Alkali-activated cements and concretes – Durability testing to underpin standardisation

Alkali-activated cements, including \u27geopolymer\u27 materials, are now reaching commercial uptake in the UK and elsewhere, providing the opportunity to produce concretes of good performance and with reduced environmental footprint compared to established technologies. The development of performance-based specifications for alkali-activated cements and concretes is ongoing in many parts of the world, including in the UK where the world-first British Standards Institute (BSI) Publicly Available Specification PAS8820:2016 has been published to describe these materials and their utilisation. However, the technical rigour, and thus practical value, of a performance-based approach to specification of novel cements and concretes will always depend on the availability of appropriate and reliable performance tests. This paper will briefly outline the requirements of PAS8820, and discuss the activities of RILEM Technical Committee 247-DTA in working to validate durability testing standards for alkali-activated materials, bringing scientific insight into the development of appropriate specifications for these materials

Performance of sodium carbonate/ silicate activated slag materials

Alkali-activated slag (AAS) materials are acknowledged as environmentally friendly due to the reduced embodied energy associated with their production. However, the use of highly alkaline solutions such as sodium silicate to promote the chemical reactions that lead to their hardening, poses potential human and environmental hazards that might constrain their utilization beyond specialized applications. It is possible to use less alkaline solution based on near-neutral salts as activators, such as sodium carbonate, to produce alkali-activated slag binders with desirable properties. However, to achieve this, the ‘right match’ between slag chemistry and activation conditions is required. The use of sodium carbonate presents several advantages compared with using sodium silicate when producing AAS, including reducing alkalinity to values comparable to that of Portland cement, and extending the setting time and improving workability, which facilitates the casting of these materials. Sodium carbonate-activated slag binders do not always meet the setting time and strength requirements for on-site concreting, which has limited the application of these materials. A recent study in pastes demonstrated that the addition of sodium silicate in these binders significantly improves the compressive strength development, while effectively controlling the kinetics of reaction, which makes AAS binders produced with a blend of activators an attractive candidate for producing concretes. In this study we report compressive strength, water absorption and durability properties of AAS concretes produced with a blended sodium carbonate/silicate activator. Shrinkage microcracking of these materials was also studied, by drying the specimens for 8 weeks at 65% relative humidity (RH) and 23ºC. The results obtained are compared with concretes produced solely using sodium silicate as alkali activator

Critical thinking on efflorescence in alkali activated cement (AAC)

Alkali-activated cement (AAC), also known as “geopolymer”, has been extensively investigated over the past 40 years and has been developed from laboratory mock ups to real structural usage in construction in the last decade. While numerous life cycle analyses and carbon accounting studies show the “green potential” of this material compared to Portland cement, some authors state that the high alkali concentration in AAC is a potentially unstable factor which may lead to, for example, efflorescence. This paper presents a critical thinking on the literature and some new experimental work regarding the possibility of efflorescence in AAC products. Subjects of the discussion include: (1) the role of alkalis in AACs, (2) the effect of alkali concentration on efflorescence, (3) the effect of solid precursor selection on efflorescence, (4) the effect of curing scheme and chemical additives on efflorescence, and (5) the impacts of efflorescence on the microstructural properties of AACs. Particular attention is given to the relationship between pore structure and efflorescence behaviour, and consequently the mechanical properties of AACs suffering from either efflorescence or alkali loss (by leaching). The changes in sodium aluminosilicate hydrate (N-A-S-H) gels due to efflorescence or alkali loss are critical to the durability of AACs. This paper emphasizes that the nature of the solid precursor and the pore structure of the resulting AAC are the two most important factors that control efflorescence rate. However, considering its alkaline nature, it seems difficult or impossible to avoid this issue in AAC products, although kinetically controlled diffusion of alkalis using phase transformation techniques may help to mitigate efflorescence. Efflorescence in AAC is a “skin issue” that needs to be carefully treated. It is recognized to be different from the visually similar, but chemically distinct, efflorescence that occurs in Portland cement based materials

Assessing the long-term structural changes of metakaolin geopolymers encapsulating strontium loaded ion-exchanger

Zeolite-type inorganic ion-exchangers are extensively used in the nuclear industry to remove fission product radionuclides from contaminated process water and in groundwater cleanup. A significant amount of ion-exchangers loaded with concentrated radioactive isotopes of Sr are generated every year, and this is a particularly pressing issue in the Fukushima Daiichi site, where minimising the environmental release of these radioisotopes is currently the focus of much work. Encapsulation of these granular radionuclide-loaded ion-exchangers, which are often stored as slurries, into a stable solid waste form (as required for disposal) with a low leaching rate of toxic ions is challenging but critical for the safety of long-term geological disposal. Metakaolin geopolymers are attracting interest in the immobilisation of nuclear wastes. However, only limited information is available from the literature regarding the stability of key ion-exchangers in geopolymer binders, and the potential modifications occurring in the binder materials as a function of interactions with the ion exchangers. In this study, an ion exchanger representing those used in the Fukushima Daiichi wastewater treatment process, loaded with inactive isotopes of Sr, was encapsulated using metakaolin-based geopolymers. Different alkali cations were used as activators and the effects of different reaction temperatures were also assessed. The phase evolution, dimensional stability, and changes in microstructure of the geopolymer binders containing Sr-loaded ion-exchanger were characterised up to 1 year, to provide important information for evaluating the partitioning of Sr between the pore solution, ion-exchangers, and the binder

The origin of the acceleration period in alkali-activated fly ash

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Electrochemical characterisation of cement hydration and properties by alternating current impedance spectroscopy

The complexity of the chemical and microstructural evolution of cement during the hydration process can be evaluated using many different characterisation techniques. One of these techniques which has been demonstrated to be useful, but not yet fully accepted by the cement research community, is Alternating Current Impedance Spectroscopy (ACIS). However, although ACIS is a non-destructive, rapid, and easily implemented technique, it has been found in the past that it has several limitations such as electrode contact, electrode area dependence, ground coupling effects, complications due to inductance at high frequencies, and a lack of mathematical and physical rigour in much of the data interpretation. Please click Additional Files below to see the full abstract

Editorial: 2022 Retrospective: structural materials

© 2024 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/Welcome to the Research Topic “2022 Retrospective: Structural Materials.” This curated Research Topic highlights esteemed spontaneous articles from the past few years, personally selected by our Chief Editor, Prof. John L. Provis. The work showcased here underscores the extensive spectrum of research conducted within the section and seeks to illuminate its primary areas of interest. All the studies presented here demonstrate significant advancements in theory, experimentation, and methodology, offering solutions to compelling problems. Therefore, the editorial team deems it crucial to give special attention to these matters.Peer reviewe

Alkali-activated mineral wools

Mineral wools –a general term for stone wool and glass wool– are the most common building insulation materials in the world. The amount of mineral wool waste generated in Europe totaled 2.3 Mt in 2010 – including wastes from mineral wool production and from construction and demolition industry. Unfortunately, mineral wools are often unrecyclable due to their fibrous nature (Figure 1) and low density. Thus, the utilization of mineral wool waste in post-consumer products remains low. Please click Additional Files below to see the full abstract

Effect of calcination method and clay purity on the performance of metakaolin-based geopolymers

The calcination of kaolinite clay to produce metakaolin can be achieved using a range of processes, including rotary, fluidised bed and flash calcination. Rotary calcination was the most popular of these processes for many years as it takes place in a rotary kiln, which is readily available, at easily attainable temperatures of 650 – 800 °C. However, in recent years’ flash calcination processes have become more widely used, and the technology has advanced to a point where commercial flash metakaolin-based geopolymers are now available. Flash calcination involves the rapid heating of clay at temperatures of around 1000 °C for less than a few seconds. The differences in these calcination methods can have a notable effect on the structural ordering of the metakaolin itself, as well as playing an important role in defining the chemical and physical properties of metakaolin-based geopolymers. The purity of the clay also plays a key role in the chemistry of the geopolymers produced. Calcined clay-based geopolymers can be used as construction materials or for the immobilisation of problematic wastes, among other applications, as they can offer desirable performance characteristics. The chemical and physical properties of these geopolymers, and thus the influence of the clay source on key performance parameters, will need to be fully understood when deciding how they can be used for many different applications. This study demonstrates the effect of the calcination method on the properties of calcined metakaolin geopolymer systems for waste immobilisation applications. A main focus of this study is the rheological properties, as the flow properties of these systems are one of the most important parameters for many geopolymer applications. The porosity, heat evolution and mineralogical development of these systems is also presented, with a view towards assessing performance in targeted applications for the immobilisation of nuclear waste

Impact of water content on the performance of alkali-activated slag concretes

In this study, we report the effect of varying the water/binder (w/b) ratio on the performance of sodium silicate activated concretes. Compressive strength development and water transport properties of these concretes were assessed, along with their resistance to carbonation. The results demonstrate that varying the water content within a reasonable range induced negligible changes in the compressive strengths of these concretes, when a constant paste content was used. A direct correlation between the w/b ratio and the amount of permeable voids in the concretes was not identified. The carbonation behaviour of these concretes changes prominently depending on the CO2 concentration of exposure, meaning that comparable accelerated carbonation rates were observed at varying w/b ratios, conversely to observations under natural carbonation conditions where w/b was significant in defining the carbonation rate