Early Hydration And Shrinkage Of Alkali-activated Slag/Fly Ash Blend Cement

Alkali-activated cement has earlier and higher strengths, lower hydration heat,higher resistance to acid and alkali corrosion, chlorine ion corrosion resistance andimpermeability frost. However, it presents many problems such as rapid setting, highshrinkage which leads to the microcracks and the possibility of alkali-aggregatereactions as well. Among them, the crack caused by the high shrinkage will do harmto the durability. When fly ash added as gel binder, the hydration development andshrinkage deformation of alkali-activated materials will be quite different. Whenunder good design of the component ratio, the costs of this kind of materials can bereduced without reducing their intensity. What’s more, the quality of molding can beimproved and conducive to the formation of the hydration products. However, thehydration process, hydration products and shrinkage are still not comprehensive.This paper aims to research the factors of hydration and shrinkage of systems ofMe2O–MeO–Me2O3–SiO2–H2O and Me2O–Me2O3–SiO2–H2O. Under the action ofalkaline agent, fly ash and slag play a synergistic effect mechanism to establish themicro development model. It has a great significant to the research on the intelligencedevelopment of alkali-activated materials. This paper researches the early hydrationand shrinkage of the mixed gel system of alkali-activated slag/fly ash blend cementwith different material composition proportions, dosage of sodium silicate modulusand temperature. By calculating hydration degree, the hydration performance andactivation energy of alkali-activated slag/fly ash blend cement are researched as well.The autogenous shrinkage, drying shrinkage and factors of alkali-activated slag/flyash blend cement are investigated. The results show that:Rate of hydration heat during different hydration stages of alkali-activatedslag/fly ash blend cement was quite different. At early stage, silicon and calcium ionsdissolved gradually out from slag and fly ash, and then hydrolysis of sodium silicateas Si (OH)4and NaOH. Within0.5~6h, the first hydration exothermic peak appearedand the higher temperature, the shorter the time span during the early hydration. Thenwithin1~12h, the second hydration exothermic peak appeared. With the samehydration temperature, the heat evolution rate sped up with the increase in dosage ofslag and the decrease in the modulus of sodium silicate. The apparent activationenergy also varied with the hydration condition. When sodium hydroxide as the activate agent, the value of autogenous shrinkagewas relatively small. When activated with the waterglass, the autogenous shrinkage ofalkali-activated slag/fly ash blend cement was much larger.The drying shrinkage of alkali-activated slag/fly ash blend cement had a linearrelation with the mass loss in the same period. When activated with sodium hydroxide,the slope of line was greater when the slag content increases.The rate of dry shrinkage decreased with the increase of slag content.Oxyalkylene alcohols based shrinkage-reducing agent could reduce the shrinkage andthe effect was obvious. While the reduction effect of magnesia expansive agent was alittle poorer.