Basic Study Of Magnesium Hydroxide Sulfate Hydrate Whisker-reinforced Magnesium Silicate Hydrate Cement

Portland cement in the traditional production process consume a large amount of non-renewable resources.Therefore,the sustainable development of the cement industry has been severely challenged as resource and environmental constraints.Green cement with the connotation of"low carbon and environmental protection"has become the only way for the development of the traditional cement industry.The production process of magnesia cementitious material is simple,CO₂ emission lowing and no other harmful gases,as a result,magnesia cementitious material has become the main direction of low-carbon green cement research.Our country has rich magnesium resources,the proven magnesite reserves account for about 29%of the world’s total reserves,of which Liaoning Province is the largest.The rich magnesium resources provide great convenience for the research of magnesium cementitious materials.MgO-SiO₂-H₂O system is a new type of hydraulic magnesium cementitious material,has low alkalinity,light weight,high temperature resistance and other characteristics.It has great development potential and practical value in the preparation of refractory insulation materials,electromagnetic shielding materials,and solidification of low and medium level nuclear waste.However,the shortcomings of MgO-SiO₂-H₂O system,such as large shrinkage deformation and poor volume stability,restrict its further application.Therefore,this article uses MgO-SiO₂-H₂O system as the base material,and magnesium hydroxide sulfate hydrate whiskers as the reinforcing material.The composition,structure,morphology and performance of magnesium hydroxide sulfate hydrate whiskers were characterized.The articles explore the effect of magnesium hydroxide sulfate hydrate whisker content and curing age on various mechanical properties of hydrated magnesium silicate cement composites,the changing law of magnesium hydroxide sulfate hydrate whiskers on the volume stability of MgO-SiO₂-H₂O system.Combining various microscopic testing methods to analyze the size,morphology,dispersion degree of whiskers in different reaction stages,as well as the changes in the microstructure of cement-based materials,and interface bonding,etc.Identify the mechanism of the effect of magnesium hydroxide sulfate hydrate whiskers on the microstructure of MgO-SiO₂-H₂O system.The main conclusions are as follows:（1）Magnesium hydroxide sulfate hydrate whiskers have excellent applicability and durability in hydrated magnesium silicate cement materials.Magnesium hydroxide sulfate hydrate whiskers are slightly alkaline materials,being placed in p H=9.5,10.5,11.5 Na OH solution for 28 days,the whiskers still maintain the needle-like structure,surface smoothness and whisker integrity.The magnesium hydroxide sulfate hydrate whiskers in the long-age（360d）test block still have a good enhancement effect,when the cement block is damaged,it still exerts its fiber reinforcement effect.（2）The incorporation of magnesium hydroxide sulfate hydrate whiskers can improve the compressive strength and flexural strength of M-S-H cement mortar,the compressive and flexural strengths of the composites increased and then decreased as the magnesium hydroxide sulfate hydrate whisker content increased.The flexural strength of samples with 4 wt.%whiskers is best,and the compressive strength of the sample with 3 wt.%whiskers is best.However,the magnesium hydroxide sulfate hydrate whiskers had a negligible effect on the toughness of M-S-H cement mortar,which still displayed brittle fracture,due to the small size of magnesium hydroxide sulfate hydrate whiskers and the limited effect of fiber bridging traction.（3）The microscale reinforcement mechanism of magnesium hydroxide sulfate hydrate whiskers involves whisker pullout,crack deflection,whisker-cement coalition pullout,and whiskers fracture.These mechanisms effectively dissipated energy and optimized the stress distribution and transfer,thus improving the compressive strength,flexural strength,and other properties of the composites.The magnesium hydroxide sulfate hydrate whiskers filled pores,which eliminated pores with adverse effects and optimized the pore size distribution,improving the compressive strength of the M-S-H cement.Magnesium hydroxide sulfate hydrate whiskers had an optimal effect at 3 wt.%whiskers.As the whisker content increased further,whisker agglomeration occurred,and the density of pores increased,which reduced the porosity,resulting in a less-homogeneous pore size distribution,and decreasing the compressive strength of the M-S-H cement.（4）Among Balshin formula,Ryshkevitch formula,Schiller formula and Hasselmann formula,Ryshkevitch equation can best describe the porosity-strength model of M-S-H cement.The model indicate that,the porosity and compressive strength of M-S-H cements were inversely proportional,and the main factor that influences mechanical properties of M-S-H cement is porosity and magnesium hydroxide sulfate hydrate whiskers dosages.（5）The drying shrinkage rate of M-S-H cement mortar increases with the drying shrinkage age,and the drying shrinkage rate changes most significantly in the early stage of the drying shrinkage test.The incorporation of magnesium hydroxide sulfate hydrate whiskers can inhibit the drying shrinkage of M-S-H cement mortar,and the inhibitory effect increases with the increase in the amount of whiskers.（6）The water loss rate of M-S-H cement mortar increases with the drying shrinkage period.The water loss rate has the most significant change in the early stage of the drying shrinkage test.As the drying shrinkage period increased further,the water loss rate of M-S-H cement mortar tends to be stable.The incorporation of magnesium hydroxide sulfate hydrate whiskers can slow down the dehydration of M-S-H cement mortar.And the water loss rate of M-S-H cement mortar mixed with magnesium hydroxide sulfate hydrate whiskers increases with the increase of the whisker content,but the water loss rate was still lower than the comparison sample.（7）The mass loss rate of M-S-H cement mortar with different magnesium hydroxide sulfate hydrate whiskers doesn’t change significantly with the increase of the number of dry-wet cycles,and the mass loss rate is stable in the range of-1%-1%.As the number of wet-dry cycles increased further,M-S-H cement mortar compressive strength is also rising.The compressive strength of growth in the wet-dry cycle is much larger than soaking in water.The reason is that high temperature helps magnesium hydroxide react with silica fume to form M-S-H gel,the compressive strengths of the composites increased as M-S-H gel increased.In addition,high temperature curing helps increase the degree of polymerization of the silicate network structure in the M-S-H gel,which is beneficial to the transformation of Q2silicon-oxygen tetrahedrons to Q3 silicon-oxygen tetrahedra.