International Concrete Abstracts Portal

Due to its predominant soda-lime composition, most post-consumer glass processed by recycling facilities would be classified as high-alkali pozzolanic glass powder (GP). In cementitious matrices, the intrinsic alkaline pore solution induces the dissolution of both silica and alkali ions. Therefore, the GP can potentially induce two similar reactions in concrete: either a deleterious alkali-silica reaction or a pozzolanic reaction. The equilibrium of the pore solution will determine which reaction will prevail in the long term. To understand the chemical stability of GP in a cementitious system, the evolution of the solubility of key elements in an alkali-rich synthetic pore solution was studied as a function of reaction time, particle size, presence of Ca(OH)₂ and CaCO₃, and binder/solution ratio (B/S). The solution was based on the R³ method, which consists mainly of lab-grade chemicals such as KOH and K₂SO₄. Although the chemical equilibrium seems to be fully reached in the first hours of hydration, the main products, such as C-S-H, are unstable because the alkali leaching/uptake in the C-S-H chains is dynamically evolving. The experiments show that both C-S-H precipitation and alkali leaching rates increase with increasing B/S ratio and decreasing particle size, and are directly related to the presence of calcium in the solution.

Due to the drastic necessity to reduce cement and binder carbon footprint and because of the increasing scarcity of traditional supplementary cementitious material, there is an increasing interest in non-traditional reactive materials with low CO₂ footprint, eventually coming from the circular economy.

There are several emerging opportunities that need to be investigated before confirming their aptitude to substitute clinker, alone or in combination with other materials. Among these opportunities, reclaimed fired clay roof tiles and bricks represent a good candidate not only because they were historically used by the antic Romans but also because they represent a significant part of demolition wastes.

These materials, gathered from different locations in France with different ages, were chemically and mineralogically characterized. Their potential pozzolanic character was assessed by mean of R³ tests with a follow-up of the evolution of hydrates suite (consumption and precipitation), doubled with mechanical strength on mortar for different formulated binders, combining materials to simulate standardized cement types according to European standards EN197-1 and -5. The overall results indicate their reactivity was good enough to conclude that these materials can be considered as potential alternative SCMs.

The use of calcined clays as a substitute for traditional cement materials has the potential to significantly reduce the carbon emissions of the cement industry. However, for their widespread adoption and urgent need to feed the cement industry, it is essential to have a comprehensive understanding of the methods involved in processing these clays to ensure maximum reactivity. Thermal treatment of these clays induces chemical reactions that transform the combined materials into reactive pozzolan by eliminating hydroxyl groups from the clay structure, resulting in the activation of alumina and silica oxides. A commonly employed industrial method for activating these clays is through the use of a rotary kiln. With an optimal temperature profile and material retention time, rotary kilns play a crucial role in ensuring the production of high-quality calcined clays. This study aims to enhance control performance and achieve a highly reactive marl by optimizing the preparation process through three steps: (1) characterizing the raw materials, (2) optimizing the kiln parameters, and (3) conducting the life cycle assessment of the process. The reactivity of the calcined marl will be evaluated using the ASTM C1897 Standard Test Methods and the strength activity index.

In this study, we use different natural clays with a variety of mineral and chemical compositions to seek to understand the influence of the secondary minerals (i.e. not kaolinite) on early-age performance. The study shows that the R³ test is not suitable to assess the early-age reactivity, and early-age properties (compressive strength, rheology). It is also shown that the early-age performance of calcined clays is not dominated by metakaolin content and that secondary phases or impurities – for example, iron phases have a significant impact. A clinker-free model system with the aim of studying early-age reactivity is introduced.

Calcined clay is a well-established supplementary cementitious material (SCM) in the cement industry. Its high pozzolanic reactivity – upon thermal treatment – and widespread availability offer the potential to reduce the clinker factor substantially. While extensively explored, research has primarily focused on muffle furnace calcination, with limited studies available on understanding the influence of varying calcination methods and scalability. Thus, it is crucial to generate new research data that allow for optimizing different calcination methods. In this study, two clays – with different kaolinite and iron impurity contents – calcined under three laboratory methods, namely a muffle furnace, a rotary kiln, and an entrained flow reactor are investigated. The calcined samples are characterized by TGA/DSC, XRD, and the R³ bound water test to assess: a) dehydroxylation degree, b) recrystallization peak formation, and c) pozzolanic reactivity. In addition, PSD and BET are used to explore structural changes in the calcined samples regarding agglomeration and specific surface area, while touching on deposit formation and color control of flash calcined clays. The results show that both soak and flash calcination techniques can deliver optimal pozzolanic reactivity for both clays. DSC has shown a good correlation by accurately indicating the recrystallization. Agglomeration and specific surface area rates were significantly affected by flash calcination, requiring after grinding. The clay containing higher iron content demonstrated a grey color after flash calcination, while at high temperatures deposit formations were limiting the material’s collection from the entrained flow reactor. Ongoing research of samples with varying iron contents will further evaluate this occurrence.