Study On Hydration,Autogenous Shrinkage And Freeze-thaw Resistance Of Early-age Cement Pastes

Cement concrete is one of the most widely used construction materials in the world nowadays.The early-age performance of cementitious materials directly determines the functional performance,durability and life cycle of structure at later ages.Thus thorough study on the early-age performance of materials is needed.The rapid development of hydration reaction accelerates the compaction of pore structure as well as self-desiccation process after casting.In order to meet the requirements of construction project,low binder-to-water ratio and supplementary cementitious materials with high reactivity are often applied in the optimization of mix design,especially for high-performance concrete and high-strength concrete,which is highly recommended being used in the modern construction projects.However,these characteristics do not only mean improved strength and low permeability,but also relate to dramatic self-desiccation effect and poor volume stability.This dissertation includes a serious of investigations on the early-age hydration evolution,autogenous shrinkage deformation and freeze-thaw resistance of various cement pastes.So as far as the dissertation is concerned,main conclusions can be drawn as follows:1.Experimental technique investigation is conducted first before studying autogenous shrinkage.The corrugated tube method with eddy current induction probe can measure the whole-stage deformation after casting.However,the accuracy of testing results may be influenced by the over-elongation of tube body during the casting as well as the ratio of axial stiffness to radial stiffness,which is determined by the material of used corrugated tube.In contrast,the prism method is easier to operate and has fewer sources of experimental error,thus showing a better data discrimination and repeatability.Therefore the prism method is more recommended to measure autogenous shrinkage after hardening of cement paste.2.The effective capillary stress is the driving force of autogenous shrinkage.The ratio of effective capillary stress to compressive strength is defined as Autogenous Shrinkage Stress Ratio.Cement paste with lower water-to-cement ratio has a higher Stress Ratio and greater autogenous shrinkage.The influence of water-to-cement ratio on effective capillary stress is stronger than that on compressive strength.3.Electrical resistivity can be used to divide early-age hydration stages and characterize setting time clearly and precisely.The critical time of second peak on differential curve of electrical resistivity can be applied to predict initial setting time while the inflection point on logarithmic curve of electrical resistivity can be applied to predict final setting time.Besides,electrical resistivity can be also applied to predict autogenous shrinkage,the effective capillary stress and compressive strength.4.Due to the inert filling effect at early age and the activated pozzolanic reactivity shown at later age,slag has a slight inhibition effect on autogenous shrinkage of cement paste in the early 5 days and promotion effect after 5 days.It shows a double peaks phenomenon on the autogenous shrinkage differential curve of cement paste blended with slag.Aggregates contribute to reducing autogenous shrinkage of cement paste.The result calculated using hyperbolic model based on Pickett model shows a good agreement with experimental data.Thus this hyperbolic model can be used to quantitatively analyze the inhibiting effect of aggregate on autogenous shrinkage of cement paste.5.The experimental data from saturation degree measurement by the mass method is much higher than the theoretical calculation value of Powers’ model.So it is recommended to use Powers’ model to calculate the saturation degree of early-age cement paste which is sealed cured.Cement paste with lower water-to-cement ratio or blended with silica fume has a better freeze-thaw resistance during single freeze-thaw cycle test because it is faster for its saturation degree to drop below the critical saturation degree.On the contrary,the addition of SAP can weaken the freeze-thaw resistance of cement paste to some extent,especially for early-age cement paste with low water-to-cement ratio and later-age cement paste with high water-to-cement ratio.All the following indicators can be applied to evaluate the freeze-thaw resistance of cementitious materials at early age:residual strain in TMA test,critical degree of saturation(85%),critical freezable water content(20%～30%)and damage index(7%)based on UPV test.If it is possible,the accuracy and reliability of evaluation conclusion can be much improved when above methods are comprehensively used to study freeze-thaw performance.