Effect Of Fly Ash On Rehydration Of Low Water-Cement Ratio Cement-Based Materials

There is a large amount of unhydrated cementitious materials inside cement-based materials with a low water-binder ratio.When it is at service in an aqueous environment,the externally infiltrated water will react with the unhydrated cementitious materials,and the reaction is also called the rehydration reaction.The rehydration reaction has a significant effect on the macroscopic and microscopic properties of concrete materials and needs to be studied.In this paper,the effects of factors such as fly ash content,fly ash fineness and rehydration temperature were considered.The material strength,hydration degree,microstructure,volume stability and mass changes were chosen as the evaluation indexes,and property evolution laws were analyzed combining the macroscopic and microscopic tests under rehydration effect.The main work and conclusions of the paper were as follows:(1)Based on the cement hydration kinetics model,considering the binding effect of hydration products on unhydrated cementitious materials,the established rehydration model of pure cement system accurately simulate the change of the rehydration degree of cement paste samples with different water-cement ratio ait different rehydration time.In analog with cement hydration process,the rehydration model of the fly ash-cement system was established by considering the interaction between fly ash particles and cement particles.The simulated values of rehydration degree were in good agreement with the experimental values.(2)The evolution laws of hydration degree and mechanical properties of fly ash-cement pastes with different fly ash contents(0%,15%,30%and 45%)under the rehydration condition of 60? water bath were studied.The total hydration degree of the paste specimens with different fly ash contents increased with the increase of rehydration time,and the rehydration degree decreased with the increase of fly ash content.There were obvious differences between change laws of in the flexural and change laws of compressive strength changes for paste specimens with different fly ash content under rehydration effect.The change law of flexural strength could be divided into three stages of strengthening-damage-repair,while the change law of compressive strength could be divided into two categories according to the different fly ash contents:the compressive strength change was divided into three stages:strengthening-damage-repair when the fly ash content was less than or equal to 30%,while the change in compressive strength was mainly the strengthening phase when the content was 45%.(3)The effects of temperature and fly ash fineness on rehydration were studied.The changes of microstructure and pore struture of the fly ash-cement paste were analyzed by scanning electron microscope and mercury intrusion test The mechanism of the rehydration effect on strength changes was illuminated.Under the rehydration environment of 20? 40? and 60? water bath,the strength change of the pastes with 30%fly ash content was divided into three stages:strengthening-damage-repairing.The rehydration degree increased with the increase of temperature at the same rehydration time.The strength change rate of the fly ash-cement paste specimens prepared by high-fineness fly ash was lower than that of the low-fineness.The rehydration effect refined the internal pore structure of the specimen,meanwhile enhanced the weak interface between the fly ash and the rehydration product,and thereby improved its strength with the rehydration effect manifesting as strengthening or repairing.However,the volume of the rehydrated product expanded,and it would coarsen the pore structure or enen cause matrix microcrack,and furthermore it would weaken the strength with the rehydration effect manifesting as damage when there was not enough space to accommodate the rehydration product.(4)The uniaxial preloading test was carried out on high performance concrete twice,and the rehydration test was carried out on the preloaded damaged concrete.The change laws of the mass,volume stability and strength of the specimen under rehydration effect were analyzed.The results showed that the pre-loaded damaged concrete cracks obviously healed and the mass increased continuously with the increase of rehydration time.Rehydration had a repairing effect on the strength of preloaded damaged concrete,and the incorporation of fly ash could enhance the repairing of rehydration.