Preperation And Modification Of The Magnesium Oxychloride Cementitious Material

Magnesium oxychloride cement?MOC?has excellent properties,such as high strength and durability,fire insulation and light environmental protection,but due to problems such as poor water resistance,poor dimensional stability,and large brittleness of products,their application in engineering is limited.The purpose of this paper is to prepare water-resistant,high-strength,and high-toughness magnesium oxychloride cementitious materials by optimizing the ratio of raw materials and curing conditions,screening high-efficiency and environmental-friendly modifiers,and optimizing the toughening pathways.Effect of the ratio of raw materials and curing conditions on the properties of magnesium oxychloride cement was examined with the main indexes of flexural strength,compressive strength,and softening coefficient.The results showed that the optimum molar ratio of MgO:MgCl₂:H₂O was 7.5:1:13.89.Under this ratio,the flexural strength of sample was 33.6 MPa,the compressive strength was 132.4 MPa,and the softening coefficient was 0.37.The best curing conditions were:Sealing curing,the water molecules in the system can participate more fully in the formation of the 518 crystal phase,so that the sample obtains higher mechanical properties.The influence of acid modifiers on the water resistance of magnesium oxychloride cement was investigated using the flexural strength and softening coefficient as the main indicators.The results showed that phosphoric acid,boric acid,potassium aluminum sulfate and aluminum phosphate had an effect on improving the water resistance of magnesium oxychloride cement,but far from reaching the engineering use requirements for related cement materials products.Organic acid modifiers such as hydroxyacetic acid,oxalic acid,tartaric acid,and citric acid can effectively improve the water resistance of magnesium oxychloride cement.The softening coefficient of the modified sample is more than 0.75.The coefficient is 0.93,which is an increase of 86%compared to the paste sample by hydroxyacetic acid modified MOC.Succinic acid has no modification effect on magnesium oxychloride cement,and the loss of bending strength and softening coefficient of the sample is serious.With flexural strength and volume shrinkage as the main indicators,the toughening effect of organic fiber and whisker on magnesium oxychloride cement was investigated.The results show that the addition of a certain amount of organic fibers or whiskers can effectively increase the flexural strength of MOC sample,and reduce the volume shrinkage.Among them,when the content is 2%,the flexural strength of the magnesium borate whisker reinforced magnesium oxychloride cement can reach 39.77 MPa,and the volume shrinkage rate is 4.63%,which is increased by 51.56%and 50.44%respectively compared to the paste sample.A three-factor and three-level response surface test of magnesium borate whisker,hydroxyacetic acid,and tartaric acid was performed using the center composite experiment method in response surface design,and the flexural strength and softening coefficient model of the MOC sample was established.The optimal ratios obtained are:1.22%glycolic acid,0.30%tartaric acid,2.19%magnesium borate whisker,the error between the prediction value and the actual value of the bending strength is 1.25%,and the error between the prediction value and the actual value of the softening coefficient is1.14%.Response surface analysis has important reference value and guiding significance for optimizing the performance of magnesium oxychloride cementitious materials.