Crack Self-healing Performance Of Concrete With Magnesium Oxide Expansion Agent

Concrete is a brittle material that is prone to crack during use and service.If it cannot be healed in a timely manner,it will have an impact on the safety and durability of the structure.The crack self-healing technology is one of the effective methods to solve the durability problem of cracked concrete,and has become a hot research topic at present.Magnesium oxide(MgO)expansive agent have the characteristics of mild hydration reaction,low water demand,and stable hydration product performance,and are usually used to control the shrinkage and cracking of concrete.In recent years,relevant studies have shown that MgO expansive agent have a certain promoting effect on the self-healing of concrete cracks.This article systematically studies the crack self-healing effect of concrete mixed with MgO expansive agent in freshwater and seawater environments from the perspectives of crack surface healing effect,water permeability recovery effect,and mechanical performance recovery effect.For the above two environments,further consideration was given to the effects of factors such as the activity of MgO expansive agent(M-type and R-type),dosage(5%and 10%),and initial width of cracks(0.2mm,0.4mm,and 0.6mm)on the self-healing effect of cracks.The composition and morphology of self-healing products were analyzed using micro testing methods such as X-ray diffraction analysis(XRD),thermogravimetric analysis(TG/DTG),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and micro infrared spectroscopy(FTIR).The self-healing mechanism of cracks in concrete with MgO expansive agent was revealed.The main conclusions are as follows:(1)In a freshwater environment,the crack surface healing effect and water permeability recovery effect of specimens mixed with MgO expansive agent are significantly better than those without MgO expansive agent.And the higher the activity and dosage of magnesium oxide expansion agent,the better the self-healing effect of concrete cracks with the same initial width under the same age conditions;For specimens with different widths,the larger the initial width,the worse the self-healing effect.When the crack width is 0.2mm,the final width repair rate and area repair rate of the specimen reach 100%;When the width is 0.4mm,the width repair rate and area repair rate of the specimen are 82.77%and 89.09%,respectively;When the width is 0.6mm,the width repair rate and area repair rate of the specimen are only 51.85%and 43.32%.(2)In the seawater environment,from the perspective of crack healing speed and final healing width,the healing effect of each specimen is better than that in the freshwater environment.When the crack width is 0.2mm,the final healing effect of the specimens with or without MgO expansive agent is relatively good,and there is no significant difference.Consistent with the freshwater environment,the healing effect of each specimen deteriorates with the increase of initial width.The specimen without MgO expansive agent exhibits the more obvious performance.When the crack width is 0.2mm,the final width repair rate and area repair rate of the specimen reach 100%;When the width is 0.4mm,the width repair rate and area repair rate of the specimen are 98.39%and 89.42%,respectively;When the width is 0.6mm,the width healing rate and area healing rate of the specimen are 63.69%and 77.13%respectively.(3)The micro analysis technology was used to analyze the healing products of each specimen in the fresh water and seawater environment respectively.It can be found that the healing products of the specimens without MgO expansive agent in the fresh water environment mainly include calcite,C-S-H gel,ettringite and calcium hydroxide,while the healing products of the specimens with MgO expansive agent include calcite,calcium hydroxide and brucite;The healing products of the specimens with and without MgO expansion agent in seawater environment are basically the same,mainly including brucite,aragonite,calcite,hydromagnesite and M-S-H gel.