Study Of Dry Shrinkage Cracking Of Geopolymers Under Cold And Dry Environment In Northwest China

Geopolymer has outstanding properties in terms of green low-carbon,mechanical characteristics,corrosion resistance,and stable use at high temperatures resistance.It is regarded as a green and environmentally friendly substitute for cement,with excellent performance in early high strength and long service life.Geopolymer has been the focus of more and more research in the fields of new building materials,solid waste treatment,etc.Despite its excellent performance,geopolymer undergoes brittle fracture under external forces and is prone to shrinkage and cracking in dry environments.This leads to a reduction in mechanical performance and also allows corrosive ions to enter the specimen along the crack,reducing the impermeability and durability of the geopolymer.Against the backdrop of optimizing building material structures,improving the shrinkage cracking of geopolymer is an important factor in promoting its practical application.This article evaluates the shrinkage,cracking behavior,and mechanical properties of geopolymer specimens under different alkali activation moduli,curing conditions,and pore structure evolution.The article aims to obtain the optimum preparation process for metakaolin-base geopolymer by comparing and studying the impact of these factors on the shrinkage and cracking behavior of geopolymer specimens.The evaluation criteria include mechanical properties,dry shrinkage rate,and cracking behavior.The research results show that:(1)The modulus of the alkali activator has a significant effect on the degree of geopolymerization reaction,dry shrinkage cracking and mechanical properties of the geopolymer.The catalytic activity of the precursors differs between modulus activators.Systems with a higher degree of reaction have a higher Si-O-Si/Al content,i.e.,more sodium chloroaluminate(N-A-S-H)gel phase skeletons are formed.The formation of gels fills the pores and reduces the degradation of the specimen due to the interfacial transition zone(ITZ).The alkali activation modulus of 1.3 exhibits the highest catalytic activity for high alumina geopolymer,and the structure of geopolymer becomes denser with the increase of age.In addition,the maximum dry shrinkage rate of each sample occurs one day after demolding,and the dry shrinkage rate on that day increases with the increase modulus of the alkali activator.(2)With the formation and deposition of geopolymer gels,the geopolymerization reaction progressed to varying degrees with the curing conditions.During wet curing,the internal voids of the sample are filled with water molecules.After transfer to the Dry Shrinkage Maintenance Box,the ambient humidity decreases and a large number of water molecules escape from the specimen,resulting in a large capillary stress on the capillary water and concave liquid surface,leading to a sudden increase in shrinkage strain.This can be approximated as self-shrinkage as the sample has almost no water loss during standard maintenance.The drying shrinkage of each specimen generally showed a decreasing trend with increasing standard conditioning hours.The drying shrinkage behavior on the first day under different conditioning environments plays a major role in the drying shrinkage behavior of metakaolin-geopolymer at long ages.A small dry shrinkage peak also occurs later in some specimens,and the delayed effect is related to the geopolymerization process.The geopolymerization process is accompanied by the production of water molecules,which peak within a week,and the capillary pressure continues to increase as the produced water escapes,inducing a further dry shrinkage strain.(3)With increasing geopolymer age,the pore structure of the geopolymer under different alkali activator modules adjusts,and the peak intensity of gel pores increases while the peak intensity of capillary pores decreases.The corresponding minimum relaxation time first decreases and then increases with increasing alkali activator modulus,and the most probable pore size first decreases and then increases with increasing modulus.As the standard curing age increases,the capillary pore structure of the geological polymer changes from a small number of large pores to a large number of small pores.The pore structure of geological polymer under different curing conditions is still dominated by gel pores,accompanied by a small number of capillary pores.At 7 days,the dry shrinkage of geopolymer is positively correlated with the total porosity and the number of gel pores,but the dry shrinkage rate decreases as the number of large pores increases.This paper studies the influence of different alkali activator modulus,curing conditions and pore structure evolution on the drying shrinkage and cracking law of geopolymer specimens from a microscopic perspective,such as observing the hydration degree and pore structure of geopolymer.The best relationship obtained by evaluating the macroscopic mechanical performance and dry shrinkage cracking behaviors can provide a reference for resource development and application of geopolymer in cold and arid environments.