Binding of external chlorides by cement pastes

BINDiNG OF EXTERNAL CHLORIDES BY CEMENT PASTES Doctor of Philosophy, 200 1 Hassan Zibara Graduate Department of Civil Engineering University of Toronto Chloride-induced reinforcernent corrosion is the dominant cause of premam deterioration of reinforced concrete structures worldwide. The need to quanti@ the durability of new and existing structures led to the development of senice life prediction rnodels. This requires a clear understanding of the mechanisms of chloride penetration into concrete cover. This research looks into chloride binding when chlorides are introduced after the hydration of cernent. Highlights of this research include the X-ray diflktion (XRD) results which provide insight into some meciianisms of chloride binding. ïhey show that several calcium aluminate hydrates (CA-H). including monosulpbate, convert to Friedel's salt. The rnonosulphate converts to Kuzel's salt at low concentrations before transforming into Friedel's salt at higher concentrations. Evidence suggests îbat ettringite starts converting to Friedel's salt at high chloride concentration. Friedel's sait p d s kept Uicmsing at chloride exposures above 1 .O M. indicating that chemical binding is not exhausted at low chloride concentrations. The results indicate the cbloride binding capacity of cement paste results fiorn the contribution of hydrates of different cernent phases. The C,A has a strong effect on cbloride binâing, especially at high chlonde concentrations (1 .O 3.0 M). Evidence h m synthetic compound pastes and cernent pastes indicate tbat C,AF, C,S, and C,S bind chiorides and significantly conûibute to the binding capacity. The binding isotherms of cernent pastes can be predicted fiom the chernid composition of cements. The resulîs show that chioride binding isothenns are non-linear. and the Freudich isothenn is the best fit in the chfonde concentration range 0.1 M3.0 M. In general, the partial replacement of cernent with fly ash, growid ,oradated blast furnace slag (GGBFS), or metakaolin. increased chloride binding. Partial replacement with silica fùme teduced chioride bindiig. An increase in the pH (13-14) of the storage solution reduced chloride bindhg. The presence of sulphate ions in the storage solution at 0.1 M concentration reduced chloride binding. Precarbonation of cement pastes greatly reduced chloride binding. An increase in temperature between 7°C and 38°C reduced chloride binding at 0.1 M chloride exposw, but increased it at 3 M exposure.