Study On The Influence Of Different Seed Crystal On The Properties Of Magnesium Oxysufte Cement

Magnesium oxysufte（MOS）cement is light,fire-resistant,green and environment-friendly,and can be used as building wall,furniture substrate and other materials,but the later strength and water resistance of MOS cement need to be improved.Whether the hydrated MOS cement can play the role of seed crystal in MOS system remains to be studied.In this thesis,the influence of seed crystal with different hydration time,particle size and dosage on the compressive strength,softening coefficient and other properties of MOS cement was studied,and the influence mechanism was analyzed by XRD,SEM,MIP and other test methods,and get a better preparation process.In addition,to enhance the early compressive strength of MOS cement,synthetic 517 phase seed crystal with heat treatment were added to MOS cement.The effect of synthetic 517 phase seed crystal with and without heat treatment on the setting time,compressive strength,phase composition,microstructure and pore structure of MOS cement was investigated.The feasibility of improving the early mechanical properties of MOS cement by using synthetic 517phase seed crystal after heat treatment was analyzed,and the range of the content of MOS cement prepared by using synthetic 517 phase after heat treatment as the seed source was determined.The seed crystal after hydration for 7 d and 28 d can promote the growth of 517crystal phase of MOS system,inhibit the hydration of Mg O to form Mg（OH）₂,reduce the total porosity of MOS cement,increase the content of gel pores,increase the density of matrix,and improve mechanical properties and water resistance of MOS cement.The seed crystal with particle size of D₅₀≈30μm（hydration 28 d）has a stronger effect on the later mechanical properties of MOS cement.The 28 d compressive strength of MOS cement with 8%the seed crystal with particle size of D₅₀≈30μm was increased by 19.5%,and the softening coefficient of MOS cement with 10%the seed crystal with particle size of D₅₀≈57μm was increased by 17.2%.The doping amount of the seed crystal after hydration for 7 d and 28 d in MOS system should not exceed 20%.The 517 phase seed crystal with particle size of D₅₀≈7μm inhibits the growth of 517 phase in MOS system,promotes the formation of Mg（OH）₂,and makes MOS cement have poor 28d compressive strength and water resistance.The seed crystal after hydration for 1 d has obvious negative effect on the mechanical properties and water resistance of MOS cement.The synthetic 517 phase seed crystal decomposed into 5Mg（OH）₂·Mg SO₄·4H₂O and 5Mg（OH）₂·Mg SO₄at 100℃and 150℃,respectively,and both of these dehydrated phases can adsorb water molecules from the MOS slurry to convert into517 crystals.This can further promote the formation of 517 crystals in MOS system due to the adhesive-seed crystal synergistic effect of these two dehydrated phases,resulting in a shortening in the initial and final setting time,an optimization in the pore structure,and an improvement in the early and late compressive strength of MOS cement.In addition,the content of synthetic 517 phase seed crystal used to prepare MOS cement should not exceed 4 wt.%.Compared with synthetic 517 phase seed crystalwithout any heat treatment,the synthetic 517 phase seed crystal with heat treated at 100℃and 150℃have a stronger positive effect on the performance of MOS cement.The compressive strength of MOS cement mixed with 2%synthetic517 phase seed crystal after heat treatment at 100℃at 1 d,7 d and 28 d was 15.0%,51.1%and 21.7%higher than that of MOS cement mixed with 2%synthetic 517phase seed crystal without heat treatment.