| Modified MOS cement,also known as basic magnesium sulfate cement,(abbreviated as BMS cement),is mainly composed of needle-like basic magnesium sulfate crystals(5Mg(OH)2·MgSO4·7H2O,517 phase)interlaced growth.Recently,it has attracted widespread attention due to its advantages such as light weight,high strength,water resistance and good corrosion resistance.However,the content of active MgO(?-MgO)in light-burned magnesia(LBM),the main raw material used to prepare BMS cement,is only about 60%,and contains 15?20%of other impurities,causing a largely excessive amount of unreactive MgO and other impurities remained in the hydration products,which will inevitably affect the performance of BMS cement.In addition,the mineral admixtures commonly used in building materials,such as fly ash,its pozzolanic activity is difficult to be excited in a weakly alkaline system of BMS cement,and the composition of fly ash is more complicated,its effect mechanism on the early hydration behavior of BMS cement is not clear.This paper aims at the above problems of BMS cement.The effects of unreactive MgO and impurities in light burned MgO on the early hydration process and performance of BMS cement was investigated;Using pure SiO2 and Al2O3 as simulated mineral admixtures,SiO2-BMS cement and Al2O3-BMS cement were prepared,the influence mechanism of SiO2 and Al2O3 content on the early hydration behavior of BMS cement,as well as the influence on the compressive strength and water resistance of cement were studied,which provided theoretical support for the preparation of high performance BMS cement.The main research contents and conclusions are as follows:(1)BMS cement was prepared by LBM and MgO derived from precipitate of brine(PBM),characterized and measured by XRD,SEM,TG-DSC,FTIR,MIP,and electrodeless resistivity measurements,etc.The effects of unreactive MgO and impurities in LBM on the hydration process of BMS cement were investigated.The results show that the addition of CA has a significant retarding effect on BMS cement.The unreactive MgO and impurities have a negative effect on early hydration,but the compressive strength of LBM-BMS cement and PBM-LBM cement is almost equal due to the microaggregate effect of excesss unreactive MgO and impurities.However,due to the instability of unreactive MgO and MgCO3 in water,PBM-BMS cement has better water resistance.From the perspective of long-term performance of building materials,PBM may be more suitable for preparing high-performance BMS cement.(2)In order to investigate the effect of active silica and alumina in fly ash on the early hydration behavior and performance of BMS cement,SiO2-BMS cement and Al2O3-BMS cement were prepared by PBM,SiO2 and Al2O3 as mineral admixtures.The effects of different mineral admixtures content on the early hydration behavior and performance of BMS cement were systematically investigated.The results show that in the early hydration reaction of SiO2-BMS cement,not only the 517 phase is formed,but also a new gelling phase M-S-H(Mg3Si2O5(OH)4)is formed,which results in a significant increase in early strength of the cement;Excessive SiO2 doping will reduce the content of strength phase 517 phase per unit volume of the hydration product,but the microaggregate effect of SiO2 is significant,and its compressive strength can still reach 65 MPa;a part of SiO2 participates in the hydration reaction,and the formation of gel phase M-S-H reduces the capillaries and macropores,the fine pore structure of hydration products is improved,and the water resistance of BMS cement is improved.The doping of Al2O3 significantly improves the water resistance of BMS cement.When the content of Al2O3 is ?10%,the mechanical properties of BMS cement can be significantly improved,but excessive doping of Al2O3 will inhibit the growth of 517 phase in BMS cement.Therefore,if solid wastes containing Al2O3 components are selected as mineral admixtures,the maximum content of Al2O3 should not exceed 20%. |
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