Research On The Micro-cracks Induction And Self-healing Mechanism By Fly Ash In The Cement Paste

Modern high strength cement materials have some striking features like lowerwater/cement ratio, bigger cement fineness, shorter interval time between initialsetting time and final setting time of cement paste. However, with a rapid speed offormation of cement paste structure, unequal distribution of internal stress hasbecome an outstanding problem, native defects such as cracks are easily appear incement paste microstructure, and the properties and durability of materials wereaffected. This paper is financially supported by National Natural Science Foundationof China. According to the principle of microscopic mechanics and cementitiousmaterials, fly ash with smooth surface, material hard, low degree of early activity, andhigh degree of late activity was used to as induction medium. With the interaction ofinduction medium and cement paste, interal structure of cement paste was adjustedand cracks were induced to a place around of induction medium, and then the crackswere self-healed in-situ in the late stage.The effects of fly ash including mixing amount, partical size and water/cementratio on the formation in time and development of cement paste microstructure andcracks by a penetration test device independent designed and manufactured werestudied. Results indicated that the water absorption by crack increased with theincreased volume fraction of fly ash, fly ash particle size and water-cement ratio, andwater absorption firstly increased rapidly and then increased linearly, which wasconsistent with the development trend of cement paste microstructure. Thedevelopment of cracks in the cement paste under restraint was observed. The cracksappeared firstly where the restraint located, and crossed with other cracks after afurther development, and the cracks with considerable length and width were formedfinally. The fly ash was proved to have an early inductive effect and later stage repairmechanism, with the microstructure observation by macroscopically mechanicalproperty and SEM.The self-healing mechanism of cement paste by introducing fly ash was explored.The self-healing of fly ash in the cement paste was characterized by analyzing theeffect of water/cement ratio, volume fraction of fly ash, activators, curing system et al. on the restoration ratio of compressive strength of cement paste. Results indicatedthat both of the restoration ratio of compressive strength and the self-healing effectincreased with the increased water-cement ratio and volume fraction of fly ash. Theactivators promoted the early hydration of fly ash and weakened the later hydration,thus decreased the restoration ratio of compressive strength. The self-healing effect offly ash based cement paste cured in the water was lower than that cured under astandard condition, which was consistent with the testing result by XRD, TG-DTGand the SEM by using a BSE detector.Crack healing technology of cement matrix was studied based on internal curingeffect. The internal curing agent SAP (Superabsorbent Polymer) immersed in water inadvance was used for internal curing of cement matrix. Results indicated that basedon the hydration effect of cement at late stage, which could provide crack healing, theintroduce of SAP could provide enough water for pozzolanic effect of fly ash and wasbeneficial to create good hydration environment, thus leading to the increase ofcompressive strength recovery rate of samples compared with that without internalcuring independent of water-binder ratio and fly ash content. Meanwhile, therestoration ratio of compressive strength of cement paste increased with theincreasing of content of fly ash, indicating that internal curing effect could improvethe crack self-healing of cement matrix effectively.