Study On Hydration And Carbonation Mechanism Of Lower-Carbon Composite Cementitious Materials Based On Reactive MgO

Research on the reduction of carbon emissions in production process of Ordinary Portland cement (abbreviated as OPC), and the development of low-carbon cement materials become hot topics. Dead burned MgO prepared under the high-temperature calcination has a swell effect on the hydration process of OPC, which controlled as the harmful ingredients in production process of OPC. In recent years, application of high reactive magnesium oxide (abbreviated as reactive MgO) prepared at low-temperature as cement expansive agent or CO2adsorption material has been paid more attention in the low-carbon cement. Focused on the rich magnesite resources in our country, reactive MgO was prepared by calcining magnesite at low-temperatures. Evaluation on the hydration reactivity and the characterization of hydration expansion, as well as carbonation characteristics of reactive MgO in OPC were explored in this dissertation. The dissertation mainly includes the following aspects:(1) The preparation of reactive MgO by calcining magnesite at low-temperatures and the evaluation on the hydration reactivityThe chapter focused on the law of different calcination process conditions of reactive MgO prepared by calcining magnesite in rotary kiln. The optimum condition was found out to prepare reactive MgO by changing the parameters of calcination temperature, holding time. On the basis of evaluation methods such as citric acid chromogenic time measurement and the MgO hydration conversion ratio, a correction method of hydration ratio was advanced for the reactive magnesium oxide prepared by calcining magnesite at low-temperatures such as600-750℃. Hydration products of calcined magnesite were characterized by XRD, SEM, TEM and other techniques. The results showed that at lower calined temperature for shorter holding time, the reactivity of MgO in per unit mass of calcined magnesite product is higher, but with lower decomposition rate of magnesite. Considering both decomposition rate of magnesite and reactivity of MgO in calcined magnesite products, parameters for preparing reactive MgO by calcining magnesite at calcination temperature700℃, for holding time180min MgO product preparation activity for cement materials in subsequent research. absorption amount of27.30%. Phenolphthalein was used to measure the carbonation depth. Completely carbonization zone with pH9.6is colorless. Non-carbonized zone with pH12.9is red. Carbonation reaction zone have a pH at range of9.6to12.9. The maximum compressive strength of mortar cured in10%CO2is sample PM3-1(30wt%fly ash and10wt%MgO). Compressive strength for14d,28d and90d are60.75MPa,53.69MPa and67.56MPa, respectively.(4) Volume deformation of alkali-activated slag cement mixed with reactive MgOAlkali-activated slag (AAS) cement is easier to crack than OPC slurry for the calcium silicon ratio and the pore size structural characteristics of the hydration products CSH gel. AAS mixed with fly ash and reactive MgO can reduce the drying shrinkage and chemical shrinkage of AAS cement and improve the crack resistance. Four factors and three levels experiments have been conducted by the surface of the response method (RSM, Response Surface Methodology), which is based on the single factor experiments on modulus and dosage of alkali activated agent, fly ash content and MgO content. Results show that:the optimal ratio of the parameters of MgO-AAS-FA system is fly ash and slag’s mass ratio (FSR) is0.4, alkali activated agent dosage is10%, modulus of sodium silicate is1.6, dosage of MgO is0.19. Mortar block prepared according to the fitting results with proportion of MgO:slag:fly ash is20:56.3:23.7, the flexural strength for28d is7.29MPa and the compressive strength is44.58MPa.