Preparation Of Magnesium-Silicate-Hydrate Cement By Replacing Silica Fume With Pulverized Fuel Ash

Our country is rich in magnesium resources, most studies about magnesium-based cements are focused on the modification of traditional magnesium cements and be short of research and development of new magnesium-based cements. This paper was firstly exploring the preparation and properties of Magnesium-silicate-hydrate cement (M-S-H), and on this basis, replacing silica fume (SF) with pulverized fuel ash (PFA) to prepare the M-S-H cement, finding out its development regulations of physical properties, including:compressive strength, pH value changes, early age shrinkage characteristics, setting time, and optimum mix proportion of M-S-H cement was put forward. Microscopic structure and thermal decomposition properties of the hydration products were studied explored by XRD, SEM, TG/DSC and other microcosmic methods.By adjusting the addition of sodium hexametaphosphate (NaHMP) and water-solid ratio (w/s) to explore the compressive strength change regularity of samples containing high grade 60 wt.% SF and 40wt.% PFA, the results showed that the addition of lwt.% of NaHMP and water-solid ratio of 0.5 was best, compressive strength decreased with w/s. When w/s less than 0.6, the compressive strength was over 45 MPa at 28 days age, though the strength was higher when w/s was 0.4, but the flowability of pastes was bad and appeared bubbles, so w/s of 0.5 was regarded as the optimal ratio.M-S-H cement was prepared by replacing SF with PFA, the compressive strength firstly increased and then decreased with PFA content increasing, when the PFA content is over 20wt.%, the compressive strength was highest, when replacement was over 40 wt.%,the strength was lower than the samples without PFA. The early age shrinkage rate and excess water increased with the PFA content. Considering the application and economic benefits, additions of 35wt.% PFA was optimum and the hydration products was complex, including M-S-H gel, magnesium-carbonated gel and brucite, there were amounts of dense mesh microstructure by SEM test.Flowability of the pastes increased with the content of PFA, to improve the compressive strength and maintain good workability, w/s ratio of 0.38 was suitable for cement with 35wt.% PFA replacement by experiments and calculation, which can reach 46 MPa after 28 days curing. It was found that alkali-activator promoted the compressive strength development, and 10wt.% Na2CO3 was best for PFA-SF-MgO system. This new M-S-H cement system has a bright future as the cost of production can be reduced by 150-200 RMB per tone.