Formation Of Ettringite And Its Effect On Shrinkage Properties Of Alkali Slag Cement Mortar

Alkali slag cementification material is composed of grinding water quenched blast furnace slag as solid dispersed phase,with a certain amount of alkali excitation components,the production process is energy saving,waste benefit,environmental protection,and has excellent strength and durability,development and utilization in line with the economic and social sustainable development policy.But the configuration of concrete with dry shrinkage rate is significantly higher than the common concrete problem,easy to lead to the structure surface cracking and the formation of dry shrinkage cracks.A certain amount of ettringite can effectively compensate the shrinkage of ordinary concrete,and its formation is related to the alkali content of concrete.Alkali slag concrete is a kind of ultra-high alkali concrete.The study of formation and stability of ettringite in ultra-high alkali concrete is very important to develop shrinkage compensation technology suitable for alkali slag concrete.The effects of the ratio of calcium,aluminum,sulfur and alkali equivalent on the hydration products,microstructure,compressive strength and expansion properties of alkali slag were studied.The formation conditions and mechanism of ettringite in the alkaline slag cementitious material system were analyzed based on the test results of XRD and SEM.The main laws revealed are as follows:(1)Sulfate content and alkali equivalent have an important effect on the formation of ettringite in alkali slag cement system.When the water-binder ratio is 0.35,the gypsum content is in the range of 5.8-14.49%,and the sodium sulfate content is in the range of2.39-8.37%.With the increase of the content,the formation amount of ettringite in the hydration products of alkali slag cement increases.In comparison,the formation rate of ettringite is faster when gypsum is added.When other conditions are fixed,the alkali equivalent increases from 0 to 6%,and the amount of ettringite formed in alkali slag system decreases.(2)With the increase of gypsum content,the morphology of ettringite gradually changes from short and thick hexagonal column to long and thin,and becomes slender needle-stick shape.With the increase of sodium sulfate content,ettringite gradually changes from slender needle-stick shape to shorter and finer fibrous shape.With the increase of alkali equivalent,ettringite gradually becomes thinner and longer.(3)The formation of ettringite can improve the compressive strength of alkali slag cement for 3 d and 7 d,but has no obvious effect on the strength for 28 d.The unreacted gypsum or sodium sulfate has a negative effect on the compressive strength.(4)The increase of curing temperature is not conducive to the formation of ettringite in the alkali slag cement system.The formation of ettringite is not seen in the alkali slag slurry rapidly cured at 70℃-100℃.(5)The pH value of pore solution decreased with the increase of gypsum or sodium sulfate content,but increased with the increase of alkali equivalent.The lower Ca ion concentration in the pore solution with sodium sulfate and the lower Al concentration in the low alkali equivalent solution may be the controlling factors of ettringite formation.(6)When alkali slag cement mortar produces ettringite,the limited expansion rate is increased,the drying shrinkage rate is reduced,and the expansion effect is more obvious in the early hydration reaction.