Preperation And Modification Of The Magnesium Oxysulfate Cementitious Material

Magnesium oxysulfate cement is a new type of environmental protection building material.It can not only take advantage of the excellent engineering strengths of magnesium cement,but also have important significance for environmental protection.Problems such as low mechanical properties,poor water resistance,and poor volume stability have hindered its development.Therefore,the development of high strength,high water resistance and high tenacity magnesium oxysulfate composites becomes significant.The effects of a single modifier on the properties of magnesium oxysulfate cement were studied with the main indexes of flexural strength,softening coefficient,and setting time.The results show that the addition of oxide modifiers such as silica and calcium oxide has no significant effect on the modification of the mechanical properties and water resistance of the magnesium oxysulfate cement.Adding inorganic acid and inorganic acid salt modifiers such as phosphoric acid,sodium dihydrogen phosphate and disodium hydrogen phosphate can effectively improve the performance of the sample,and the better was sodium dihydrogen phosphate.The flexural strength of the specimens modified with1.0%sodium dihydrogen phosphate reaches 22.92 MPa,and the softening coefficient is0.72.Adding organic acids and organic acid salt modifiers such as malic acid,malonic acid,succinic acid,gluconic acid,sodium malonate,and sodium malate can improve the performance of magnesium oxysulfate cement in various degrees.Among them,the modification effect of gluconic acid is the best.The flexural strength of the specimens modified with 1.0%gluconic acid reaches 26 MPa,and the softening coefficient is 0.83.The effects of compounding modifiers and modifiers and compounding modifiers with whiskers on the properties of magnesium oxysulfate cement were studied with the main indexes of flexural strength and volume shrinkage,and a series of high-strength water-resistant excellent compound formula were selected.The results show that the addition of composite modifiers can significantly improve the flexural strength of magnesium oxysulfate cement.Among them,four composite modifiers have greatly improved the mechanical properties of magnesium oxysulfate cement,which are 1%sodium dihydrogen phosphate and 1.5%malic acid,0.5%sodium malonate and 2%gluconic acid,1%sodium malate and 2%malonic acid,and 1%sodium malate and 2.5%gluconic acid.Their flexural strength is above 30MPa,equal to that of magnesium oxychloride cement.Gluconic acid is compounded with K₂Ti₆O₁₃whiskers,Zn O whiskers and Ca CO₃whiskers,respectively,which can play a better modification effect.Not only mechanical properties but also volume stability was improved.A 153 type(MgSO₄·5Mg(OH)₂·3H₂O)Magnesium Hydroxide Sulfate Hydrate Whiskers was prepared by hydrothermal synthesis.The influence of the preparation conditions on the basic magnesium sulfate whiskers was investigated to obtain the basic magnesium sulfate whiskers with regular morphology and high purity.By adding the Magnesium Hydroxide Sulfate Hydrate Whiskers into 1.5%gluconic acid magnesium oxychloride cement,the effect of basic magnesium sulfate whisker on mechanical properties of magnesium oxychloride cement was studied with the main indexes of flexural strength and volume shrinkage.The results showed that the basic magnesium sulfate whisker synthesized under this conditions have straight morphology,smooth surface and good dispersion:the activity of magnesium oxide is 84%,the concentration of magnesium sulfate is 10%,the molar ratio of magnesium oxide to magnesium sulfate is 1.06,and the stirring rate is 300 r/min,and the prepared Magnesium Hydroxide Sulfate Hydrate Whiskers has a regular morphology and high purity.Magnesium Hydroxide Sulfate Hydrate Whiskers can significantly improve the mechanical properties,water resistance and volume stability of magnesium oxysulfate cement.After adding 3%of basic magnesium sulfate whiskers,the flexural strength of magnesium oxychloride cement samples is increased by 26.96%,the softening coefficient is increased by 9.64%,and the volume stability is improved by 41.63%.