Environmental controls and reaction pathways of coupled de-dolomitization and thaumasite formation Florian Mittermayr a, ⁎ ,1 , Andre Baldermann b,1 , Claudia Baldermann a , Georg H. Grathoff c , Dietmar Klammer b , Stephan J. Köhler d , Albrecht Leis e , Laurence N. Warr c , Martin Dietzel b a Institute of Technology and Testing of Building Materials, Graz University of Technology, Inffeldgasse 24, 8010 Graz, Austria b Institute of Applied Geosciences, Graz University of Technology, Rechbauerstraße 12, 8010 Graz, Austria c Department of Geography and Geology, Ernst-Moritz-Arndt-University Greifswald, Friedrich-Ludwig-Jahn Straße 17a, 17487 Greifswald, Germany d Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Lennart Hjelms väg 9, 75007 Uppsala, Sweden e JR-AquaConSol GmbH, Steyrergasse 21, 8010 Graz, Austria abstract article info Article history: Received 18 February 2016 Received in revised form 16 February 2017 Accepted 23 February 2017 Available online xxxx Deteriorated concrete and interstitial solutions (IS) were collected from Austrian tunnels to elucidate potential connections between de-dolomitization caused by coupled alkali carbonate reactions (ACR) and thaumasite form of sulfate attack (TSA). A conceptual reaction model for the portlandite–CSH phases–dolomite–calcium sul- fate–calcite–brucite–thaumasite system was developed based on experimental data, hydrochemical modelling, IS chemistry and apparent concrete compositions. During the initial stage of sulfate attack, ettringite and gypsum formation weakened the concrete's microstructure and initiated ACR. Leaching of hydrated cement phases result- ed in IS with a pH ~ 12-13, which promoted incongruent dolomite dissolution. Infiltration of Ca–SO 4 –type ground water into the de-dolomitization zone facilitated calcite and brucite neo-formations at 13 N pH N 10.5 during ad- vanced states of concrete deterioration and subsequently resulted in thaumasite precipitation at pH ~ 8.7. In this contribution, the reaction mechanisms and environmental controls of de-dolomitization are discussed in relation to the durability of concrete under sulfate attack. © 2017 Elsevier Ltd. All rights reserved. Keywords: Dolomite (D) Degradation (C) Sulfate attack (C) Pore solution (B) FIB-SEM (B) Alkali-Aggregate Reaction (C) 1. Introduction Down to the present day, deterioration of concrete and cement- based materials used in various underground structures is frequently related to chemical alteration. Although external sulfate attack has been considered to be a major threat, significant gaps in knowledge per- sist regarding the environmental controls and reaction mechanisms linked to external sulfate attack on concrete. Sulfate attack and alkali ag- gregate reactions (AAR) are strongly reducing the durability of cemen- titious materials and typically result in of ettringite (Ett) formation, precipitation of sodium sulfate efflorescence and gypsum (Gp), thaumasite form of sulfate attack (TSA), delayed Ett formation (DEF), bacteriogenically-induced sulfuric acid attack, alkali silica reactions (ASR) or alkali carbonate reactions (ACR). The above processes result in severe (micro)structural and mechanical damage and, probably most importantly, chemical alteration of the concrete through multiple and partly overlapping mineral dissolution and replacement reactions [1–9]. TSA is one of the most hazardous processes causing concrete corro- sion and typically occurs at temperatures b 10 °C, a pH around 9 and during multiple wetting-drying cycles, in particular at high concentra- tions of aqueous SO 4 2- (up to 30 g L -1 ), silicic acid, Ca 2+ and CO 3 2– ions (total dissolved solids, TDS from 15 to 60 g L -1 ) in the interstitial solu- tions [10–12]. TSA is a rather slow concrete damaging process and it is commonly initiated by cracking and softening of the concrete due to previous Gp and Ett formation. Other possibilities for thaumasite forma- tion are via direct precipitation or the reaction of calcium carbonate with severely Ca-leached CSH phases [11,13–27]. TSA is known to cause (micro)structural damage to the cement paste by generating ex- pansive forces in particular in the small pores of the microstructure [18,28,29]. While the influence of limestone fillers on TSA have been ex- tensively studied in lab experiments, the effects of dolomite (Dol) fillers in this context have not been clearly identified [7,13,18,19,30–33]. In acidic environments, the apparent dissolution rate of Dol [CaMg(CO 3 ) 2 ] positively depends on H + concentration, but is notably slower than that of calcite [34]. Under strongly alkaline pH found in pore solutions of most cementitious materials, it is currently believed that magnesium carbonate aggregates such as Dol are on the long term transformed into brucite [Mg(OH) 2 ] and calcite [CaCO 3 ] through liberation of alkali and calcium hydroxide ions from the cement paste Cement and Concrete Research 95 (2017) 282–293 ⁎ Corresponding author. E-mail address: f.mittermayr@tugraz.at (F. Mittermayr). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.cemconres.2017.02.011 0008-8846/© 2017 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres