| Cement-based materials with low water-cement ratio/water-binder ratio has a dense structure,high strength and excellent durability.Therefore,it plays a significant role in national defense,nuclear power,marine engineering and other special projects.Cement-based materials with low water-cement ratio usually contain a large amount of unhydrated cement particles.External water enters the interior of the matrix and rehydrates with unhydrated cement,which would lead to the cracking of hardened cement paste and influence the long-term performance of the cement-based materials.The influence of rehydration on the properties of Cement-based materials with low water-cement ratios was studied,considering the influence of water-cement ratios,temperature and curing environment in this paper.The influence mechanism of rehydration was analyzed,and then the rehydration model was established through hydration degree,X-ray diffraction,and pore structure distribution.The main works and conclusions of the paper were as follows:(1)The variation laws of mechanical properties of cement-based materials with different water-cement ratio under 30? water soaking condition were studied.And the mechanism was analyzed based on the hydration degree and the changes of pore structure.The hydration degree of specimens with water-cement ratios of 0.15,0.20,0.25,0.30,and 0.40 increased with the increase of rehydration time.As the water-cement ratio increased,the growth rate of the hydration degree increased first and then decreased,and the specimen with water-cement ratio of 0.25 had the highest growth rate of hydration degree.The highest growth rate of hydration degree of different water-cement ratio specimens was 35.82%after 360-day rehydration.The variation laws of cement-based materials strength with different water-cement ratios could be divided into three categories.The first category was those with ultra-low water cement ratio(0.15),their damage was notobvious;The second category was those with low water cement ratio(0.20,0.25),their strength had gone through three stages of repair,damage and re-repair;The cement-based materials with high water cement ratio(0.30,0.40)was the third category,they would be also damaged after long-term rehydration.During the repair phase,the porosity decreased,and the most probable pore diameter decreased,whereas the porosity increased,and the most probable pore diameter increased during the damage phase.(2)The influence of temperature and curing environment on rehydration were analyzed.Under 20?,30?,40?,60? and 80? water soaking,the strength changes of specimens with water-cement ratio 0.20 could be divided into three stages:repair,damage and re-repair.At the same rehydration time,the hydration degree of the specimens increased with the increase of soaking temperature.The recommended temperature for rehydration acceleration test was 60 ?.It was difficult for rehydration to take place under indoor natural conditions.(3)The content of C3S and C2S in unhydrated cement and AFt and CH in the hydration product during rehydration was analyzed by X-ray diffraction test.After rehydration for 180 days,reintegration intensity of C3S and C2S respectively decreased by 18.46%and 21.36%,and the CH content increased from 12.81%to 14.32%.The proportion of AFt in the hydration product was lower,and it increased first and then decreased along with rehydration.Under the effect of rehydration,the hydration degree was greatly increased.(4)Based on the assumption of independent hydration of cement clinker minerals,a single mineral rehydration model was established using the R.Krstulovic hydration kinetics theory.Rehydration rate of C3S and C2S was much higher than that of C3A and C4AF.C3S and C2S played a key role in the rehydration reaction.The single mineral rehydration model could accurately predict the theof hydration degree of C3S and C2S at different rehydration periods.Under different temperature,the rehydration reaction coefficients KD1 and KD2 of C3S and C2S satisfied the Arrhenius law. |
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