Study On The Mechanism Of Type And Dosage Of Superplasticizers Increasing The Early Shrinkage Of Cement-based Materials

With the development of modern concrete technology and the demand of pumping construction,admixtures are added to concrete as the "fifth group" to enhance its properties.Superplasticizers(SPs)have become an essential component of concrete by its advantages of improving concrete workability,mechanical properties,and durability of the concrete.However,the addition of superplasticizers will significantly increase the early shrinkage of concrete,resulting in the occurrence of early cracks,seriously affecting the durability and safety of concrete.The early shrinkage of cement-based materials increased by the addition of superplasticizers has been widely concerned by experts and scholars in the past ten years,but its mechanism is still unclear.To further study the mechanism of the effects of SPs on the early-age shrinkage of cement paste,an experimental investigation was carried out to determine the influence of the type and dosage of SPs on water loss,hydration degree,elastic modulus,and pore size distribution(PSD).Three types of SPs-polycarboxylate-based,naphthalene-based,and aliphatic-based-were used to prepare cement pastes that were employed to measure the drying shrinkage.The early-age shrinkage was investigated by self-made equipment.The results indicated that the addition of SPs increased the early-age drying shrinkage of cement paste considerably,and drying shrinkage increased with the dosage of SPs.Further,the drying shrinkage varied with the type of SP.However,the water loss of the cement pastes during the period of shrinkage measurements did not change significantly when different types of SPs were added.Furthermore,the addition of SP did not significantly influence hydration degree.The calculation results showed that the change in the elastic modulus was not consistent with the rule of drying shrinkage,which means that the change in the elastic modulus is not the main reason for the increase in the drying shrinkage of cement pastes with SPs.The PSD results showed that the addition of SPs refined the pore size distribution of cement paste.However,the existing works of mechanism analysis on early-age drying shrinkage of cement pastes with SPs are limited since they use a single influencing factor.We introduce a parameter rs(the radius of the pores where the meniscus forms),which is composed of two factors,i.e.,water loss and cumulative pore size distribution(PSD),to provide a better explanation of the influence of SPs on early-age drying shrinkage.In addition,this paper also carried on the study on the early-age internal humidity of cement paste mixed with polycarboxylate-based superplasticizer(PC).Through the experimental study of early internal humidity,it was found that the addition of superplasticizer reduced the early internal humidity in 72h;A shrinkage model was introduced to quantify the relationship between rs,internal humidity,hydration degree,elastic modulus and other influencing factors and early drying shrinkage.Finally,on the basis of the existing research,the author provides the idea for further research.