Study On Chloride Corrosion Resistance Of Alkali-activated Concrete In Severe Environment

Reinforced concrete is the most widely used material in structural engineering,but the durability problem is particularly prominent in harsh environments such as salt lakes and oceans,especially the corrosion of steel bars caused by chloride ions is the main problem affecting the safety and service life of concrete structures.In order to improve the durability of concrete in harsh environments and alleviate the environmental problems caused by cement production and application,alkali-activated concrete,a low-carbon and environmentally friendly material with high strength and good durability,is being extensively studied.At present,there are many studies on the durability of alkali-activated concrete under the single action of chloride,sulfate,and carbonation,but the service performance under the action of multiple factors is still unclear.The simple superposition of factors,this paper studies the influence of various factors on the anti-chloride ion erosion performance of alkali-activated materials,which provides a theoretical basis for the application of alkali-activated materials in harsh environments.In this thesis,alkali-activated fly ash（AAFA）,alkali-activated slag（AAS）and alkali-activated fly ash-slag（AAFS）were selected,and ordinary cement concrete（OPC）was used as the control group.Salt-sulfate coupling,chloride-carbonation coupling,and chloride-sulfate-carbonization three-coupling dry-wet cycle in four different erosion environments;according to the transmission mode of chloride ions in concrete,the unsteady migration,capillary pores Permeation and natural diffusion are used as indicators of chloride ion transmission capacity,and the relationship between pore structure and chloride ion transmission capacity is established;by analyzing the distribution of chloride ions in concrete,the relationship between p H,specific surface area and chloride ion binding capacity is established.Changes in microscopic properties,to find out the reasons for the decrease in the stability of combined chloride ions.The molecular model of the gel after erosion is established according to the change of material microstructure and elemental composition,and the interaction between the erosion medium and gel molecules is simulated based on molecular dynamics.The main research findings are as follows:The initial chloride ion diffusion coefficient is AAFA>AAFS>OPC>AAS,but as the erosion time increases,the diffusion coefficient and porosity of the alkali-activated material（AAM）decrease gradually with the increase of erosion time in the chloride-sulfate environment,indicating that when AAM is exposed to the coupling environment of chloride-sulfate under dry-wet cycle conditions,due to the filling of salt crystals in the pores and the excitation effect of Na₂SO₄,the pore structure is refined and the chloride ion transport resistance is improved.The chloride ion diffusion coefficients of AAFS and AAS increased to different degrees in the chloride salt-carbonization and the three coupling environments.After 32 cycles,the chloride ion permeability resistance of AAFS was close to or even exceeded that of AAS.The porosity and harmful pores（10-50 nm）content of both AAFS and AAS specimens were higher than those before erosion due to gel decalcification and pore structure deterioration due to carbonization.For the relatively low strength AAFA and AAFS specimens,salt crystallization caused surface spalling and strength reduction.The coupling effect of carbonization will further accelerate the deterioration of AAFA,but the carbonization of AAFS specimens can inhibit the surface peeling.The appearance of AAS specimens did not change much under various erosive environments,and only slight signs of deterioration appeared when subjected to the coupled action of chloride-sulfate-carbonization.The chloride ion binding capacity of each group of samples before erosion is AAFA>AAFS>AAS>OPC,and the residual chloride ion binding capacity after being exposed to the coupling environment of chloride-sulfate-carbonization under dry-wet cycle conditions is OPC>AAFS>AAS>AAFA.Since the way of OPC binding chloride ions is mainly chemical combination,it is relatively stable.The binding ability of AAM mainly comes from physical adsorption.After the coupling effect of AAFA and AAFS,especially after sulfate erosion,the crystallization of Na Cl encapsulated in the sample becomes less,and the length of the gel molecular chain becomes shorter;the layer of hydrotalcite in the AAS sample the spacing changes,SO₄^2-and CO₃^2-replace the Cl^-between the hydrotalcite layers,resulting in the re-release of bound chloride ions.Through molecular dynamics simulations,it was found that the structural oxygen atoms of C-A-S-H and C-（N-）A-S-H gels were the main binding sites,and the adsorption of chloride ions in C-A-S-H gels was higher than that of C-（N-）A-S-H gels,but the binding stability of C-（N-）A-S-H gel is higher.The migration rate of Cl-on the gel interface is generally higher than that of SO₄^2-,the SO₄^2-migration coefficient of C-（N-）A-S-H gel is smaller than that of C-A-S-H gel,and the high cross-linking degree of C-（N-）A-S-H gel contributes to Improve SO₄^2-resistance.The absolute value of the adsorption energy of the N-A-S-H gel is the largest,indicating that the particles in the salt solution are more stable on the surface of the N-A-S-H gel,while the absolute value of the adsorption energy of the C-A-S-H gel is the smallest,and the possibility of ion desorption becomes larger.