Study On Drying Shrinkage Of Alkali Activated Fly Ash/Slag

Alkali activated materials?AAM?are a class of green building materials with great potential in environmental energy efficiency?i.e.low CO₂ emissions,no calcined raw materials and no grinding of clinker?.However,its large-scale engineering application is still at an exploratory stage.One of the important obstacles that limit the application of AAM is it's large shrinkage,and the cracks generated by the shrinkage accelerate the intrusion of various harmful substances?such as chloride ions,carbon dioxide,and acid-base ions?in the external environment.This leads to a decrease in the strength and rigidity of the materials,which endangers the durability and service life of the materials.Therefore,correctly understanding the drying shrinkage of alkali activated materials and improving their volume stability is one of the problems that need to be solved before the alkali activated materials can be put into large-scale industrial production and applications.In this paper,alkali activated fly ash/slag blended system was studied.Firstly,effects of different raw materials and mixture ratios?fly ash content:0,30%,50%,70%;the modulus of activator:0,1,1.5,2;Na₂O content:4%,5%,6%;water to powder mass ratio:0.35,0.4,0.45?on the setting time and mechanical properties of alkali activated fly ash/slag blended system were studied,and a series of suitable mix proportions were obtained.On this basis,effects of different raw materials and mixture ratios on the drying shrinkage and the weight loss of alkali activated fly ash/slag pastes during the drying process were studied.The free water content,the progress of reaction and pore structure of alkali activated fly ash/slag were investigated using isothermal calorimeter,mercury porosimeter and nitrogen adsorption instrument,respectively.Based on the above experimental results,the mechanism of drying shrinakage of alkali activated fly ash/slag was investigated from the aspects of macroscopic properties,the degree of reaction and microstructure,respectively.The results showed that the setting time of alkali activated fly ash/slag increased as the increase of fly ash content or the decrease of the modulus of activator or the decrease of Na₂O content or the increase of water to powder masst ratio.The compressive strength of alkali activated fly ash/slag decreased as the increase of fly ash content or the decrease of Na₂O content or the increase of water to powder mass ratio or the decrease of the modulus of activator.The reaction rate curve of alkali activated fly ash/slag was similar to that of cement.,since both of them had the first exothermic peak and second exothermic peak.The cumulative heat release decreased as the increase of fly ash content or the decrease of Na₂O content or the decrease of water to powder mass ratio.Effects of water to powder mass ratio on the cumulative heat release in alkali activated fly ash/slag were different from those of cement systems.When a certain reaction factor changed,there was a linear relationship between the 7-day compressive strength and the 7-day cumulative heat release in alkali activated fly ash/slag.The drying shrinkage of alkali activated fly ash/slag increased as the increase of fly ash content or the increase of the modulus of activator or the increase of water to power mass ratio,respectively.When the activator modulus was not zero and constant,the drying shrinkage of alkali activated fly ash/slag increased as the increase of Na₂O content.When the activator modulus was zero?SiO₂ content was 0?,there was no influence on the drying shrinkage of fly ash/slag with the increase of Na₂O content.When the SiO₂ content was not zero and constant?eg SiO₂ content=5.81%?,the drying shrinkage of alkali activated fly ash/slag decreased as the increase of Na₂O content.Increasing the amount of fly ash,the reactivity of alkali activated fly ash/slag was reduced.This led to the free water content increase and the chemically bound water content decrease.The modulus of the activator increased,the free water content and the chemically bound water content of the alkali activated fly ash/slag decreased.The Na₂O content increased,the free water content of the alkali activated fly ash/slag decreased and the chemically bound water content decreased.The water to powder mass ratio increased,the free water content of alkali activated fly ash/slag increased and the chemically bound water content decreased.Through pore structure tests,it was found that the amount of mesopores of alkali activated fly ash/slag increased as the increase of fly ash content or the increase of the modulus of activator or the increase of water to powder mass ratio.When the Na₂O content increased,the number of mesopores decreased in the alkali activated fly ash/slag.Through statistical analysis,within our investigation conditions,there was no linear relationship between free water content or the degree of reaction or the development of strength and the dry shrinkage of alkali activated fly ash/slag.The mesopore volume and weight loss during the drying process were important factors affecting the drying shrinkage of alkali activated fly ash/slag.The mesopore volume had the most significant effect on the drying shrinkage of alkali activated fly ash/slag.When the fly ash content or the modulus of activator or the water to powder mass ratio changed,the mesopore volume was positively correlated with the drying shrinkage of the alkali activated fly ash/slag.When the Na₂O content changed,the mesopore volume was negatively correlated with the drying shrinkage of alkali activated fly ash/slag.In fact,the drying shrinkage is also related to the shape and connectivity of pores.The relationship bewteen the mesopore volume and the drying shrinkage in alkali activated fly ash/slag with different raw materials and mixture ratios was different.The reason may be the significant change of microstructure due to the change Na₂O content.It was found that the shape and connectivity of pores might change significantly through nitrogen adsorption test,when Na₂O content changed.Namely,the drying shrinkage was related to the mesopore volume well as the shape and connectivity of pores.