| Green and environment-friendly idea has gained much attention due to the increasing environmental problems such as global warming.In view of the energy consumption and CO2 emissions in the process of Portland cement(PC)production accounting for considerable proportion of that in the world,to adhere to the strategy of sustainable development,and also to meet the needs of the construction industry for cement,alkali-activated cementitious materials(AAMs)have been emerged as a new type of energy conservation and environment-friendly materials.Among various AAMs,alkali-activated slag(AAS)has been paid much attention.Although it has been investigated for 70 years and lots of research about its basic properties has been reported,the investigation of its durability,especially the corrosion of steel in AAS,is very limited and the mechanism of steel corrosion has not yet reached a consensus.Considering the marine environmental characteristics of the South China Sea Islands and the coastal region,the corrosion behavior and mechanism of steel bars in AAS were systematically studied through experiments of simulated pore solutions and reinforced concretes,which would provide certain theoretical and experimental basis for the use of AAS in structures.In the experiment of simulated pore solutions,the pore solutions of AAS and PC mortars were obtained by filter-press method and then analyzed via inductively coupled plasma optical emission spectrometry(ICP-OES)and ion chromatograph.Based on the real concentration of ions in solutions,four parts were set to study the effect of different solutions on steel corrosion after adding chlorides:(1)the effect of solutions containing S-ions(S2-,S2O32-,SO32-and SO42-)at different pH values(9.56,11.03,12.62,13.64 and 14.47(The concentration of OH-for pH=14.47 is about 2.95 mol/L.Here,pH value is used as a temporary marking.));(2)the effect of solutions containing different concentration of AlO2-;(3)the effect of solutions with co-existence of S containing ions,AlO2-and SiO32-;(4)the effect of traditional corrosion inhibitor Ca(NO2)2(dosage was[NO2-]/[Cl-]=0.67)and green corrosion inhibitor K2HPO4(dosage was[PO43-]/[Cl-]=1).In order to assist the analysis of corrosion mechanism of steel,the Pourbaix diagrams(potential-pH diagrams)of different ions in Fe-H20 system was drew using HSC Chemistry thermal chemical fluid simulation software.The results showed that proper concentration of aluminates and silicates could both improve the resistance of steel bars to chloride ingress.AAS simulated pore solution possesses greater capacity to protect steel bars from corrosion than PC simulated pore solution.This is due to the existence of higher concentration of silicate in AAS pore solution.The ability of AAS simulated solution to passivate steel bars was stronger than that of PC.The addition of Ca(NO2)2 significantly improved the resistance ability of steel bars to chloride erosion in AAS simulated solution,while K2HPO4 exhibited the opposite effect.Additionally,Pourbaix diagrams based on AAS system were consistent with experimental results which verified the applicability of Pourbaix diagrams to steel corrosion study in AAS system.In the experiment of reinforced concretes,seawater,sea sand and two kinds of corrosion inhibitors of steel bars in concrete is consisten(Ca(NO2)2 and K2HPO4)were introduced into AAS concretes.Results showed that the corrosion behaviort with that in simulated solutions,which indicated that using simulated pore solutions to represent simplified concrete environment is reliable when studying the corrosion mechanism of steel bars in AAS system.During the process of wet-dry cycles for simulating marine environment,the service performance of AAS reinforced concrete is equivalent or even better than that of PC reinforced concretes.Seawater and sea sand mixed AAS concretes had higher strength The application of seawater and sea sand had a little but not notable influence on the corrosion behavior of steel bars in AAS concretes,which indicated that seawater and sea sand can be applied to AAS system and provide AAS better resistance to the erosion of severe marine environment.The effect of corrosion inhibitors on steel corrosion in seawater and sea sand mixed AAS concretes was similar with that in simulated solutions.Between these two corrosion inhibitors,Ca(NO2)2 effectively improved the protection of reinforcement.The results of accelerated carbonation indicated that concretes of AAS system had lower capacity to resist carbonation than PC concrete.Seawater and sea sand enhanced the carbonation resistance of AAS concrete,but may lead to more severe corrosion when concretes reached total carbonation.Additionally,two kinds of inhibitors improved the resistance to carbonation attack of steel bars in AAS concrete,even though they made AAS concrete have higher carbonation depth.Above results suggested that seawater,sea sand and inhibitor Ca(NO2)2 can be applied to AAS concretes under certain circumstances to improve the durability of AAS reinforced concretes. |
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