Corrosion Behaviour Of Steel Reinforcement In Alkali-activated Materials Under Simulated Marine Environment

With the continuous development of the construction industry,its consumption of resources and energy and carbon emissions have increasingly become important obstacles restricting its development.The use of environmentally friendly cementitious materials to replace Portland cement is an effective way to solve this problem.Alkali-activated material is a cementitious material using the activator to activate aluminosilicate solid waste.It has the advantages of good corrosion resistance,rapid strength growth,and compact structure.It mainly uses industrial by-products such as granulated blast-furnace slag,fly ash,and steel slag.The production process does not require high-temperature calcination.Its production and application reduce carbon emissions and contribute to the sustainable development of the construction industry.At present,there have been certain researches on the performance and reaction mechanism of alkali-activated materials,but the research on its durability,especially the corrosion behaviour of steel reinforcements in marine environments,is still insufficient.Corrosion of steel reinforcements can lead to the deterioration of reinforced concrete structures,and this problem is the focus of the design,construction and later maintenance of concrete structures.Under corrosive environments such as marine environments or deicing salt,chlorides can penetrate the concrete.The corrosion will occur on the steel reinforcements,and the expansive corrosion products produced by it will damage the concrete protective layer,cracking and destroying the concrete protective layer,until it completely fails,bringing huge safety hazards and economic losses.In response to the above problems,this paper has researched on the basic performance of alkali-activated cementitious materials,the analysis of the corrosion behaviour of alkali-activated materials in a simulated marine environment,and the mechanism analysis of the corrosion process of alkali-activated materials.First,the hydration process and performance characteristics of granulated blast-furnace slag,steel slag,and ferronickel slag under the action of an alkali activator are studied.The results show that the calcium phase in the precursor materials such as slag will first dissolve and combine with the active silicon in the water glass to form a C-A-S-H gel.This process is mainly controlled by the concentration of Ca²⁺,and as the reaction proceeds,due to the decrease of silicon content in the solution,the reaction will be inhibited.At this time,the Ca²⁺in the solution will combine with the dissolved Al O^2-to form C₂ASH_S.The silicon phase in the precursors will continue to dissolve as the hydration reaction progresses,and more C-A-S-H will condense.Steel slag and ferronickel slag are difficult to fully dissolve in the early stage,and therefore have a low contribution to the early chemical reaction.As the chemical reaction continues,the partially dissolved precursors will react with the active ingredients in the liquid phase,to generate C-A-S-H gel to make the mortars more compact.The incorporation of steel slag and ferronickel slag can increase the fluidity of the mortars,extend the setting time,reduce the heat of hydration,and reduce the shrinkage of the mortars and the chloride diffusion coefficient,which helps to resist the corrosion of chlorides in the marine environment.Subsequently,the effect of using copper slag instead of natural sand as fine aggregate on the performance of alkali-activated slag and fly ash mortar and the corrosion behaviour of steel reinforcements were studied.It was found that due to the large density difference between copper slag and paste,under the condition of a higher substitution rate of 60%,segregation and bleeding will occur,and the matrix will be uneven and structural defects will be produced.Under the conditions of a suitable substitution rate of 20%and 40%,the incorporation of copper slag can increase the fluidity and improves the strength of the mortars,reduces the drying shrinkage and makes the pore structure denser.Under dry and wet cycle conditions,the 20%and 40%copper slag replacement rates showed higher corrosion potential,and the corrosion risk of steel reinforcements was lower.Next,the pore solution of the alkali-activated slag-steel slag paste with different ratios was obtained by the filter press method,and the content of various ionic components was tested by inductively coupled plasma photoelectron spectroscopy.At the same time,ion chromatography is used to analyze the relative content of sulfide ions in different valences.The slag contains a large amount of sulfur.In the early dissolution process,HS^-exists,as time goes by,HS^-will undergo redox reactions and generate higher valence states of S₂O₃^2-,SO₃^2-and SO₄^2-.The Fe-H₂O system in the presence of different ions is analyzed by HSC Chemistry 9 thermochemical analysis software Pourbaix diagram（Eh-pH diagram）,it is found that HS^-can significantly change the stable phase composition of the system,and react with the steel reinforcements in the early age to inhibit the formation of the passive film.Although AlO₂^-and SiO₃^2-have little effect on the Pourbaix diagram,However,they can form a gel to adhere to the surface of the steel reinforcements,preventing the steel reinforcements from contacting with chlorides and reducing the risk of steel corrosion.The corrosion behaviour of low-alloy reinforcing steel reinforcements in the pore solution and low-carbon steel reinforcements in the presence of chloride ions are compared.Because Cr in the corrosion-resistant steel bars will accumulate near the pitting pits to form a dense corrosion product film.The low-alloy reinforcing steel reinforcements have higher corrosion resistance.Finally,in combination with the actual marine environment,submerged zone,tidal zone,and atmospheric zone were selected as the research areas,and the effects of different marine areas on the performance changes of alkali-activated mortars and the corrosion behaviour of steel reinforcements were investigated,and compared with Portland cement mortar of the same strength level.The results show that after 480 days of exposure to the simulated marine environment,the strength of the samples in the submerged zone is the highest,followed by the atmospheric zone,while the strength of the mortar in the tidal zone has the most significant decrease.The alkali-activated mortar has a higher volume change than the Portland cement mortar,especially in the tidal zone,and the incorporation of steel slag can reduce this change to a certain extent.The alkali-activated mortar shows a better steel protection property than Portland cement mortar in the submerged zone.And in the tidal zone,due to the drying shrinkage and carbonation of the alkali-activated mortar,the steel reinforcements have a greater risk of corrosion.