The Model Of Multi-Species Ionic Transport In Concrete And Its Effect On Durability

Due to the impact of physics and chemistry,the performance of concrete materials and structures deteriorate,which will directly reduce the service life of concrete.One of the main aspects is that the Mg²⁺,SO₄^2-and Cl^-in the environment gradually transport into the concrete,changing the components and structure of concrete,and decreasing the performance of the concrete.Those durability problems are particularly significant in the environment such as the marine,the saline soil of the western salt lake and the groundwater.Up to now,there are many investigations on ions attack and its effect on concrete durability.However,most of them focused on the transport of single ions,e.g.Cl^-or SO₄^2-,in concrete,and the durability problems caused by steel corrosion and sulfate attack.Although they are great of significance for the study of concrete durability,due to the existence of multi-species ions in the environment,the interaction between various ions under the influence of charge action,ion activity and other factors,the multi-ions transport process,which is closer to the actual environmental effect,is more complicated than the single ion transport process.Therefore,this paper focuses on the transport process of multi-species ions in concrete,in order to reveal the ionic interaction between multi-species ions transport process in concrete and the effect of mineral admixture on the ions attack durability of concrete,and establish a multi-species ions transport model.The main works are as follow.?1?The one-dimensional ions attack process in the concrete was investigated by using Na₂SO₄,MgSO₄,MgSO₄/NaCl as the etching solution and saturated Ca?OH?₂ as the reference solution.The interaction of Mg²⁺,SO₄^2-and Cl^-in the transport process was revealed by comparing the relative content,surface ion concentration and diffusion coefficient at different corrosion ages.The results show that the Mg²⁺has the greatest influence on the transport of SO₄^2-,which can hinder the SO₄^2-of the external solution from entering the interior of the concrete,and promote the transport of the SO₄^2-that has entered the interior of the concrete.The presence of Cl^-can promote the entry of the Mg²⁺from the external solution into the concrete and accelerate the diffusion rate,while it has an indirectly effect on the transport of SO₄^2-by affecting the transport of Mg²⁺.?2?To reveal the effect of different mineral admixtures on the ions attack resistance of concrete,the corrosion of three kinds of mineral admixture composite concrete in MgSO₄/NaCl solution was investigated.The results show that the high content?50 wt.%?of fly ash can effectively inhibit the transport of Mg²⁺,SO₄^2-and Cl^-in the concrete.In contrast,the incorporation of slag accelerates the transport of Mg²⁺and SO₄^2-in the concrete and accelerates the rate of concrete degradation.The incorporation of fly ash together with slag can effectively inhibit the transport of Cl^-in the concrete,but shows insignificant influence on the transport of Mg²⁺and SO₄^2-.?3?To study the attack mechanism of multi-species ions in concrete,various methods were used to characterize the apparent damage of concrete,the composition and morphology of surface corrosion products.The results show that the apparent damage of concrete in MgSO₄/NaCl solution is in the form of pitting corrosion,and the corrosion products are mainly composed of brucite and ettringite,which can form a stable brucite protective layer on the concrete surface.When the high content of mineral admixture is incorporated,most calcium hydroxide inside the concrete is consumed due to the pozzolanic reactions of the admixture,resulting in a low alkalinity environment.The surface peeling occurs due to that the dense brucite protective layer cannot form on the the concrete surface,meanwhile the corrosion product of SO₄^2-and the hydration product is converted into gypsum.?4?The Nernst-Planck equation is used to describe the effect of ion concentration gradient and the self-induced electric field.By modifying the surface ion concentration and diffusion coefficient,a multi-ion transport model in concrete was established.The ion transport law of concrete in MgSO₄/NaCl composite solution system was simulated by using MATLAB software to solve the model.The absolute value of the potential calculated by the model is between 26.42 mV and 63.8 mV.In addition,the model calculation results of the erosion depth and distribution law of ions at different erosion ages are basically consistent with the experimental results.