Fracture dolomite as an archive of continental palaeo-environmental conditions

The origin of Quaternary dolomites in continental environments (e.g. karst and lakes) is barely constrained compared to marine dolomites in sedimentary records. Here we present a study of dolomite and aragonite formations infilling young fractures of the ‘Erzberg’ iron ore deposit, Austria, under continental-meteoric and low temperature conditions. Two dolomite generations formed shortly after the Last Glacial Maximum (~20 kyr BP): dolomite spheroids and matrix dolomite. Clumped isotope measurements and U/Th disequilibrium ages reveal formation temperatures of 0–3 °C (±6 °C) and 3–20 °C (±5 °C) for the both dolomite types, and depositional ages around 19.21 ± 0.10 kyr BP and 13.97 ± 0.08 kyr BP or younger, respectively. Meteoric solution and carbonate isotope compositions (δ18O, δ13C and 87Sr/86Sr) indicate the dolomites formed via aragonite and high-Mg calcite precursors from CO2-degassed, Mg-rich solutions. Our study introduces low temperature dolomite formations and their application as a sedimentary-chemical archive.ISSN:2662-443

Report of RILEM TC 281-CCC: outcomes of a round robin on the resistance to accelerated carbonation of Portland, Portland-fly ash and blast-furnace blended cements

Many (inter)national standards exist to evaluate the resistance of mortar and concrete to carbonation. When a carbonation coefficient is used for performance comparison of mixtures or service life prediction, the applied boundary conditions during curing, preconditioning and carbonation play a crucial role, specifically when using latent hydraulic or pozzolanic supplementary cementitious materials (SCMs). An extensive interlaboratory test (ILT) with twenty two participating laboratories was set up in the framework of RILEM TC 281-CCC ‘Carbonation of Concrete with SCMs’. The carbonation depths and coefficients determined by following several (inter)national standards for three cement types (CEM I, CEM II/B-V, CEM III/B) both on mortar and concrete scale were statistically compared. The outcomes of this study showed that the carbonation rate based on the carbonation depths after 91 days exposure, compared to 56 days or less exposure duration, best approximates the slope of the linear regression and those 91 days carbonation depths can therefore be considered as a good estimate of the potential resistance to carbonation. All standards evaluated in this study ranked the three cement types in the same order of carbonation resistance. Unfortunately, large variations within and between laboratories complicate to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions on the carbonation performance of the specimens tested. Nevertheless, it was identified that fresh and hardened state properties alone cannot be used to infer carbonation resistance of the mortars or concretes tested. It was also found that sealed curing results in larger carbonation depths compared to water curing. However, when water curing was reduced from 28 to 3 or 7 days, higher carbonation depths compared to sealed curing were observed. This increase is more pronounced for CEM I compared to CEM III mixes. The variation between laboratories is larger than the potential effect of raising the CO concentration from 1 to 4%. Finally, concrete, for which the aggregate-to-cement factor was increased by 1.79 in comparison with mortar, had a carbonation coefficient 1.18 times the one of mortar

Mechanisms and Processes of Concrete Corrosion in Sewers

Concrete corrosion in sewers is caused by the combination of chemical and biological processes including sulfide and carbon dioxide generation and partition in wastewater, sulfide oxidation, neutralizing reactions of carbon dioxide, hydrogen sulfide, and its oxidation products (mainly sulfuric acid) with concrete. Wastewater is a sulfate-rich environment with sufficient carbon sources. The metabolism of sulfate-reducing bacteria leads to the formation of hydrogen sulfide in wastewater under anaerobic conditions. During wastewater transport through the sewers, depending on the dissolved oxygen concentrations and pH in wastewater, hydrogen sulfide can be chemically or biologically oxidized in wastewater, or partition into sewer gas in gravity sewers. Due to the alkaline and porous nature of concrete sewer pipes, the hydrogen sulfide reacts with intact concrete and reduces the concrete pH, lowering the concrete surface pH through chemically induced corrosion. The additional outgassing of CO2 from the wastewater further accelerates these processes. The reduction of surface pH facilitates the colonization of sulfide-oxidizing microorganisms on concrete surfaces. Sulfide-oxidizing microorganisms can further biologically oxidize hydrogen sulfide into sulfuric acid, leading to microbiologically influenced concrete corrosion. This chapter describes and discusses the mechanisms of these processes in sewers

Control of MgSO40(aq) on the transformation of amorphous calcium carbonate to high-Mg calcite and long-term reactivity of the crystalline solid

International audienceThe Mg and SO 4 content of naturally occurring calcite are routinely used as paleoenvironmental proxies. Yet little is known about the mechanisms governing the presence of these ions in carbonate minerals when their formation proceeds via an amorphous precursor. To address this, the transformation of Mg-free amorphous calcium carbonate (ACC) into nanocrystalline high-Mg calcite (HMC) was experimentally studied in solutions containing 27 mM of Mg and a range of 10-90 mM of SO 4. The obtained results suggest that ACC is stable for several minutes in the experimental solutions and this amorphous phase actively uptakes Mg and SO 4 that are incorporated in its structure. Additionally, the obtained results suggest that the stabilization of ACC is not affected by its Mg content and that the transformation to HMC is effectively controlled by the abundance of the free Mg 2+ (aq) ion. The transformation of ACC to HMC occurs earlier at elevated SO 4 concentrations because SO 4 limits the availability of Mg 2+ (aq) due to the formation of the MgSO 4 0 (aq) complex. The HMC that is formed from ACC appears as aggregates composed of nanocrystallites and exhibits Mg and SO 4 contents up to 8 and 2 mol% depending on the initial SO 4 concentration in the reactive solution. The precipitated HMC was kept in contact with the reactive solution in order to assess its reactivity for up to 1 year of reaction time. Over time, a continuous exchange of Mg and SO 4 between calcite and reactive solution was observed resulting in enrichment of Mg and depletion of SO 4 affecting the total mass of the aggregates with the distribution of these elements to appear homogeneous in the crystalline solid. The high reactivity and the continuous exchange of solutes between the nanocrystalline calcite and the reactive solutions limits the use of Mg and SO 4 content of these HMCs as environmental proxies

Concrete Corrosion Characterization Using Advanced Microscopic and Spectroscopic Techniques

The aim of this chapter is to give an overview of basic and advanced state-of-the-art microstructural and spectroscopic analytics to investigate inorganic material corrosion in the context of biochemically aggressive sewers. The chapter covers optical methods, electron beam, X-ray and neutron techniques (SEM, MLA, XRF, XRD, CT, Neutron radiography and tomography), and spectroscopic methods (MAS-NMR, FT-IR, and Raman). For each technique, a short section on the fundamental scientific background of the method precedes and examples of data output from the latter in respect to the corrosion of cementitious materials including reinforced concrete is presented

Optical pH imaging of concrete exposed to chemically corrosive environments

Major types of chemical concrete degradation such as carbonation, leaching and acid attacks are strongly associated with decreasing internal or external pH. Thus a precise determination of the latter is crucial for the assessment regarding the degree of corrosion and corresponding development of prevention strategies. Conventional pH measure methods for concrete, such as phenolphthalein indicator, pore solution extractions and flat surface electrodes have proven to contain significant limitations and inadequacies. This contribution presents the application of sensor foils based on luminescent pH sensitive dyes entrapped in a polymeric hydrogel matrix, to quantify and image the distribution of surface pH of concrete based construction materials. An imaging technique called time-domain dual lifetime referencing (t-DLR) was used, which suppresses artifacts from scattering of the background and other light inhomogeneities. Using this methodology high-resolution pH profiles of concrete samples exposed to carbonation and biogenic acid corrosion were recorded

Optical pH imaging of concrete exposed to chemically corrosive environments

Major types of chemical concrete degradation such as carbonation, leaching and acid attacks are strongly associated with decreasing internal or external pH. Thus a precise determination of the latter is crucial for the assessment regarding the degree of corrosion and corresponding development of prevention strategies. Conventional pH measure methods for concrete, such as phenolphthalein indicator, pore solution extractions and flat surface electrodes have proven to contain significant limitations and inadequacies. This contribution presents the application of sensor foils based on luminescent pH sensitive dyes entrapped in a polymeric hydrogel matrix, to quantify and image the distribution of surface pH of concrete based construction materials. An imaging technique called time-domain dual lifetime referencing (t-DLR) was used, which suppresses artifacts from scattering of the background and other light inhomogeneities. Using this methodology high-resolution pH profiles of concrete samples exposed to carbonation and biogenic acid corrosion were recorded

Microbial induced acid corrosion from a field perspective - Advances in process understanding and construction material development

Microbial induced acid corrosion is accounted for ~40 % of the degradation of subsurface wastewater infrastructure globally. While fundamental process understanding has increased significantly within the last decades, to date no sustainable building material exists, which meets the long-term requirements in such aggressive and corrosive environments. This work describes a novel model based on field studies, conducted in various Austrian sewer networks, intertwining biological, mineralogical and hydro-chemical factors. Additionally, an extensive field testing campaign over the duration of 18 months, conducted on different geopolymer concretes (GPC), particularly designed for the latter environments will be presented. Innovative GPCs were tested regarding their microstructural behavior, microbial accessibility and hydro-chemical alterations over time and compared to commercially produced cement based products, including OPC and CAC concretes. Additionally, the system specific environmental parameters, such as relevant gas concentrations (H2S, CO2, CH4), relative humidity and temperature were constantly monitored

Wide-range optical pH imaging of cementitious materials exposed to chemically corrosive environments

The pH of concrete-based material is a key parameter for the assessment of its stability and durability, since a change in pH is usually associated with major types of chemical degradation such as carbonation, leaching and acid attacks. Conventional surface pH measurements with potentiometric flat surface electrodes have low spatial resolution, whereas optical pH visualization with indicator dyes (phenolphthalein) only indicates the areas with higher or lower pH than the pKa of the indicator. In this regard, it is key to develop wide-range imaging systems, enabling accurate and spatially resolved determination of pH variability for an advanced knowledge of degradation mechanisms. This contribution presents the enhancements made for a high-resolution optical pH imaging system based on fluorescent aza-BODIPY indicator dyes. The measurement range was increased to 6 pH units (pH 6.5 to pH 12.5) by a combination of two indicator dyes. Moreover, background scattering effects were sufficiently eliminated. With the improved sensor foils steep pH gradients (up to 3 pH units within 2 mm) were successfully recorded in various concrete specimens using a macro lens reaching a resolution of down to 35 µm per pixel

Fracture dolomite as an archive of continental palaeo-environmental conditions

The origin of Quaternary dolomites in continental environments (e.g. karst and lakes) is barely constrained compared to marine dolomites in sedimentary records. Here we present a study of dolomite and aragonite formations infilling young fractures of the ‘Erzberg’ iron ore deposit, Austria, under continental-meteoric and low temperature conditions. Two dolomite generations formed shortly after the Last Glacial Maximum (~20 kyr BP): dolomite spheroids and matrix dolomite. Clumped isotope measurements and U/Th disequilibrium ages reveal formation temperatures of 0–3 °C (±6 °C) and 3–20 °C (±5 °C) for the both dolomite types, and depositional ages around 19.21 ± 0.10 kyr BP and 13.97 ± 0.08 kyr BP or younger, respectively. Meteoric solution and carbonate isotope compositions (δ18O, δ13C and 87Sr/86Sr) indicate the dolomites formed via aragonite and high-Mg calcite precursors from CO2-degassed, Mg-rich solutions. Our study introduces low temperature dolomite formations and their application as a sedimentary-chemical archive.ISSN:2662-443