| Bischofite was used as raw material (two-step method) for the preparation of Magnesiumoxychloride cement (MOC) by microwave technology and thermal decomposition. The waterresistance of MOC was investigated by the addition of silicates in this paper. Bischofite and MOCwere applied effectively by above method, and more time and energy saving at the same time.The decomposition of bischofite was studied by microwave at first. Microwave weight lossrate, Cl-/Mg2+molar ratio, microwave decomposition temperature and microwave product werediscussed through the chemical analys is, TG and XRD. Then the optimal microwave power andmicrowave time of MgCl2·6H2O into MgCl2·nH2O (n≤2) were determined by above methods. Themolar rate of MgO/MgCl2of the compound after calcination of MgCl2·nH2O (n≤2) was measuredby EDTA titration method and mohr method and then MOC was prepared by mixing thecompound of the molar ratio of MgO/MgCl2greater than5with the appropriate weight of water,and compared with the method of preparation of MOC by thermal decomposition directly. Thehydration products in the neat MOC paste specimens were identified by X-ray diffraction analys is(XRD), the morphology and microstructure of the MOC pastes were characterized by scanningelectron microscopy (SEM). Then the optimal calcination temperature and calcination time weredetermined through above methods.By incorporation of ammonium phosphate, TEOS, sodium silicate, sodium hydroxide,ammonium carbonate, ammonium oxalate into MOC, and explore the effects of the dosage ofmodifier on the properties of MOC. MOC paste specimens after modifying were identified byX-ray diffraction analysis (XRD) and scanning electron microscopy (SEM).The results indicate that when microwave power was constant, the weight loss rate ofMgCl2·6H2O firstly increasing then keep unchanged, Cl-/Mg2+molar ratio firstly decreasing thenkeep unchanged, microwave decomposition temperature first decreased then increased to aconstant, with the increase of microwave irradiation time; When microwave irradiation time wasconstant, weight loss rate of MgCl2·6H2O had a gradual decreasing trend as well as microwavedecomposition temperature, which contrary to Cl-/Mg2+molar ratio. The microwavedecomposition of MgCl2·6H2O was active and useful to the subsequent decomposition reaction of MgCl2·6H2O. The optimal microwave power and microwave time of MgCl2·6H2O intoMgCl2·nH2O (n≤2) were700W and10minutes respectively. In addition, microwavedecomposition was more energy and time saving than thermal decomposition. The28dcompressive strength of MOC specimens prepared by the calcination of MgCl2·nH2O (n≤2) wasup to77.1MPa when the molar rate of MgO/MgCl2/H2O was5.9:1:14, and the optimalcalcination temperature and calcination time were550℃and50minutes respectively.9420kilojoules of energy was consumed by two-step method and10886.4kilojoules of energy wasconsumed by the method of preparation of MOC by thermal decomposition directly in the sametime.The effect of ammonium phosphate, TEOS, sodium silicate, ammonium oxalate, ammoniumcarbonate and sodium hydroxide on the water resistance of MOC reduced in turn. The optimizedmodifiers were TEOS and sodium silicate. By incorporation of TEOS and sodium silicate intoMOC, a high water resistance and high compressive strength material can be formed, and thecompressive strength and strength retention coefficient were improved. The strengths andwater-resistance of hardened MOC had a firstly increasing and then decreasing trend with theincrease of TEOS and sodium silicate. When TEOS and sodium silicate dosage were1.5%, thestrength retention coefficient could be increased by48%and45%. The compressive strength andwater resistance of MOC were affected by sodium silicate through the effect of the generation of5phase and3phase of MOC in the chemical reaction equilibrium. The compressive strength andwater resistance of MOC were affected by TEOS through its hydrolysis to generate thecross-linking of silicone cohesion. |
PDF Full Text Request