Fabrication Of Flue Gas Desulphurization Gypsum Based Steel Slag Cementitious Materials

Flue gas desulphurization (FGD) gypsum is a material with excellent performance, which has far better performance than that of natural gypsum. It is one of the best alternatives instead of natural gypsum. FGD gypsum based steel slag is a new type of cementitious materials with FGD gypsum as basal body and steel slag as fill body. This material has the performance of gelation capacity due to a certain amount of cementitious minerals, which could raise the utilization rate of these two kinds of industrial solid wastes. However, the steel slag is not widely used because of its complex formation condition and few cementitious mineral, FGD gypsum has also a low utilization rate because of large water content, large standard consistency water consumption, particle size distribution difference and so on. Therefore, we take methods of adding exciting agent, retarder, water reducer to improve the performance of FGD gypsum-based steel slag cementitious materials, which aims to increase the utilization rate of these two kinds of materials.Firstly, the effect of steel slag on mechanical property, setting time, crystallization and other performance of FGD gypsum based steel slag were discussed to get the optimum dosage of steel slag. The results show that the cementitious materials exhibit a more excellent performance on the the mechanical properties, setting time, and hardened crystal morphology when the dosage of steel slag is 15%.In this paper, various performance of cementitious materials was improved through stimulating the activity of steel slag, whose potential activity was mainly stimulated by adding single and compound exciting agents. Studies have shown that variety of stimulating agents could stimulate the potential activity of steel slag in different level, improve the mechanical performance of cementitious materials, increase the amount of needle-like hydration products of steel slag in cementitious materials hardened body and shorten the setting time. From SEM photos and X-ray diffraction patterns of cementitious materials, it can be seen that the hydration products of cementitious materials are mainly calcium sulfate dihydrate crystals, C-S-H gel, AFt, AFm and so on. Research indicated that 8 activator (cement clinker dosage of 2%, water glass dosage of 0.2%, aluminum sulfate dosage of 3%, triethanolamine dosage of 0.8%) can effectively improve the phenomenon of pan-cream in the surface and enhance the mechanical performance of cementitious materials. Conservation method is an important factor in the performance of cementitious materials after the addition of activators. The test analysis concludes that the best way for conservation of cementitious materials is nature conservancy (Temperature:20±2℃, Humidity:40%-50%). As rate of hardening of FGD gypsum is very quick and the admixture of activator can shorten the setting time of cementitious materials. Hence, we add retarders such as sodium tripolyphosphate, tartaric acid, citric acid to prolong the setting time. We found that all these three kinds of retarders could moderate setting time in different degree, but all have negative impact on the mechanical performance of cementitious materials. Citric acid, tartaric acid combined with calcium ions in the liquid phase to form complex compound, which covers in the surface of embryos of dihydrate gypsum, reduces the surface activity of nuclei, increases the nucleation barrier, reduces the number of crystal nucleation of dihydrate gypsum and delayes hydration of cementitious material further. Sodium tripolyphosphate reacts with gypsum to form a precipitation membrane that is deposited in the surface of crystal nucleus of semi-hydrated gypsum and dihydrate gypsum to hinder the dissolution of semi-hydrated gypsum and to delay the formation of nuclei.Appropriate amount of water reducer may enhance water-reducing rate, but water-reducing effect is no longer significant and the mechanical performance decreased when excessive water reducing agent is added. The water-reducing effect of polycarboxylate superplasticizer is better than that of FDN. The water-reducing effect of sika viscocrete is mainly achieved by electrostatic repulsion effect and steric effect, what's more, the water-reducing effect of FDN is carried out by electrostatic repulsion effect.Water retention agent generally reduces the early strength of cementitious materials, increases the needle-like crystals in the hardened body and extends the time of hydration. The water retention effect of carboxymethyl cellulose is worse than that of the methyl cellulose. The influence of methyl cellulose on the performance of cementitious materials is reflected in thickening, viscosity, water conservation and strengthening. The effect of cellulose glycolate represent in retardation and water holding.