Study On Correlations Of Hydration Properties Of Early-age Cementitious Materials

In recent years,with the advancement of construction level led the concrete material develops toward high-strength,high-performance direction.However,this also leads to a common cracking phenomenon caused by early shrinkage of high performance concrete.Cracks of concrete will reduce the performance,security,durability,thus shorten the service life of concrete.To optimize concrete mix ratio,control the early shrinkage cracking of high performance concrete and improve its durability,the need for in-depth study of the early shrinkage mechanism of high performance concrete is necessary.Electrical performance is the basic properties of materials;hence,it is available to use the resistivity method to predict the early autogenous shrinkage of cementitious materials through the study of electrical properties and combined with the property of autogenous shrinkage.Based on electrical resistivity method,this paper mainly tested the shrinkage and resistivity developments of cementitious materials with different water-cement ratios and aggregate contents,the mechanisms of different hydration properties and the correlations between them were researched.The main contents and conclusions can be summarized as follows.Measured the chemical shrinkage(CS),autogenous shrinkage(AS)and electrical resistivity(ER)in 7 days of 4 cement pastes specimens which the water-cement ratios(W/C)are 0.3,0.35,0.4,and 0.45.Results show that the CS of unit mass cement increases with W/C,and the AS and the ER decrease with W/C.The specimens of 0.3and 0.35 W/C show consistently shrinkage in 7d,while the specimens of 0.4 and 0.45 W/C show a temporal expansion in 7.7h and16.8h,respectively.The early-age volume changes of cement pastes are mainly controlled by the AS development,compressive strength development(restricting shrinkage)and thermal expansion.The differential curves of ER of sample P0.35 shows three peaks,which stand for the ER value starts to increase from decrease,the AFt transforms to AFm,and the fastigium of hydration reaction ends,respectively.The hydration process of the cement pastes can be divided into four distinct stages: dissolution period,induction period,hydration acceleration period and hydration deceleration period by ER curves and the differential curves of ER.The relation between autogenous shrinkage and chemical shrinkage shows that the AS value and CS value differ by two orders of magnitude,since there are small quantities of internal pores caused by CS could produce AS which can affects the macroscopically volume of cement paste.Meanwhile,the AS/CS values were merely affected by age,but strongly affected by W/C.Therefore,the AS/CS values increase with the W/C decrease.The equation between AS(?)and ER(?): ?=aeb? shows that the AS value manifests a exponential function movement with respect to ER value,furthermore,the value of a decreases and the value of b increases with W/C increases,which indicates that there is a feasibility to predict the early-age autogenous shrinkage of cement pastes used by electrical resistivity method.Tested the ER changes and AS changes of concrete specimens of 0?10%?30%? 50% aggregate volume fractions and 0.4 W/C in 7 days.Results show that the aggregate incorporation leads to increase of ER;the differential curves of ER manifest the ER of higher aggregate content concrete decreases faster during dissolution stage and increases faster during later stages;the total ER value of 7d has positively exponential correlation with aggregate content.The incorporation of aggregate results in lower AS;the inhibition effect on AS of aggregate in 1d is much obvious when the aggregate volume fraction is greater than 30%.The AS value also changes exponentially with respect to ER value of concrete,which is a demonstration of the prediction of the early-age autogenous shrinkage by electrical resistivity method of cementitious materials.Measured AS and ER changes in 7 days of concrete with aggregate sizes of 5-10 mm and 10-20 mm,the volume fractions of aggregate are 10%?30% and 50%,respectively.Meanwhile,fixed the aggregate content to be 30%,the interfacial transition zone(ITZ)thickness of different aggregate sizes' concrete of 7 days' age was tested by micro hardness instrument and the volume fraction of ITZ was calculated.Results show that the thickness of ITZ around a single aggregate in concrete with different aggregate sizes is both 0.05 mm.When aggregate content is consistent,the volume fraction of ITZ increases with the decrease of the size of aggregate,and the larger the volume fraction of ITZ,the smaller the resistivity and the greater the autogenous shrinkage of the concrete.However,with the increase of the aggregate content,the difference of volume fraction of ITZ in concrete with different aggregate size diminishes,thus the impact on hydration properties reduces.