Thermodynamics of calcined clays used in cementitious binders: origin to service life considerations

The use of calcined clays in construction materials has attracted significant attention in the last few years. Based on the continued need for sustainable construction to meet global development challenges, the green transition of the cement industry is an urgent necessity. The use of clay-blended cements will keep increasing to meet the need for mass quantities of materials and the prospect of reducing their embodied CO2, as traditional supplementary cementitious materials are expected to decline in availability. To enable the necessary rapid increase in the fraction of clays that can be used in cements, the use of modeling tools which provide insights into the clays and their reactivity in cementitious systems is of increased interest. The aim is to predict the properties of the calcined clays based on the original rock and calcination conditions, the phase evolution, material properties, and durability of construction materials. This is crucial to reduce the time needed for development and commercialisation, whereas extensive empirical work has been used in the past to achieve material transition in the construction sector, which can be extremely time consuming. This review article therefore aims to provide an overview of available thermodynamic data, issues with database integration, modelling of process parameters, and properties prediction for cementitious materials

Mapping circular economy practices for steel, cement, glass, brick, insulation, and wood – a review for climate mitigation modeling

Circular economy (CE) practices pave the way for the construction sector to become less material- and carbon-intensive. However, for CE quantification by climate mitigation models, one must first identify the CE practices along a product (or material) value chain. In this review, CE practices are mapped for the value chain of 6 construction materials to understand how these practices influence and can be considered in climate mitigation modelling. The main sub-categories of steel, cement, glass, clay-brick, insulation materials, and wood were used to identify which Rs are currently addressed at the lab and industrial scales: refuse, reduce, rethink, repair, reuse, remanufacture, refurbish, repurpose, recycle, and recover. The CE practices were reviewed using scientific repositories and grey literature, validated by European-wide stakeholders, and mapped across the life-cycle stages of the six materials – extraction, manufacturing, use, and end-of-life (EoL). The mapping was limited to the manufacturing and EoL stages because materials could be identified at these stages (the extraction phase pertains to resources, and the use phase to a product, for example, buildings). All reviewed CE practices identified at the industrial scale were quantified at the European level. For example, EoL reinforcement steel is 1–11 % reused and 70–95 % recycled; manufacturing CEM I is up to 60 % reduced; remanufacturing flat glass is 26 % remanufactured while less than 5 % EoL flat glass is recycled. A major barrier to closed-loop recycling is the need for sorting and separation technologies. Open-loop recycling synergies are found at the industrial scale between, for example, flat glass and glass wool value chains. Climate mitigation models are proposed to be augmented to include these practices requiring an explicit link between building use and the other construction materials' value chain stages

Influence of sulfuric acid on the early hydration kinetics and phase assemblage in a stabilization/solidification context

Cementitious binders have been used for the safe management of hazardous wastes. Acid concentrations of liquid waste that are intended to be solidified for storage in cementitious binders are relevant for the casting and long-term durability. In this contribution, the solidification of liquid waste that contains different quantities of sulfuric acid is reported with respect to the influence on early hydration kinetics. Additionally, the phase assemblage is characterized and compared to thermodynamic modeling. The data gathered shows that the quantity of sulfuric acid solution used has a clear influence on the phase assemblages and accelerates the hydration reactions. The balance between AFm phases and ettringite is also changed towards more ettringite formation for higher acid contents

Phase evolution and mechanical performance of an ettringite-based binder during hydrothermal aging

Little is known about the performance of ettringite-based binders in hydrothermal conditions. This investigation aims to gain insights into the phase evolution and corresponding mechanical performance of an ettringite-based binder considering crystallization pressure caused by late-reaction products. Additionally, the role of fiber reinforcement on the strength retention of the binder was investigated. When aged at an elevated temperature under water-saturated conditions, hard-burned MgO hydrated to form brucite. The precipitation and growth of the brucite crystals led to a crystallization pressure of approximately 200 MPa calculated using thermodynamic modelling. Damage was observed after 4 months of aging with cracks in the microstructure and eventually a failure at the macro scale. Ettringite remained stable at 60 °C due to the water-saturated conditions. Polypropylene fiber delayed crack propagation and thus reduced the damage caused by crystallization pressure. The fiber improved the flexural performance of composite attaining deflection-hardening behavior regardless of aging conditions

Influence of bicarbonate ions on the deterioration of mortar bars in sulfate solutions

This work investigates the influence of bicarbonate ions on the deterioration of cementitious material exposed to sulfate ions. Mortars based on a CEM I and on a CEM III/B cement were investigated. Experimental investigations were compared to thermodynamic modeling and phase characterization to understand the differences in deterioration. The presence of bicarbonate ions significantly reduced the expansion of the CEM I mortars. Thermodynamic modeling showed that at high concentrations of bicarbonate ettringite and gypsum become unstable. Microstructural characterization combined with information from thermodynamic modeling suggests that conditions of high supersaturation with respect to ettringite are unlikely in the samples exposed in solutions containing bicarbonate. Consequently, expansive forces are not generated by the crystallization pressure of ettringite. There was little expansion of the CEM III/B sample even in the sodium sulfate solution. In the bicarbonate solution this mortar showed a highly leached zone at the surface in which calcite was observed. (c) 2012 Elsevier Ltd. All rights reserved

On the retardation mechanisms of citric acid in ettringite-based binders

This study aims to obtain insights into the retardation mechanism of citric acid in an ettringite-based binder from ladle slag and gypsum. The hydration kinetics and phase assemblage of the binder were experimentally investigated and thermodynamically modelled. Additionally, the effects of citric acid on synthetic ettringite were studied to obtain further understanding of the interaction between this organic ligand and the crystal. Experimental results reveal that citric acid works as an inhibitor of ettringite's formation leading to the precipitation of monosulfate and gypsum; the ettringite surface blockage by citrate ligand effectively prevents precipitation of this crystal. This leads to an overestimation in the precipitation of ettringite in the thermodynamic model due to this kinetic barrier imposed by the ligand. Thermodynamic modelling suggests ettringite, monosulfate, aluminum hydroxide, and strätlingite as main hydrates in this binder, whereas an intermixed C–(A–)S–H gel was observed experimentally instead of strätlingite

Cementitious materials for oil-well abandonment and numerical simulations of cement durability at oil well conditions

In this work, we investigate Portland cement hydration and durability at elevated temperatures of 60°C – 80°C and at pressures up to 300 bar in water and in artificial brine solutions that mimic the exposing environments of cement plugs used for abandonment of out-sourced off-shore oil wells. A principal aim is to follow the structural and compositional evolution of the C-S-H phase, and thus, solid-state 27Al and 29Si NMR are used as the main analytical tools. The NMR results indicate that hydration at high pressure (300 bar) has no significant impact on the C-S-H phases formed at 60 and 80 °C, however, it may retard the decomposition of ettringite and the AFm phases at 80 °C. Interestingly, the exposure of the hydrated cement paste to the laboratory-made brine solution significantly impacts the microstructure of the formed C-S-H product as a cross-linked C-S-H phase is formed at a high ratio of solution to solid. The samples are additionally cured in CO2-injected brine, and the effect of particle size on the carbonation degree is studied. The experimental phase assemblages from the investigations will be used in thermodynamic reactive-transport modeling to estimate the durability and physical properties of the hardened cement on a prolonged time scale

Oxidation of sulfides from secondary materials in cementitious binders as a function of environmental conditions

The process of mining produces large quantities of waste, which may be utilized as secondary materials in the building sector. Mine tailings have been investigated in the context of construction materials in different forms. One way of using these materials is to reduce the clinker quantity of cement used in concrete or mortars to reduce the “embedded” CO2 footprint of infrastructure projects. Corresponding requirements have been implemented for example in the Danish construction regulations, with a planned reduction for allowable CO2 quantities per m2 and 50 years of operation.Utilizing secondary materials can be seen in this context as a step towards the circularity and symbiosis of industrial sectors while also reducing embedded CO2 in concrete. However, many mining ores contain sulfide minerals, which may have a negative influence on the performance and service life of cementitious materials. Hence, it is crucial to understand the oxidation of sulfides in cementitious binders. This study looked at two storing conditions of cementitious materials, underwater and in the air at varying relative humidities, with the aim of investigating the oxidation of sulfides in cementitious materials in aqueous environments and during carbonation. Storing the samples underwater, with partial solution exchange, lead to steady sulfate release during the period of testing. Changing the relative humidity and allowing the carbonation of cement hydrates, which occurred simultaneously with the oxidation of the sulfides led to higher concentration of sulfates in the leachate solution and small increase of ettringite in the cement pastes