Supplementary cementitious materials: New sources, characterization, and performance insights

Conventional supplementary cementitious materials (SCMs), such as blast furnace slags or fly ashes, have been used for many decades, and a large body of knowledge has been collected regarding their compositional make-up and their impacts on cement hydration and concrete properties. This accumulated empirical experience can provide a solid, confident base to go beyond the status quo and develop a new generation of low-clinker cements composed of new types and combinations of SCMs. The need for new sources of SCMs has never been greater, as supplies of traditional SCMs are becoming restricted, and the demand for SCMs to reduce CO2 emissions from concrete production is increasing. In this paper, recent research on emerging SCM sources is reviewed, along with new developments in characterizing and qualifying SCMs for use and improved knowledge of SCMs on long-term concrete performance and durability

Advances in near-neutral salts activation of blast furnace slags

The utilisation of near-neutral salts as activators to produce alkali-activated slag cements offers several technical advantages, including reduced alkalinity of the binders, minimising the risk associated with handling of highly alkaline materials, and better workability of the fresh paste compared to that of sodium silicate-activated slag cements. Despite these evident advantages, the delayed setting and slow early-age mechanical strength development of these cements have limited their adoption and commercialisation. Recent studies have demonstrated that these limitations can be overcome by selecting slags with chemistry which is more prone to react with near-neutral salts, or by adding mineral additives. A brief overview of the most recent advances in alkali-activation of slags using either sodium carbonate or sodium sulfate as activators is reported, highlighting the role of material design parameters in the kinetics of reaction and phase evolution of these cements, as well as the perspectives for research and development of these materials

Layered Double Hydroxides Modify the Reaction of Sodium Silicate-Activated Slag Cements

The impact of adding two types of layered double hydroxides (LDHs), commercial hydrotalcite (HT) and its thermally treated form (CLDH), on the reaction kinetics and phase assemblage development of a sodium silicate-activated slag cement was investigated. The reaction kinetics of LDH-modified cements was assessed by isothermal calorimetry, and the results were correlated with in situ attenuated total reflection Fourier transform infrared spectroscopy results collected over the first days of reaction, to identify the structural evolution of the main binding phase forming in these cements: a sodium-containing calcium aluminosilicate hydrate (C-(N)-A-S-H)-type gel. The addition of either HT or CLDH into sodium silicate-activated slag paste accelerates the precipitation of reaction products and increases the formation of HT in these cements, without causing significant changes to the C-(N)-A-S-H binding phase. This is extremely relevant in terms of the durability of alkali-activated slag cements, as a higher content of the HT-like phase has the potential to reduce their chloride permeability and enhance carbonation resistance

Circular economy strategies for concrete: Implementation and integration

Concrete is the world's most widely-used anthropogenic material, and Circular Economy strategies will be key to addressing the myriad challenges that face its use today and into the future. Despite a rapid growth of research interest in developing Circular Economy strategies for concrete, this has mostly focussed on technical and environmental issues at the material and product scale. Holistic approaches considering wider social and political aspects as well as system-scale perspectives have been relatively neglected. This article uses a narrative review to investigate three outstanding questions to help address this gap: how concrete's material, product and system-scale attributes influence the interpretation of Circular Economy principles; how the full range of Circular Economy strategies can be implemented for concrete; and what the likely implementation issues will be when integrating different Circular Economy strategies (such as design for durability, component reuse and material recycling). From a product-scale perspective, it is argued that greater specificity is needed around the growing diversity of concrete materials and products in Circular Economy discourse - their properties are often distinct and hence specific strategies are not necessarily universally applicable. At the same time, a solely product-centric Circular Economy perspective is insufficient for concrete, and only joint consideration of structural and systemic perspectives will yield satisfactory solutions. ‘Soft’ perspectives of social, political and legal aspects cannot be viewed simply as an added bonus, but are essential to reconciling the ‘hard’ issues of technical, environmental and economic aspects that dominate discussions. Whilst concrete can and should have a key role in a Circular Economy, its success will require more than just extensions of linear economy thinking

Structural Ordering of Aged and Hydrothermally Cured Metakaolin Based Potassium Geopolymers

This study evaluates the potential correlation between natural aging and hydrothermal curing (accelerated aging), related to the crystallisation of zeolites in potassium-based metakaolin geopolymer binders. 7-year old sealed-cured specimens, formulated with varying silicate contents, were evaluated. The effect of different accelerated aging durations on the mineralogy of these potassium-based geopolymers was also assessed. The results show that although zeolite formation is favoured under both natural and accelerated aging in potassium-based geopolymers, different types of zeolites are formed depending on the silicate content added to the mix, and the curing conditions of the specimens

Advances in alkali-activation of clay minerals

To future-proof alkali-activation technology, there is a need to look beyond well-established precursors such as fly ash and blast furnace slag, due to resource competition, geographical distribution and technical limitations. Clay minerals are abundant and diverse aluminosilicate resources available around the world. However, due to the mineralogical complexity amongst the most common 1:1 (kaolinite, halloysite) and 2:1 (montmorillonite, illite) clay minerals, and practical issues such as workability, their use has been more limited. Recent advances have improved understanding both of pre-activation treatments (thermal, mechanical, chemical), and of the factors influencing clay reactivity, phase assemblages and properties of final products. This opens new opportunities for the exploitation of these resources to produce sustainable cements. A one-size-fits-all approach for processing and activating clay minerals is not viable. Instead, activation routes need to be tailored according to the clay mineralogy to achieve the binder properties required for key applications

Use of supplementary aggregates in mortars produced using a novel lime drying technique

Formulated Lime mortars suitable for long-term storage in silos or bags can be produced by adding quicklime to wet, as-received quarried sand. However, sands with high water content may require the addition of so much quicklime that would alter the mortar proportioning. This work investigates the possibility to replace part of the lime-dried sand with dry crushed mixed glass cullet and calcium carbonate aggregate to allow greater control over mortar formulations. It is shown that the use of glass yields a similar or slightly weaker product, depending upon curing regime, than the control whilst calcium carbonate generates the strongest mortar

Slag-Based Cements That Resist Damage Induced by Carbon Dioxide

The use of sodium carbonate as an activator to prepare alkali-activated cements from blast furnace slag and calcined hydrotalcite offers many attractive performance and environmental benefits. However, the understanding of the long-term performance of these cements is limited. In this study, the resistance of sodium carbonate-activated slag cements to carbonation attack was determined under natural (0.04%) and elevated (1.0%) CO₂ concentrations. Two calcium carbonate polymorphs, calcite and vaterite, were formed as carbonation products at a longer time of CO₂ exposure. A cross-linked alkali aluminosilicate gel and a Ca-deficient calcium (alumino)silicate hydrate gel were identified to form by decalcification of the main binding phases initially present in these cements. However, despite these carbonation-induced mineralogical changes, the mechanical strength after carbonation was comparable to that of noncarbonated specimens, which is contrary to previous observations of strength loss due to carbonation of slag-rich cements. The high carbonation resistance of sodium carbonate-activated slag cement indicates these materials have the potential to resist attack by atmospheric CO₂ in service with sustained mechanical performance