Influence And Mechanism Of Polyether Amphiphiles On The Properties Of Cement-Based Materials

Shrinkage deformation caused by water dissipation is one of the main causes of cracking in cement-based materials.Shrinkage reducing agents containing polyether amphiphiles as one of the main components can be uniformly dispersed into cement-based materials with the mixing water,which can mitigate shrinkage and have good application prospects.As for the shrinkage reducing mechanism,although it is widely accepted in the academic community that shrinkage reducing agents reduce shrinkage by lowering the surface tension of the capillary pore solution,not all substances that can lower surface tension have shrinkage reducing effects,and the above-mentioned shrinkage reduction mechanism needs to be improved.This study focuses on small-molecule polyether amphiphiles(such as diethylene glycol monobutyl ether,polyethylene glycol,etc.)and large-molecule polyether amphiphiles(long straight-chain fluoropolyethers,long straight-chain block polyethers,T-structure polyether-modified silicone oil)as the main research objects.By comparing the surface activity,contact properties,and their effects on the performance of cement paste,combined with dynamic light scattering(DLS),transmission electron microscopy(TEM),and mercury intrusion porosimetry(MIP)and other micro-detection methods,the shrinkage mechanism was explored and improved,and an effective method for selecting shrinkage components was summarized based on this.The main research results are as follows:(1)Polyether amphiphiles have a low degree of adsorption on cement particles,and have little effect on the workability of cement paste.They can reduce the surface tension of the pore solution of cement paste and do not significantly change the contact properties between the pore solution of cement paste and the pore wall,which is in line with the basic criteria for selecting shrinkage reducing agents and the assumption of capillary tension theory.Small molecule polyether amphiphiles do not form stable micelles in the pore solution,while long straight-chain fluorinated polyether,long straight-chain block copolymer polyether,and Tstructure polyether modified silicone oil have extremely low critical micelle concentration(CMC)and can self-assemble into stable micelles with a diameter of more than 10 nm in the pore solution.(2)The ability of small molecule polyether amphiphiles to reduce surface tension is positively correlated with their shrinkage reduction effect,which is in line with capillary tension theory.However,the ability of long straight-chain block copolymer polyether and Tstructure polyether modified silicone oil to reduce surface tension is weakly related to their shrinkage reduction effect,indicating that reducing surface tension is not the dominant factor in shrinkage reduction.(3)The mechanism of micelles affecting the shrinkage reduction effect of polyether amphiphiles is proposed and verified.Large micelle particles can block capillaries that affect shrinkage,hinder the effective spreading of monomers of polyether amphiphiles on the surface of the pore solution,reduce the effective concentration of polyether amphiphiles in the pore solution,and cause the actual surface tension of the pore solution to be higher than the test value,with the capillary additional pressure maintained at a higher level.Therefore,the shrinkage reduction effect does not comply with the inference of capillary tension theory.The larger the micelle particle size,the higher the degree of overlap between the particle size distribution and the capillaries with a diameter of 3-50 nm,and the more micelles are formed,the greater the weakening effect on the shrinkage reduction effect.(4)Based on the above mechanism,an effective method for selecting polyether amphiphiles as shrinkage reducing agents for cement-based materials is proposed:(1)stable dispersion in cement paste solution without precipitation;(2)low degree of adsorption on cement particles,without drastic changes in adsorption properties with varying concentrations;(3)low tendency to form micelles(or high CMC),or the ability to form small-sized micelles(at low CMC),with significant reduction in the surface tension of the pore solution.