Evolution of phase composition and microstructure in various dolomite containing mortars due to the alkali-carbonate reaction

Sutormina, Elena (author), Meden, Anton (mentor), Marinšek, Marjan (comentor)

The alkali-carbonate reaction (ACR) in concrete is a combination of chemical reactions, which occur mainly at the phase boundary of dolomitic aggregates and binding paste. The appearance of new phases in concrete due to the progressing ACR affects the microstructure of the concrete, its mechanical properties and its durability, which spurred interest in thoroughly understanding the process and its long-term effects on concrete behavior. In this work, we studied the evolution of the phase composition and microstructure, the dynamics of secondary reaction product formation, their chemical composition and possible structure, and the effect on the mechanical properties of various dolomite-containing mortars with respect to the aggregate and binder types, and ageing conditions. Twelve types of mortar mixtures were prepared using ordinary Portland cement, blastfurnace cement or lime and two types of dolomitic aggregates. Raw materials and dolomite-containing mortars were characterised by X-ray diffraction in combination with Rietveld analysis, thermogravimetric analysis and scanning electron microscopy with X-ray microanalysis. The results showed that the dedolomitisation degree depended on the alkaline environment of binder paste, the amount and mobility of the available hydroxide ions, and the characteristics of the initial dolomitic aggregates (calcium content, structure ordering, the presence of admixture phases). The interaction of dolomite with Al- and Si-containing components of the cement paste resulted in the formation of secondary reaction products such as the hydrotalcite-like phase and further Mg-Si-Al-containing clinochlore-like phase. The dedolomitisation process was found to promote the dissolution of Si and Ca from the slag, accelerating its hydration and leading to the formation of the multi-rim structure of hydration products. The processes of dissolution/reprecipitation of Ca/Mg and carbonate ions diffusion in lime-dolomite aggregate mortars resulted in the formation of secondary calcite, hydromagnesite or eitelite during ageing under various conditions. The formation of secondary reaction products during the ACR reduced the differences between the initially dense dolomite aggregates or slag grains and the less dense cement paste and densified the microstructure, thereby increasing the compressive and flexural strengths of the mortars.

• Language: English
• Keywords: -
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2025
• PID: 20.500.12556/RUL-167091
• COBISS.SI-ID: 226183427
• Publication date in RUL: 07.02.2025

Secondary language

• Language: Slovenian
• Title: Razvoj fazne sestave in mikrostrukture v različnih dolomitnih maltah zaradi alkalno-karbonatne reakcije

Abstract

Alkalno-karbonatna reakcija (ACR) v betonu je kombinacija kemijskih reakcij, ki se večinoma pojavijo na mejni površini dolomitnih agregatov in vezivne paste. Pojav novih faz v betonu zaradi napredovanja ACR vpliva na mikrostrukturo betona, njegove mehanske lastnosti in trajnost, kar je spodbudilo zanimanje za temeljito razumevanje procesa in njegovih dolgoročnih učinkov na obnašanje betona. V tej raziskavi smo preučevali razvoj fazne sestave in mikrostrukture, dinamiko nastajanja sekundarnih reakcijskih produktov, njihovo kemijsko sestavo in možno strukturo ter vpliv na mehanske lastnosti različnih malt, ki vsebujejo dolomit, glede na tipe agregatov in veziv ter pogoje staranja. Pripravili smo dvanajst vrst mešanic malt z uporabo navadnega portlandskega cementa, žlindrinega cementa ali apna ter dveh vrst dolomitnih agregatov iz različnih regij Slovenije, ki ne vsebujeta silicij-vsebujočih komponent. Surovine in malte, ki vsebujejo dolomit, smo karakterizirali z rentgensko praškovno difrakcijo v kombinaciji z Rietveldovo analizo, termogravimetrično analizo in vrstično elektronsko mikroskopijo z rentgensko mikroanalizo. Rezultati so pokazali, da je stopnja dedolomitizacije odvisna od alkalnega okolja vezivne paste, količine in mobilnosti razpoložljivih hidroksidnih ionov ter značilnosti začetnih dolomitnih agregatov (vsebnost kalcija, urejenost strukture, prisotnost primesi). Interakcija dolomita s komponentami cementne paste, ki vsebujejo Al in Si, je povzročila nastanek sekundarnih reakcijskih produktov, kot je hidrotalkitu podobna faza in nadaljnja klinokloru podobna faza, ki vsebuje Mg-Si-Al. Ugotovili smo, da postopek dedolomitizacije pospešuje raztapljanje Si in Ca iz žlindre, pospešuje njeno hidratacijo in vodi do oblikovanja strukture produktov hidratacije z več reakcijskimi obroči okrog zrn. Procesi raztapljanja/precipitacije Ca/Mg in difuzije karbonatnih ionov v apneno-dolomitnih maltah so med staranjem v različnih pogojih povzročili nastanek sekundarnega kalcita, hidromagnezita ali eitelita. Tvorba sekundarnih reakcijskih produktov med ACR je zmanjšala razlike med prvotno gostimi dolomitnimi agregati ali zrni žlindre in manj gosto cementno pasto ter zgostila mikrostrukturo, s čimer sta se povečali tlačna in upogibna trdnost malt.

• Keywords: -