A Study On The Preparation And Properties Of Alkali-activated Slag Cementitious Material

Alkali-activated slag cementitious material has become a research hotspot of cementitious materials field at present, due to its excellent performances compared with cement, such as higher intensity, lower hydration heat, and higher performances of rapid hardening, corrosion resistance, frost resistance and reinforcing steel bar protection, coupled with its production of simple process, low investment, low energy consumption, little pollution, high utilization of slag and so on.Cementitious material was prepared by using blast-furnace slag as raw material, as well as sodium hydroxide, sodium carbonate, sodium sulfate, sodium silicate, and a mixture of sodium hydroxide and sodium silicate as activators. Effects of activator content, water-solid ratio, curing time, temperature, and granularity of slag powder on the properties of cementitious material such as hydration degree, density, compressive and flexural strength, setting time, stability and sulfuric acid corrosion resistance, etc. were studied.Phase composition, chemical composition and microstructure of blast-furnace slag were characterized by using XRD, XRF, SEM and TEM. Density, granularity and grinding performance of slag were studied. Also basicity factor, quality coefficient, activity coefficient, hydraulic coefficient and radiation of slag were analyzed according to the Chinese national standard documents. The results showed that the slag was mainly composed of glassy materials with a small amount of Gehlenite, which could meet the requirement of preparation of cementitious material.When sodium silicate content was 8 wt.%and water-solid ratio was 0.25, cementitious material prepared from slag specific surface area of 663.40 m2/kg, cured at a atmospheric cabinet of temperature 20±1℃and relative humidity 5±5%, developed 37.43MPa and 6.10MPa compressive and flexural strength at 3 days, 57.22MPa and 7.59MPa at 28 days. Moreover, the density was 2.31g/cm3, the initial and final setting time was 52 minutes 111 minutes respectively, the stability and sulfur resistant was relatively good.The type and content of activators affected the performance of cementitious material obviously. Cementitious material using sodium silicate as activator obtained higher 28 days compressive and flexural strength, as well as higher hydration degree, density, sulfuric acid corrosion resistance and stability compared with sodium hydroxide, sodium carbonate and sodium sulfate. Using sodium hydroxide as activator obtained a relatively shorter setting time and higher 3 days compressive and flexural strength. XRD patterns showed that the produced cementitious material was mainly calcium silicate hydrates gel (CSH gel) mixed with un-hydrated slag, which indicated that the hydration reaction of the slag was not fully completed.To the sodium silicate-activated slag cementitious material, the higher the water-solid ratio, the smaller the density, and, the longer the setting time, the lower the compressive and flexural strength.With the increase of curing time, the hydration degree, density, compressive and flexural strength of sodium silicate-activated slag cementitious material all represented enhancement respectively.High curing temperature was proved to be effective in improving the 3 days compressive strength and density of sodium silicate-activated slag cementitious material.The results also showed that, the smaller the granularity of the slag, the shorter the setting time of the sodium silicate-activated slag cementitious material. Granularity of 0.05mm-0.074mm was found to be more suitable for the compressive strength and density of cementitious materials when compared to any other chosen granularity.