Characterization of chloride induced steel corrosion processes in blended cements

Doktorska disertacija obravnava področje karakterizacije korozijskih procesov jekla v betonih iz mešanih cementov, kjer je cementni klinker delno nadomeščen z mineralnimi dodatki žlindre, filtrskega pepela, naravnega pucolana ali apnenca. V okviru preiskav na preizkusnem polju v Luki Koper smo 4,5 let spremljali korozijske procese jekla v betonskih stebrih iz portlandskega cementa in cementa z dodatkom žlindre. Stebri so bili ves čas izpostavljeni naravnemu plimovanju, pršenju morske vode in zunanjim spremembam temperature. Za spremljanje korozije smo uporabili uporovne senzorje in sklopljene elektrode, razporejene po višini stebra. V okviru laboratorijskih preiskav smo izdelali večje število maltnih vzorcev iz portlandskega cementa in treh mešanih cementov z dodatkom filtrskega pepela, naravnega pucolana, žlindre ali apnenca. Polovico vzorcev iz vsakega cementa smo pospešeno karbonatizirali in na vseh kombinacijah malte in karbonatizacijskega stanja smo ob različnih časih izpostavitve ugotovili nekatere lastnosti malt. Vzorce smo za 1 leto izpostavili cikličnemu močenju s 3,5 % raztopino NaCl in med izpostavitvijo analizirali sestavo pornih vod posameznih cementov. Za spremljanje korozijskih procesov v maltah smo uporabili tehniko galvanostatskega pulza, uporovnih senzorjev in sklopljeno mrežo elektrod. Za določitev korozijskih lastnosti jekla v pornih vodah smo merili potencial odprtega kroga, linearno polarizacijo in potenciodinamsko ciklično polarizacijo. Z uporabo večjega števila komplementarnih in inovativnih merilnih tehnik smo uspešno spremljali razvoj in napredovanje korozijskih procesov v maltah iz mešanih cementov in te rezultate povezali z izmerjenimi fizikalnimi in kemijskimi lastnostmi malt. Pri cementih z večjo vsebnostjo cementnega klinkerja se je izkazalo, da imajo daljši čas iniciacije kot cementi z manjšo vsebnostjo klinkerja, saj do iniciacije korozije lahko pride pred zadostno reakcijo mineralnih dodatkov. Po iniciaciji korozije so bile korozijske hitrosti v portlandskem cementu manjše od nekaterih mešanih cementov, kasneje pa so cementi z mineralnimi dodatki bolje zavirali korozijske procese. Izkazalo se je tudi, da imajo lahko jekla v mešanih cementih in v karbonatiziranem stanju korozijske poškodbe z manjšimi globinami od tistih, ki v enakem času nastanejo v portlandskem cementu.Doctoral dissertation discusses the topic of chloride induced steel corrosion processes in blended cements, where cement clinker is partially replaced with slag, fly ash, natural pozzolan and limestone mineral admixtures. During the on-field investigation in Port of Koper for 4.5 years, corrosion processes in concrete columns made of ordinary portland cement and portland slag cement were monitored. The columns were exposed to natural tiding, salt-water spray and environmental temperature changes throughout the exposure period. Electrical resistance sensors and coupled multi-electrodes were used as corrosion monitoring techniques. As part of the laboratory experiments, multiple mortar specimens were made of portland cement and three blended cements using fly ash, natural pozzolan, slag and limestone as partial clinker replacement. Half the specimens were subjected to accelerated carbonation and some properties of mortars in both carbonation states were characterised at different exposure periods. The specimens were exposed to 3.5 % NaCl solution for 1 year and during the exposure, pore solutions were extracted for each cement. Galvanostatic pulse, electrical resistance sensors and coupled multi-electrode array were used to monitor corrosion processes in mortar. In order to determine corrosion properties of steel in pore solutions, cyclic potentiodynamic polarization, linear polarization and open circuit potential were measured. Using multiple complementary and innovative monitoring techniques we were able to successfully monitor initiation and propagation of steel corrosion in mortars made of blended cements and link those results to physical and chemical mortar properties. Using cements with higher clinker content, it was shown that the initiation time is longer than cements with mineral admixtures. After initiation, corrosion rates in portland cement can initially be slightly lower than specific blended cements, but later blended cements offer reduced corrosion rates compared to portland cement. It was also shown that mortars made of cements with mineral admixtures and in carbonated state can have corrosion damage with lower maximum depth than those that appear in portland cement mortars in the same timeframe