Effect Of Reactive Magnesium Oxide Expansive Agent On Selfhealing Of Cracks In Strain Hardening Cementitious Composites

Concrete is a brittle material with the disadvantage of low tensile strength and easy cracking.Microcracks are inevitable during the process of serving,and these cracks could affect the durability of structure without strictly controlled.Therefore,how to consider and reduce the impact of cracks on durability of structure in the design stage is an urgent problem.To deal with this problem,research on the self-healing of cracks has become a hot topic in recent years,which has great significance to the sustainable development of society and economy.Strain Hardening Cementitious Composites(SHCC)is a kind of green environmental material with good ability of controlling crack width.It can exhibit characteristics of strain hardening and multiple cracks when subjected to load,and its cracks can self-heal under certain condition.However,the healing process of SHCC is slow,and because of some obstacles such as lack of water supply and shortage of unhydrated cement particles,etc.,the self-healing process developes poorly.To overcome this problem,in this paper,Mg O expansive agent with the characteristics of delayed expansion was used to improve the self-healing property of SHCC.At present,the researches and applications of Mg O expansive agent in China mainly focus on compensating the shrinkage of concrete,which is based on its advantages of low water demand for hydration and stable expansion property,etc.And this advantage is also applicable to the self-healing process of cracks,but the relevant researches are relatively rare.In this paper,Mg O expansive agent with three different reactivity were mixed into SHCC.Quantitative analysis of the variation of crack width was carried out on mesoscopic scale,also combining with capillary water absorption test and microscopic test analysis,the promotion mechanism and influence of Mg O expansive agent on selfhealing cracks in SHCC were systematically analyzed.The main contents and the relevant conclusions are as follows:(1)Four different curing conditions(dry condition,water fog,tap water,and saturated calcium hydroxide solution)were selected to analyze the effect of curing condition on the self-healing of SHCC cracks with Mg O expansive agent.The results show that water is indispensable for self-healing of cracks.,in the dry condition,the cracks can not seal completely even if with the addition of Mg O expansive agent.The more water,the better the healing effect of cracks.Compared with control group without Mg O expansive agent,the curing condition of saturated Ca(OH)2 solution was not suitable for self-healing of cracks in SHCC with Mg O expansive agent.(2)Three kinds of Mg O expansive agents with different reactivity values(110s,200 s,240s)were selected,and the dosage was kept at 5%,the effect of different reactive Mg O expansive agent on the self-healing of cracks was analyzed.The results show that the self-healing ability of cracks in SHCC can be effectively improved by adding reactive Mg O expansive agent,and this result has been more obvious under water fog condition.Under the same curing condition,the relationship between the reactivity of the Mg O expansive agent and the promotion of crack healing is showing a trend of SHCC-CON<MEA-240s<MEA-200s<MEA-110 s.The Mg O expansive agent with reactivity value of 110 s shows the most promising effect for crack healing due to the relatively higher reactivity.(3)Different reactive Mg O expansive agent were mixed into SHCC according to different dosages(5%,10%),and the effects of different dosages of Mg O expansive agent on the self-healing of cracks was analyzed.The results show that the self-healing effect of the 10% dosage was not as good as that of the 5% dosage.This is because the 0.25 water-binder ratio used in the test is very low,so the pore water content is little,which restricted the hydration of Mg O.In this case,excessive amount of the Mg O expansive agent was not conductive to the hydration of Mg O,thus reducing the selfhealing efficiency of cracks.For the Mg O expansive agent whose reactive index is 110 s,the 10% dosage was too much,leading to poor volume stability.