Study On Preparation Of High Magnesium Expansive Cement From Powdered Magnesite Waste Rock

Magnesite is one of the dominant mineral resources in China,and its reserves,output and exports all rank first in the world.Magnesite in the process of separation will produce a large amount of powdery waste rock,commonly known as"weathering material",And the yield of magnesite is about 1:1,the total amount is large.However,due to its complex composition and uncertain composition and structure,the utilization rate of weathered material is very low.In this paper,high magnesium expansive cement was prepared by using powdered magnesite waste rock,limestone,copper slag and gypsum as raw materials.The hydration kinetics process,expansion property,mechanical strength and steaming stability of cement material were studied.The results show that:When MgO content is in the range of 3.25%-8.2%,Krstulovic model can be used to simulate the hydration kinetics process of high magnesium expansive cement material.The hydration process still goes through three reaction stages:NG→I→D,but with the increase of MgO content,the reaction rate K₁′value decreases,and the reaction time of NG process decreases.When the value of K₂′decreases,the reaction time of process I increases.The control time of process D has little change,and its simulation error increases with the increase of MgO content.The dilatability of high magnesium expansive cement mortar varies with time.The results show that with the increase of MgO content,the expansion rate of cement mortar increases with time,the growth rate was fast in the first 28 days,and the expansion rate slowed down after 28 days,but the slow expansion lasted for a long time.The MgO content has the greatest influence on limiting expansion rate,followed by curing temperature,and the water-binder ratio is the least influential factor.The variation law of mechanical strength of high magnesium expansive cement cementitious material with time was studied.The results show that,with the increase of MgO content,the hydration process of MgO produces Mg（OH）₂,which makes the overall volume expand,and makes the total pore volume,multiple harmful pores and harmful pores increase,while the mechanical strength decreases.Increasing temperature will lead to the increase of harmful pores and harmful pores at different ages of hardened slurry.At 40℃,the mechanical strength of 28d is lower than that of normal temperature curing.The decrease is more obvious at 60℃.The hydration of MgO is not only related to the expansion,but also related to the internal pore structure of hardened slurry.The high temperature leads to the coarsening of the pore structure inside the hardened slurry,which reduces the strength of cement mortar.For the high magnesium cement clinker with different MgO content,the stability test is conducted after curing according to the water-binder ratio of0.5 and the required curing conditions.The test results show that:When the proportion of powdery magnesite waste rock in the raw material is no more than 17%,that is,when the content proportion of MgO in the clinker is less than 6.52%,the steamer stability test is qualified.When the proportion of magnesite powder waste rock in the raw material exceeds20%,that is,when the content of MgO in the clinker exceeds 7.44%,the pressure steamer stability test specimen breaks,and the stability is judged to be unqualified.