| Using industrial solid wastes to produce cement material is not only an effective way to achieve their reduction, resource and harmless utilization, but also has a great economic, social and environmental significance. Electrolytic Manganese Dioxide Slag (EMDS) is a calcium sulfate-rich industrial sludge, which can be used as a sulfate activator in the production of composite cementitious material. Blast Furnace Slag(BFS) is a ironmaking waste with potential hydraulic properties and will form Calcium-Silicate Hydrate(C-S-H) under alkaline hydraulic condition. In this study, based on the characteristics of EMDS, the treated EMDS is mixed with alkali activator and BFS to obtain EMDS-alkali complex activated BFS cementitious material. The strength and activated mechanism of the cementitious material are investigated. The results obtained are as follows:XRD, SEM and EDS analysis results showed that the main components of EMDS were CaSO4·2H2O and SiO2. Calcining caused calcium sulfate crystals in EMDS transformed. Calcium sulfate formed CaSO4·0.67H2O after thermally treated at 120℃, CaSO4·0.5H2O at 250℃and anhydrous calcium sulfate over 350℃. The solubility of anhydrous calcium sulfate in treated productor was in the highest value after treated at 350-450℃and the solubility decreased after the calcination temperature over 550℃. EMDS pretreatment and mechanical milling were conducted before blending the contents of the cementitious material to enhance the activation properties. Potential hydraulic properties of BFS was better activated after EMDS milled in a ball mill for 18 min and treated at 350℃for 1h.The main contents of the cementitious material in this study were BFS, EMDS, Ca (OH)2 and clinker. EMDS played a role as sulfate activator and Ca (OH) 2 and clinker as alkali activator. The experimental results showed that Ca(OH)2 effectively activated BFS early activity which contributed to early strength development. Clinker had a heavy impact on later strength development so that a reasonable increase of clinker in 30-70% enhanced later strength. A better performance of BFS cement was achieved in 20-30% of activating agent, which the doped ratio was (20-30):(4-6):12 for EMDS:Ca(OH)2:clinker based on the orthogonal test. Increasing component fineness by ball milling also improved the strength of cement and the reasonable milling times were 0.5h for BFS and 0.4h for clinker. On the other hand, increasing the ratio of water to cement led to cement strength lost so that the ratio should be controlled in the range of 0.45-0.50. A constant temperature and humidity maintenance was an effective way for the development of strength.The hydration process of EMDS-alkali complex activated BFS cementitious material was different from PC425 cement's, which reflected a collaborative effect of alkali and sulfate activated. The results of analysis for hydration products of each stage showed that BFS was not hydrated effectively in pure water. When activated by Ca(OH)2 and clinker, BFS hydrated like PC425 cement and produced hydrated calcium silicate and a small amount of crystal Ca(OH)2. Ettringite was formed in the function of alkali and sulfate activated and its phase changed continuously in the hydration process, which did not disappear untill 28d of hydration. |
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