Study On Preparation And Properties Of Basic Magnesium Sulfate Cement Using Dilute Sulfuric Acid

Magnesia cement has the advantages of green environmental protection,light weight,wear resistance,fast setting and early strength compared with ordinary Portland cement.It is of great significance to vigorously develop Magnesia cement for comprehensive utilization of magnesium resources,energy saving and emission reduction.New Basic magnesium sulfate(BMS)cement has attracted wide attention due to its excellent mechanical properties and good water resistance.The method of preparing BMS cement by dilute sulfuric acid instead of magnesium sulfate can not only reduce the cost,but also overcome the problem of heating and dissolving magnesium sulfate.In this paper,active magnesium oxide(?-MgO)and dilute H2SO4 are used as raw materials,and ultrafine fly ash(UFA)is used as a mineral admixture.A one-pot method is used to prepare high-performance BMS cement directly.The effects of factors such as molar ratio,UFA activation conditions,and dosage of UFA on the setting time,mechanical properties,and water resistance of BMS cement were studied.It provides a theoretical basis for the preparation of high-performance BMS cement from industrial waste sulfuric acid and salt lake magnesium resources.The main research contents and conclusions are as follows:(1)Fix M(?-MgO/MgSO4 molar ratio)to 5 and fix H to 18(H20/MgSO4 molar ratio)respectively,and study the influence of different H and different M on the performance of BMS cement.The results show that the initial and final setting time of BMS cement increases with the increase of H when M is constant.The compressive strength decreases with the increase of H.The flexural strength and water resistance coefficient increases first and then decreases with increasing H.When the optimal H is 18,the compressive strength of the prepared BMS cement is 79.5 MPa cured for 28 days,the flexural strength is 11.7 MPa,and the water resistance coefficient is 0.65.This is because sufficient dissolution of MgSO4 at normal temperature is guaranteed when H is 18.At this time,the W/C of the cement is 0.95 obtained the better fluidity of the cement slurry,which is conducive to the full reaction of the raw materials.Although the BMS cement cured for 28 days when H is 14 gets a compressive strength of 112.7 MPa,the flexural strength is reduced to 1.2 MPa.This is because the H content is too small,the raw material MgO is not completely hydrated,and MgSO4 is precipitated in the later curing process,which makes the BMS cement prone to brittle fracture.When H is greater than 18,the BMS cement will have a longer setting time and lower water resistance.The initial and final setting time of BMS cement decreases with the increase of M when H is constant.The compressive strength?the flexural strength and water resistance coefficient of BMS cement increases with the increase of M.The BMS cement test block prepared when M is 9 has a maximum compressive strength of 99.4 MPa and the highest flexural strength of 13.5 MPa.The maximum water resistance coefficient is 0.74.Mg(OH)2 and MgO are detected in the hydration products,indicating that the increase in compressive strength may be due to the filling effect of excess MgO.Soaking water will cause the hydrolysis of MgO,which will adversely affect its performance.The results show that BMS cement has excellent mechanical properties with M of 5.Comprehensive consideration,the optimal molar ratio of raw materials ?-MgO:MgSO4:H2O is 5:1:18.(2)The optimal activation conditions for UFA are:the mass concentration of H2SO4 solution is 20%with 2 h activation time.The compressive strength of UFA-BMS cement increased first and then decreased with the increase of PCUFA content.The compressive strength after soaking water decreased with the increase of soaking time and PCUFA content.The maximum compressive strength is 65.8 MPa when the PCUFA content is 20%.However,its water resistance coefficient is 0.29,indicating that UFA-BMS cement with PCUFA is not suitable for the preparation of water-resistant materials.UFA-BMS cement was prepared with CFBUFA as the raw material in order to improve the water resistance of UFA-BMS cement.The experimental results show that the compressive strength of the prepared UFA-BMS cement decreases with the increase of CFBUFA content and the compressive strength after soaking water also shows the same conclusion.However,the compressive strength of UFA-BMS cement cured with 10%CFBUFA for 28 days and the pressure of BMS cement without UFA are 79.1 MPa and 79.5 MPa,respectively,which indicates that adding 10%CFBUFA will not affect its mechanical properties.The compressive strength of 20%CFBUFA is 68.2 MPa and the water resistance coefficient of both after soaking in water are 0.57,and that without UFA BMS cement is closer.In a word,only CFBUFA with a content of less than 20%can be used to prepare UFA-BMS cement.