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

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

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 binder

On the relevance of volume increase for the length changes of mortar bars in sulfate solutions

The ingress of sulfate ions into cementitious materials leads to the formation of ettringite, gypsum and other phases. The increase in solid volume through the formation of these phases is often assumed to be the only reason for expansion. In this paper we systematically compare the volume increase predicted by thermodynamic modeling to macroscopic expansion for mortars made with CEM I in different sulfate solutions and for mortars made with a range of blended cements in sodium sulfate solution. It is shown that the length changes cannot be explained by simple volume increase alone. A more plausible explanation of expansion lies in the theory of crystallization pressure, in which crystals forming from a supersaturated solution may exert pressure on their surroundings. It is observed that expansion occurs in systems where thermodynamic modeling predicts the co-existence of ettringite with gypsum. In such a case, if monosulfate and gypsum are both present locally, the solution can be highly supersaturated with respect to ettringite, whose formation in confined conditions (such as within C-S-H) can then exert expansive forces. (C) 2013 Elsevier Ltd. All rights reserved