3%with the softening coefficient 0.663.The incorporation of sulphoaluminate cement decreased the strength of the system,but the water resistance increased to some extent.The impact of some polymer emulsions on magnesium oxychloride cement was also investigated.The results showed that the water resistance of magnesium oxychloride cement was improved by adding styrene-acrylic emulsion and styrene butadiene latex.The addition of waste glass fiber reinforced plastic?WGFRP?fiber had a positive effect on the mechanical properties and water resistance of cementitious materials to some extent.It showed that the addition of WGFRP fiber of 10-15mm increased the compressive strength by 22.55%compared to the blank.The softening coefficient of magnesium oxychloride cement containing 5¹0mm WGFRP fiber reached 0.519,which was 25.06%higher than that of the blank.Two modified systems,phosphoric acid based modifier?I?and citric acid based modifier?II?were selected and to be further optimized.The optimal proportioning of modifier I was obtained by mixing phosphoric acid and sodium silicate?1:1,molar ratio?,and the softening coefficient was up to 0.988.The optimal material ratio of citric acid based modifier?II?was:citric acid 1.3%,silica fume 6%,styrene-acrylic emulsion 0.5%,gypsum 11.95%.In this case,the softening coefficient of magnesium chloride cement reached 0.981.The modified magnesium oxychloride cement had excellent mechanical properties.The composition of hydration products of hydration paste was analyzed by using XRD,SEM,FTIR and XPS.The results were highly consistent with each other.The main reason for the poor water resistance of magnesium oxychloride cement without additive was the formation of Mg?OH?₂ from hydrolysis of 5-phase crystal network.It was found that5-phase crystals still existed in large quantities after the modified magnesium oxychloride cement was soaked in water,indicating that the addition of modifier could protect the5-phase crystals and prevent the hydrolysis of 5-phase crystals,thus ensuring the high mechanical strength and good water resistance of cementitious materials.