Research On Steel Corrosion Mechanism Of Alkali-activated Slag Concrete In Carbonation Environments

Alkali-activated slag concrete（AASC）has the advantages of rapid hardening,high strength,excellent pore structure and good durability.It has gradually become a new type of green cementing material with the most research prospects.However,AASC has poor performance in carbonation resistance compared with ordinary Portland cement concrete（OPC）,leading to the result that the steel bars embedded in AASC are easily rusted.Therefor,it is necessary to conduct in-depth research on its carbonation resistance and the subsequent corrosion of steel bars.The effects of alkali components（Na OH and waterglass）,Na₂O dosages（3%,5%and 7%）,modules of waterglass（1,1.2 and 1.5）and CO₂concentrations（natural condition,3%and 20%）on the carbonation depth,open circuit potential,linear polarization curve,electrochemical impedance spectroscopy and p H value of pore solution around steel bars were studied.Steel corrosion mechanism of AASC in carbonation environments was analyzed by XPS,XRD,MIP,SEM and ²⁹Si MAS NMR.The following main conclusions are drawn:It takes a shorter time for steel bars to form a stable passive film in AASC.It takes 120 h for steel bars embedded in OPC to form a stable passive film,while it takes 72 h and 54 h in WG5-1.5 and NH5 respectively.The corrosion potential of AASC is lower than that of OPC after forming a stable passive film.After sputtering for 1 minute in XPS test,the content of Fe in steel bars embedded in WG5-1.5 and NH5 is less than 50%and that in OPC is as high as 66%,indicating that the steel bars in AASC can form a thicker passive film.2.The carbonation depth of AASC and OPC exposed to natural condition is much shallower than that exposed to accelerated carbonation condition（3%,20%）.Exposed to 3%CO₂concentration environment,the carbonation depth of the NH group is smaller than that of the WG group at the same Na₂O dosage.The carbonation depth of AASC decrease with the increase of Na₂O dosage.The carbonation depth attain the minimum when the module of waterglass is 1.2.Exposed to 20%CO₂concentration environment,the carbonation depth of NH5 begins to exceed that of WG5-1.5 after 28 d carbonation.The carbonation depth of AASC exposed to three different CO₂concentration environment is deeper than that of OPC.3.The steel bars in AASC and OPC are not rusted exposed to natural condition for 224 d.Exposed to 3%CO₂concentration environment for 112 d,steel bars in AASC show different degrees of corrosion while steel bars in OPC are passivated.The alkali components,Na₂O dosages and modules of waterglass have no significant effect on the corrosion degree of steel bars.Exposed to 20%CO₂concentration environment for 28d,steel bars in both WG5-1.5 and NH5 begin to rust,but steel bars in OPC still keep passivated.During the entire accelerated carbonation process,the potentials of uncorroded steel bars in AASC are in the range of-0.25 V～-0.5 V,which are significantly lower than OPC.4.The hydration products of AASC are mainly Tobermoite with low Ca/Si ratio.The silicate tetrahedron are mainly Q¹,Q²and a small amount of Q³and Q⁴.Compared with NH group,the C-S-H gel of WG group have a higher polymerization degree,lower total porosity and denser pore structure.Carbonation reaction results in decalcification of C-S-H gel and significant increase of Q²,Q³and Q⁴,so the polymerization degree increased.And the total porosity of AASC decreases because the pore is filled with calcite,vaterite and aragonite.Carbonation reaction also leads to poor pore structure distribution,resulting in the reduction of harmless and little harmful pore volume,and the increase of harmful pore volume,which is caused by the shrinkage of AASC and the extrusion of pore by carbonized products.Poor pore structure is also the main reason of high carbonation rate of NH group.