Deterioration of mortar bars immersed in magnesium containing sulfate solutions

Mortars prepared with a CEM I and a CEM III/B binder were investigated in different magnesium sulfate solutions. The main deterioration mechanism for the CEM I was expansion, while surface erosion was dominant for CEM III/B. The presence of sodium, potassium and calcium in a magnesium sulfate solution led to less expansion and less surface deterioration for both, CEM I and CEM III/B, than which was observed in solutions containing only sodium or magnesium sulfate. The presence of a mixture of different cations seems to lower both the surface deterioration and the expansion and might explain why sulfate attack damages are not as frequent in the field as in laboratory tests. Sulfate binding before cracking/expansion is similar in the presence of all different solutions investigated, indicating that the speed of sulfate ingress and the amount of bound sulphate depends during the first months mainly on the binde

Chloride binding in Portland composite cements containing metakaolin and silica fume

This paper investigates how the composition of Portland composite cements affects their chloride-binding properties. Hydrated cement pastes prepared with a reference Portland cement and composite Portland cements containing metakaolin and/or silica fume were exposed to NaCl or CaCl2 solutions. Chloride-binding isotherms were determined and the hydrate assemblage was investigated using TGA, XRD, 27Al NMR, 29Si NMR and thermodynamic modelling. Compared to the reference Portland cement paste, silica fume replacement did not alter the chloride-binding capacity. The metakaolin replacement resulted in the highest chloride-binding capacity. When combining silica fume with metakaolin, the chloride binding is similar to the reference Portland cement. In this study the differences in chloride binding were linked not only to changes in the AFm content, but also to alterations in the Al-uptake and chain length of the C(-A)-S-H

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

Influence of limestone on the hydration of ternary slag cement

The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cements have been investigated. Commercial CEM I 52.5 R was blended with slag and limestone; maintaining a clinker to SCM ratio of 50:50 with up to 20% slag replaced by limestone. The sulphate content was maintained at 3% in all composite systems. Hydration was followed by a combination of isothermal calorimetry, chemical shrinkage, scanning electron microscopy, and thermogravimetric analysis. The hydration of slag was followed by the implementation of QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different w/s ratios. Thus, the method was used to analyse hydrated cements without dehydrating the specimens. The results show that the presence of limestone enhanced both clinker and slag hydration. The pore volume and pore solution chemistry were further examined to clarify to the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus lowering of the aluminium concentration in the pore solution led to the improved slag hydration

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

The material recycling options and current research activities for the circular use of the construction composite concrete

Construction materials account for a large fraction of materials handled by humans, and a correspondingly large footprint, ranging from extraction/mining of raw materials, embodied CO2 through materials production /refinement, and waste of construction activities and demolition. Concrete, after water the second most used material globally, is a staple when it comes to infrastructure in general. It is a composite material made from cement, sand, gravel water, and possibly other admixtures and additives, and their recipes/proportions can be as complex as their value chains. Concrete can be understood as man-made geology as it is an artificial stone, which is ideally tailored to its functionality, but its components may differ in chemical composition and therefore have different properties. Additionally, the material may change according to its environmental conditions, which may reduce the circular economy (CE) options for reuse and recycling. Integrating CE principles in the construction sector would ideally lead to the material reuse of construction elements as this provides the biggest CO2 benefit. However, the material properties and their possible determine the lifetime, maintenance costs, and risk for owners, operators, investors, and construction activities. Reduce, reuse, and recycle, for example, have all different meanings for the design, maintenance, and end-of-life of concrete structures depending on the type of composite/concrete and the proposed reuse strategy. It is then crucial to understand concrete properties at the end of life so we can define appropriate CE avenues (reuse, recycle, etc). This manuscript explains the most common resources (cement, sand, gravel water, and other admixtures and additives) used, and their influence on the CE-relevant design choices which ultimately also define the possible reuse or recycling routes. Additionally, a literature overview of current research topics will be provided that allows insight into reuse and recycling options for concrete. This becomes increasingly interesting as the traditional linear economy practices are threatening the supply chain of seemingly abundant materials such as sand and water as well. Hence the transition from a linear to a circular economy might not only help with reducing CO2 emissions but resource uses, even though additional work/energy/CO2 will be required to produce first-grade recycling materials that can replace virgin materials 1:

Binary and Ternary Shale Binders with High Replacement Levels

This paper investigates mortars with fifty percent cement replacement of supplementary cementitious materials in binary and ternary blends, according to DS/EN 197-5: 2021. A new standard that allows for up to 50% of cement replacement levels than previously. Different aspects ranging from rheology, mechanical properties, and mineralogical changes were measured. The selected shale was ground in a laboratory disk mill, blended and tested in binary blends (only shale), and together with limestone filler as ternary blends. As expected, the mechanical properties of these mortars are lower than the mortar made only with Portland cement. The binary binder, with 50% cement replacement by calcined shale alone, developed larger compressive strengths and larger reductions in portlandite than the ternary binder, due to the additional pozzolanic reactions. The replacement of one-third of the shale by limestone filler, with a total cement replacement of 50%, had the lowest compressive strength values but less superplasticizer demand for the target workability. This allows, when judged by the rheology and mechanical properties alone, a mixture of both SCMs might be beneficial, for example where no risk of corrosion would be expected (X0, XC1). Furthermore, one might consider the optimization of the relation between the calcined shale and limestone where CO2 emissions are being reduced