Corrosion Behavior And Mechanism Analysis Of High-performance Cement-based Composites

With the development of the social economy and the deterioration of the global environment,the requirements for building materials are getting more serious.However,due to the low flexural strength and durability,traditional cementitious composites have been unable to meet the needs of contemporary building materials.Therefore,the development of high performance cementitious composites with potentials of sustainable development and excellent properties have become one of the important directions for the green development of future construction industry.Durability is an important properties of cementitious composites.Among various durability issues,calcium leaching is a common corrosion.It usually occurs in soft water environments,e.g.rainwater and groundwater.Also,it occurs in infrastructure such as bridge piers,dams,reservoirs,and sewage pipes.In general,calcium leaching can cause the strength reduction,porosity increase,and the durability reduction of the materials when it takes longer.Nowadays,calcium leaching of traditional cementitious composites has been widely investigated.However,calcium leaching of high performance cementitious composites,especially graphene oxide-cement composites and slag-based alkali activated composites,still remains unclear.Therefore,this thesis selects two types of high performance cementitious composites to conduct the leaching research and mechanism analysis.The main results are as follows:(1)The effects of different amounts of graphene oxide(GO)(0,0.10,and 0.20 wt.%)on calcium leaching of ordinary Portland cement pastes under deionized water were studied.The results show that:a)Compared with Al and Si,Ca element was main leaching element and its leaching amount decreased with increasing GO content;b)Compared with calcium silicate hydrate(C-S-H)and ettringite,calcium hydroxide(CH)was the main dissolved substance and its dissolved amount decreased with increasing GO content;c)The increment of total porosity decreased with increasing GO content,since GO inhibited the formation of pores with a diameter of 1?m to 5?m in leached pastes;d)GO significantly improved the microstructure of the leached paste.(2)The effects and mechanism of different amounts of GO(0,0.05,0.10,0.15,and 0.20wt.%)on calcium leaching of ordinary Portland cement pastes under 6 mol/L ammonium chloride solution were studied.The results show that:a)The leaching depth and the loss of compressive strength decreased with increasing GO content,and the latter can be attributed to the fact that GO inhibited the CH dissolution which mitigated the formation of additional capillary pores;b)The increase of leaching products and the number of micropores in the leaching area decreased with increasing GO content;c)The loss rate of the electrochemical parameter R_CCP decreased with increasing GO content,and the leaching depth and the loss of compressive strength can be accurately predicted by RCCP;d)The reinforcing mechanism of GO addition can be attributed to the fact that GO can adsorb Ca²⁺ions in the pore solution and improve the microstructure of the leached pastes.(3)In order to simulate the real application scenario of alkali-activated materials in landfills,Pb²⁺ions were introduced into the pastes.The effects and mechanism of calcium leaching on slag-based alkali activated pastes containing Pb under deionized water were studied.The results show that:a)The leaching process was first controlled by diffusion and then by dissolution;b)During the leaching process,the leaching concentration of Pb gradually increased,indicating that calcium leaching can impair the stabilization/solidification performance of the pastes;c)Calcium leaching did not affect the crystalline composition of the pastes;d)Both of Pb addition and calcium leaching reduced the content of calcium aluminosilicate hydrate(C-A-S-H)gel of the pastes;e)Both of Pb addition and calcium leaching increased the content of the Q¹(0Al)sites,indicating that leaching can destroy the end-of-chain silicates of C-A-S-H gels,and release[Pb(OH)4]^2-ions;f)Pb addition increased the total porosity of the pastes,and calcium leaching resulted in a larger increment in the total porosity of the pastes containing Pb;g)Compared with the release of[Pb(OH)₄]^2-ions from the end-of-chain silicates,the surface dissolution of Pb(OH)2 precipitates was dominated in the degradation of the pastes under calcium leaching;h)Both of Pb addition and calcium leaching increased the impedance of the pastes,and the electrochemical impedance Rs value can accurately predict the cumulative leaching concentration of Pb.