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Plastic shrinkage cracking propagating at early ages inevitably impairs the performance of concrete structures. To mitigate this, using a recently developed shrinkage-reducing polycarboxylate superplasticiser (SR-PCA) with outstanding shrinkage-reducing and water-reducing effectiveness can be a promising approach. The present work aims to investigate the effect of SR-PCA on plastic shrinkage cracking and to clarify the underlying mechanisms. For comparison, a traditional polyether type shrinkage reducing admixture (SRA) and a polycarboxylate superplasticiser (PCA) were also used. Based on the investigations on horizontal shrinkage, settlement, bleeding, evaporation mass loss, cement hydration, surface tension of pore fluid and capillary pressure, the underlying working mechanisms of SR-PCA were identified. The results indicate that SR-PCA is able to reduce the crack area, maximum crack width and average crack width by up to 55, 48 and 50%, respectively. Furthermore, prolonging the bleeding duration and initial time of capillary pressure build-up caused by retarding the cement hydration, lowering the evaporation mass loss, and a lower development rate of capillary stresses and capillary pressure peak value induced by decreasing the surface tension of pore fluid are responsible for preventing the plastic shrinkage cracking of mortars containing SR-PCA. [ABSTRACT FROM AUTHOR]

• The influence law of clay content on properties related to plastic shrinkage cracking is revealed. • The addition of clay reduces the permeability of mortar. • The inclusion of clay decreases the plastic tension strength. • The interaction between clay and PCE keeps the surface tension of the pore solution the same at consistent fluidity. • The action mechanism of clay in exacerbating plastic shrinkage cracking is elucidated. Clay minerals in manufactured sand exacerbate plastic shrinkage cracking of cementitious materials. The impact of clay content on plastic shrinkage cracking and the underlying mechanism were comprehensively investigated. The results indicate that clay considerably mitigates the plastic settlement while significantly aggravates the plastic shrinkage, triggering more severe cracking. Water absorption of clay substantially lowers the permeability of mortars, which accounts for the reduction in bleeding, evaporation and plastic settlement. The intensified plastic shrinkage can be attributed to the accelerated build-up of capillary pressure and the self-contraction of clay. Furthermore, the critical cause of the more severe cracking with higher clay content is the rapid increase of plastic shrinkage and the reduction in plastic tensile strength. [ABSTRACT FROM AUTHOR]

• A novel SR-PCA is prepared via free-radical copolymerization. • The retardation effect of SR-PCA on cement hydration is studied. • The adsorption and dispersion behaviors of SR-PCA in cement paste are studied. • The shrinkage-reducing mechanism of SR-PCA in cement paste is studied. The incorporation of polycarboxylate superplasticizers may increase the risk of shrinkage cracking of cement-based materials. Co-adding shrinkage-reducing admixtures (SRAs) may decrease the compressive strength or arise the incompatibility problem. Designing a novel superplasticizer with excellent shrinkage-reducing function by modifying molecular structures of polycarboxylate superplasticizers can be a promising approach. This study aims to synthesize a novel shrinkage-reducing polycarboxylate superplasticizer (SR-PCA) possessing shrinkage-reducing and water-reducing functions and to investigate its performance and working mechanisms in cementitious systems. The results indicate that SR-PCA improves the fluidity of cement paste and has a distinguished fluidity retention capability. SR-PCA prolongs the cement hydration induction period, hinders the nucleation and growth of C-S-H in the acceleration period, while enhances hydration degree and refines the pore structure at later ages. Moreover, it can also be deduced that most PCA adsorbs on the cement surfaces, however, SR-PCA is mainly in the pore solution. The shrinkage-reducing mechanism of SR-PCA in cement-based materials is attributed to lowering the surface tension of the pore solution, due to the presence of the unreacted esterification macromonomers (AA-C4-PE-600) and the unadsorbed SR-PCA molecular. [ABSTRACT FROM AUTHOR]