Effects Of Different Active MgO On Performance And Mechanism Research Of Basic Magnesium Oxysulfate Cement

Basic magnesium sulfate cement(BMSC)is a magnesium cementing material with 5·1·7 phase(5Mg SO4·Mg(OH)2·7H2O)as the main hydration phase.It has excellent properties of early strength,high strength,good toughness and high temperature resistance.At present,the active MgO used in the preparation of BMSC is mainly from the light fired magnesite powder,and the active MgO can be prepared by brine-lime milk method,brine-ammonium carbonate method,magnesium chlorideammonium bicarbonate method,etc.However,the preparation of MgO from all kinds of MgO precursors(such as magnesium chloride,magnesium hydroxide and basic magnesium carbonate,etc.)goes through calcination process,and the magnesium oxide produced by different calcination process has different specific surface area and crystallization degree,and presents different activities,which affects the performance of BMSC.In this paper,basic magnesium carbonate was used as raw material and calcined at 600-1000℃ to obtain MgO with different reactivity,and BMSC with different MgO/Mg SO4(M)molar ratios were prepared.The effect of MgO reactivity on the performance of BMSC was studied.Through XRD,SEM,MIP,hydration exotherm and pore liquid composition,the mechanism of activated magnesium oxide reaction was analyzed.Experimental results show:(1)As the calcination temperature of basic magnesium carbonate increases,the BET surface area of MgO continues to decrease and the crystal grain size becomes larger.(2)When MgO/Mg SO4(M)is 5,the early strength first increases and then decreases with the increase of MgO activity,and the later strength increases with the decrease of MgO activity;When M=7,the third exothermic peak of Mg(OH)2 nucleation and growth appeared in the hydration process of basic magnesium sulfate cement,which resulted in the shrinkage of the strength.(3)The main hydration product of BMSC prepared with high activity MgO is Mg(OH)2,and the main hydration product of BMSC prepared with low activity MgO is 5·1·7phase.The pores in BMSC with 5·1·7 phase as the main hydration phase are mainly intercrystalline pores,and with the increase of the 5·1·7 phase crystallinity,the larger the average pore diameter.(4)The water resistance of BMSC is mainly related to the content of free Mg SO4 in cement specimens and the degree of crystallinity of 5·1·7phase.This article has carried out a study on the effect of MgO activity on the properties of mineral admixture BMSC.Experimental results show:(1)The effect of MgO activity on BMSC with mineral admixtures is the same as that without mixing,that is,the lower the activity,the lower the early strength,and the higher the later strength.(2)When mineral admixtures was added,no new hydration products of basic magnesium sulfate cement were produced.(3)When fly ash is added,the number of gel holes in BMSC is more and the average pore size is smaller.(4)Regardless of whether high activity(T800)or low activity(T1000)MgO is used,the softening coefficient of the mineral admixture BMSC increases with the increase of the content after 30 days of soaking in water.