Study On Preparation Of Reactive Magnesium Oxide And Expansion Property Of Eco-cement Made From Reactive Magnesium Oxide

In order to solve the problem of high environmental impact, high energy consumptionand high emission of greenhouse gases in Ordinary Portland cement (OPC) industry,Magnesia-based eco-cement (MEC) made from reactive magnesium oxide, fly ash(FA),and Portland cement was studied in the thesis. However the expansion of MEC is a focusquestion because of the low hydration rate of magnesium oxide. In this thesis, a systematicexperimental study was carried out on expansion characteristics of Magnesia-basedeco-cement under different curing conditions. Expansive behaviors of MEC added withvarious content of MgO and fly ash under different autoclaving conditions was studied.On this base, proper autoclaving condition and its evaluating standard was put forward.The main research contents are as follows:1. Preparation of reactive MgO and its activity evaluationReactive magnesium oxide was prepared by calcining magnesite in rotary kiln. In thisthesis, the decomposition behaviors of magnesite at different temperatures, and differentresidence time were investigated. The hydration reactivity of the calcined products at lowtemperature was studied. The result showed that, the magnesite decomposing rate wasgetting higher with the increasing calcined temperature and residence time. However thereactivity of MgO in the product decreased as the calcined temperature and residence timeincreased. When the magnesite was calcined at700℃for60min, the activity of MgOobtained was optimal, but the decomposing ratio of magnesite was low. Concerning boththe decomposing rate of the magnesite raw material and the hydration reactivity of themagnesia calcined products, the optimized calcination condition is750℃for60min.2. Study on the strength growing under three curing conditionsA systematic experimental study was carried out on the compressive strength andexpansion characteristics of Magnesia-based eco-cement under different curing conditions,i.e. standard curing, carbonization curing and steam curing. Results showed that: Instandard curing, Main mineral phase of the cement is CSH, Ca(OH)2and Mg(OH)2. Thecompressive strength at60d is27.9MPa, and the expansion is0.22%. In carbonization curing, Main mineral phase of the cement is MgCO3and CaCO3. The compressivestrength at60d is55.4MPa, and the expansion is0.38%. In steam curing, Main mineralphase of the cement is CSH, tobermorite and Mg(OH)2. The compressive strength of thecement is37.0MPa at180℃for8h, and the expansion is2.12%.The hydration rate of the reactive magnesia was slow in standard and carbonizationcuring. The hydration of MgO-based expansive agent was effectively accelerated in steamcuring.3. The optimization research on MEC in autoclave conditionsThe expansive behaviors of MEC, added with various contents of reactive magnesiumoxide and fly ash, autoclaved under different autoclaving condition, were studied.Theresults showed that: the autoclave expansion of MEC appeared rising trend with thereactive magnesium oxide content increasing, but reduced with fly ash content increasing.It showed that fly ash added may restrain expansion of MEC and improve its soundness.The autoclave expansion of MEC rised when autoclave temperature raised from80℃to140℃, but reduced with the increasing temperature from140℃to180℃. Thevarying trend of the compressive strength was reverse.The autoclave expansion of MECrised when autoclave time prolonged from4h to6h, but reduced with the extending timefrom6h to8h. The varying trend of the compressive strength was reverse.The hydration of MgO is the theoretical basis for its expansion. The research shows thatcuring temperature has a major impact on the hydration rate of periclase. The extent ofhydration of magnesium oxide rised when the curing temperature increased.In this thesis, research on preparation of reactive magnesium oxide by calciningmagnesite was carried out. This thesis makes a systematic experimental study on thecompressive strength and expansion characteristics of Magnesia-based eco-cement underdifferent autoclaving condition. Optimal autoclaving condition of MEC was obtained. Theresults in this thesis provided some technical support for expansion evaluating standard ofMEC.