Effect Of Shrinkage-reducing Polycarboxylate Superplasticizer On Shrinkage Cracking Of Cement-based Materials And Its Mechanism

As a quasi-brittle material,modern concrete has severe shrinkage and cracking problems when applied in engineering construction.Shrinkage reducing admixture(SRA)is a typical chemical shrinkage reducing admixture to control shrinkage deformation of concrete.However,its application in concrete is limited because of its large dosage,high unit price,adverse effect on strength development,poor adaptability with water-reducing admixtures,etc.Therefore,considering the designable molecular structure of polycarboxylate superplasticizer and the similarity in molecular structure between the polyether side chain of polycarboxylate superplasticizer and low molecular polyether shrinkage-reducing admixture,the researchers graft alkyl polyether with shrinkage function into the main copolymer chain using the modern molecular tailoring technology and develop a new type of polymer shrinkage reducing material-shrinkage-reducing polycarboxylate superplasticizer.Shrinkage-reducing polycarboxylate superplasticizer is a simple,economical and effective way to control concrete cracks.It is urgent to explore its action mechanism in cement-based materials.The effect of a novel shrinkage-reducing polycarboxylate superplasticizer(SR-PCA)on the shrinkage and cracking of cement-based materials and its working mechanisms are investigated in this study.Firstly,its impact on the composition,structure and morphology of the main hydration product calcium-silicate-hydrate(C-S-H)gel are discussed.The interaction mechanism between SR-PCA and cement-based materials at the nanoscale is disclosed.Subsequently,the effect and underlying mechanism of SR-PCA on the water-reducing and strength-enhancing in cement-based materials are studied,which supports the further application of SR-PCA in the field.In addition,the effect of SR-PCA on the capillary pressure and pore solution characteristics is revealed,the significance of which on the shrinkage mitigation in plastic and hardened cement-based materials are clarified.It provides a reliable theoretical basis for the development and utilization of shrinkage-reducing polycarboxylate superplasticizers.The main conclusions of this study are as follows:(1)The effects of SR-PCA and polycarboxylate superplasticizer(PCA)on the composition,structure and morphology of C-S-H are studied,which reveals their working mechanism in cement-based materials at the nanoscale.The results show that SR-PCA increases the proportion of micropores(pore size<2 nm),which improves the durability of cement-based materials.Meanwhile,it decreases the proportion of mesopores(pore size in the range of 2 to 50 nm),mitigating shrinkage deformation of cement-based materials.Additionally,SR-PCA and PCA delay the C-S-H growth and the conversion to aluminum foil morphology.Also,both can adsorb at the C-S-H surface,where SR-PCA has a wilder adsorption effect.By molecular dynamics simulation,it can be found that the reduction of interfacial Ca-O bond strength between SR-PCA and C-S-H leads to the difference in adsorption capacity between SR-PCA and PCA.(2)The influence of SR-PCA on the fluidity and mechanical properties of cementbased materials is studied,and the existing form of SR-PCA in cement-based materials and the underlying mechanism of SR-PCA on the water-reducing and strengthenhancing in cement-based materials are disclosed by exploring its adsorption and dispersion behavior,hydration process and micro structure evolution of cement-based materials.The results show that SR-PCA significantly improves the fluidity of cement pastes and has an excellent fluidity retention ability.Compared with PCA,SR-PCA has a lower density of the main chain polycarboxylic acid group and a stronger hydrophobicity of the side chain.Thus,only about 20%of SR-PCA is adsorbed on the cement particle surfaces to induce dispersion,while the unadsorbed SR-PCA is dissolved in pore solution to reduce the surface tension.In addition,the early strength of cement-based materials is decreased slightly due to the retardation effect of SR-PCA on cement hydration.However,SR-PCA significantly improves the hydration degree of cement-based materials at 28 days and refines the pores,increasing the 28d strength by 17.8%.(3)The influence of SR-PCA on plastic shrinkage cracking,evaporation and bleeding of water and capillary pressure of mortar is investigated,and its shrinkagereducing mechanism in cement-based materials is revealed.The imaging analysis shows that the cracking area,maximum cracking width and mean cracking width are reduced by 55%,48%and 50%,respectively.In the induction period of capillary pressure,SR-PCA increases the bleeding water mass and reduces the water evaporation loss of mortar,which delays the induction period of capillary pressure by 2h.During the development period of capillary pressure,the menisci are formed on the mortar surface,capillary pressure,viscosity of pore solution,permeability of solid skeleton and water evaporation mass can establish the relationship as Js=D/ηL·P=mw/ρw.It indicates that SR-PCA can reduce the surface tension and water evaporation,and enhance the viscosity of pore solution,thereby reducing the capillary pressure and plastic shrinkage cracking of mortar.(4)The effect of SR-PCA on drying shrinkage and autogenous shrinkage,together with its shrinkage-reducing mechanisms of pure cement system and the ternary cementitious material system is investigated.The results show that SR-PCA significantly reduces the drying shrinkage and autogenous shrinkage of pure cement paste and ternary cementitious material system.According to the factorial design method results,the most significant shrinkage-reducing effect of SR-PCA is observed in the system with 50%fly ash,whereas the shrinkage-reducing effect of SR-PCA is the weakest in the system with 50%slag.Accordingly,it is better to lower the slag content to ensure volume stability when designing the cement-fly ash-slag ternary cementitious systems,regardless of whether SR-PCA is incorporated or not.SR-PCA delays the cement hydration,and reduces the relative humidity depression and capillary pressure,thus decreasing the autogenous shrinkage and drying shrinkage.SR-PCA controls the drying shrinkage by lowering the hydrous compressibility factor(CF)and the volume fraction of the mesopore.Meanwhile,it can improve the expansion stress and decrease the capillary pressure,synergically restricting the autogenous shrinkage evolution.In addition,based on a hygro-thermo-chemo-mechanical multi-field coupling model,the cracking risk of the T-shape concrete slab of an expressway is reduced significantly by adding SR-PCA.