Alkali-Activated Concrete (AAC) is considered as a promising alternative to conventional Portland Cement Concrete (PCC) due to its potential to reduce environmental impacts. However, its application in practical engineering is limited by, among others, insufficient understanding of the long-term structural behaviour of reinforced and prestressed AAC elements. To address this, a series of experiments were conducted on composite girders to investigate the long-term flexural behaviour. The composite girder is formed by a prefabricated prestressed AAC inverted-T girder with cast-in-situ ACC topping concrete. The midspan deflection of two composite girders, subjected to self-weight and additional sustained loading, were measured over a 9-month period. Subsequently, flexural tests under four-point bending configuration were performed at the age of 9 months and the reference age of 28 days. The results showed that the specimens tested at 9 months exhibited reduced initial stiffness, decreased cracking load and larger crack widths in the precast prestressed girder compared to those tested at 28 days. The reduction in stiffness likely stems from decreased elastic modulus and structural cracking. Meanwhile, the lower cracking load arises from prestress losses caused by ongoing (restrained) shrinkage and creep, consistent with AAC material test observations. Larger crack widths observed in the precast girder may result from a degradation of bond between AAC and prestressing strands over time. The distinct failure patterns of the 9-month specimens (anchorage failure for sample subjected to self-weight only and flexural failure for sample exposed to additional sustained load), highlighted the role of creep on bond behaviour between prestressing strands and AAC, particularly as a function of varying stress levels at the level of strands. Finally, analytical models were applied to evaluate the prestress loss and flexural behaviour of the specimens. The effective prestressing force and cracking loads at both testing ages were overestimated when the effects of (partially) restrained deformations between precast and cast-in-situ AAC were neglected. More accurate analytical predictions were achieved when these long-term effects and the level of restraint in the composite girder were considered.

In de ambitie de uitstoot van broeikasgassen te verminderen is een centrale rol weggelegd voor groen beton, met een aanzienlijk kleinere CO2-voetafdruk dan gewoon beton [1]. Om een sectorbrede overstap naar dit materiaal mogelijk te maken, moeten economische, institutionele, sociale en technische obstakels worden overwonnen [1, 2 en 3]. Op de TU Delft is de dynamiek rondom deze betontransitie in Nederland onderzocht. Dat onderzoekt richtte zich op hoe opdrachtgevers de integratie kunnen versnellen. Daarbij zijn drie casussen geanalyseerd.

The equivalent conductivities of two aqueous silicate species, [SiO(OH)3]− and [SiO2(OH)2]2−, are fundamental to understanding many physico-chemical phenomena of silicate materials in electrolyte solutions. These phenomena include diffusion, adsorption, and phase transformations. But significant inconsistencies have been presented in published equivalent conductivities of the two silicate aqueous ions. Also, little work has so far been undertaken to discuss how aspects, such as temperature and solution composition, may influence electrolytic conductivity of silicate aqueous solutions. This work presents the equivalent conductivities of the two silicate species, measured with electrochemical impedance spectroscopy (EIS) from 277.85 K to 308.45 K. A conductivity model for mixed electrolytes of high alkaline was first established. This model was then verified with the electrolyte conductivities of NaOH-H2O solutions and NaOH-Na2CO3-H2O solutions. Next, the equivalent conductivities of [SiO(OH)3]− and [SiO2(OH)2]2−, were calculated by solving the overdetermined equation groups for different temperatures, based on electrolyte conductivities of NaOH-Na2SiO3-H2O solutions. The accuracy of both calculations and measurements are examined in depth from various viewpoints. This work presents essential inputs for quantitatively understanding multiple physico-chemical properties of silicate materials in electrolyte solutions.

Compared with blast furnace slag (BFS), the less reactive MSWI bottom ash (MBA) plays a minor role in alkali-activated blends. This research optimized the use of MBA as a precursor by enhancing its contribution to strength and microstructure development. The proposed strategy combines pre-treatment with pre-activation processes, enabling MBA to react before BFS addition. The NaOH-based pre-treatment led to the oxidation of metallic aluminum and the partial dissolution of the amorphous phase in MBA. The subsequent pre-activation resulted in the generation of C-A-S-H gel, which promoted later-stage gel formation in the paste. The reacted bottom ash particles exhibited distinct features in alkali-activated pastes. Compared with 100 % slag-based system, blending slag with MBA accelerated the slag reaction at late ages and facilitated the formation of a more polymerized C-(N-)A-S-H gel. The compressive strength results indicate that MBA is a promising alternative to Class F coal fly ash in BFS-based alkali-activated blends.

Existing research on the mechanisms affecting the strength of cementitious materials primarily focuses on the composition and properties of cement hydration products, often overlooking the interactions between different products. This study presents a systematic experimental and theoretical investigation into the mechanical properties of cementitious materials, emphasizing the interactions between crystal and gel products driven by crystallization pressure. A new mechanism based on crystallization pressure is proposed to explain the impact of the interactions between hydration products on the strengths of cementitious materials. Experiments were conducted by immersing specimens in solutions with tailored ion concentrations (including water, isopropyl alcohol, ethanol, and solutions of calcium hydroxide and calcium acetate) to vary the crystallization pressure. The flexural and compressive strengths of these specimens were then tested. An analytical model was developed and validated against the experimental data. Both experimental and calculated results demonstrate a negative correlation between crystallization pressure and strength. Specimens subjected to crystallization pressures of 101.7 MPa and 147.8 MPa showed reductions in flexural strength of 19.34 % and 30.65 %, respectively, and decreases in compressive strength of 10.00 % and 14.41 %, compared to control specimens with zero crystallization pressure. These results suggest that ion concentrations in the pore solution alter the crystallization pressure, which in turn affects the interactions between crystal and gel products and strength of cementitious materials. This study provides insights into the mechanisms of strength degradation due to moisture in porous materials.

While ensuring the long-term integrity of wellbore sealants is critical for the success of geological carbon storage (GCS), the chemical degradation of conventional materials under CO₂-rich conditions remains a major challenge. This study investigates the carbonation behavior of a one-part granite-based geopolymer, integrating a novel pore-scale simulation framework with experimental validation. A new model, ReacSan, is developed to simulate CO₂ transport and carbonation reactions within the evolving microstructure of the geopolymer under GCS-relevant conditions. The framework incorporates CO₂ dissolution using the Redlich–Kwong equation of state, gel dissolution via transition state theory, ion transport using the Lattice Boltzmann Method, and chemical reactions through thermodynamic modeling. The model was validated through experiments exposing equivalent geopolymer samples to CO₂ under in-situ conditions. The experimentally observed rapid carbonation, leading to a decrease in pore fluid pH and the precipitation of CaCO₃ matched the numerical simulations well, demonstrating the ability of the novel ReacSan framework to capture both temporal and spatial variations in the microstructure and carbonation mechanisms of alkali-activated materials (AAMs) exposed to supercritical CO₂. Based on the demonstrated validity of the model, the model is capable of providing detailed predictions of carbonation progression of AAMs or any other sealants over longer time- and length-scales required to ensure long-term GCS integrity.

Alkali and alkali earth metal ions are normally present in the gels of alkali-activated materials as well as blended PC-based materials. Previous studies have revealed that the leaching of these cations can trigger the change in gel structure and even the gel decomposition. However, the dissolution of cations was rarely known and the underlying mechanisms remained unclear. To address this issue, five calcium-(sodium, potassium-)aluminum-silicate hydrates (C-(N,K-)A-S-H gels) with different Ca/Si ratios (0.8–1.2) and Al/Si ratios (0.1–0.3) were synthesized to investigate the leaching behaviour of Ca, Na and K. For the first time, the dissolution free energies of Ca, Na and K in C-(N,K-)A-S-H gels were calculated using molecular dynamics simulations with the metadynamics method. Experimental results showed that Na showed the highest leaching ratio, followed by K and Ca, attributed to the lowest dissolution free energy of Na. The gel with a higher Ca/Si ratio or a lower Al/Si ratio showed higher charge positivity on the surface, resulting in reduced leaching of the three cations. Additionally, the presence of K was found to promote the dissolution of Na in gels.

This study presents an extended numerical approach based on GeoMicro3D to simulate the reaction kinetics and three-dimensional (3D) microstructure evolution of alkali-activated fly ash (AAFA). Dissolution experiments were conducted under varying NaOH concentrations and temperatures to formulate predictive rate functions for Si and Al release. These experimentally derived kinetic functions, alongside a thermodynamic dataset for N-(C-)A-S-H gels, were incorporated into the GeoMicro3D model to capture the chemical reactions and 3D microstructure evolution of AAFA. The model well captured reaction degree of fly ash, formation of solid products, evolution of pore solution compositions, and porosity over time. Notably, it is the first to predict the time-dependent spatial distribution of phases within the 3D AAFA microstructure by integrating kinetic and microstructural modeling. Dual validation using both dissolution data and microstructural metrics demonstrates the model's reliability and robustness. This integrated framework provides new insights into the coupled chemical–microstructural evolution of alkali-activated materials.

The recycling of municipal solid waste incineration (MSWI) bottom ash as a supplementary cementitious material (SCM) has attracted global attention, driven by the increasing availability of this by-product and the demand for sustainable SCMs to lower CO₂ emissions from cement production. Currently, the widespread use of MSWI bottom ash in the cement industry is hindered by the lack of guidelines to regulate material composition, optimize pretreatment processes, and specify mix design requirements. This review compiles and analyzes literature data on mix design, microstructural evolution, fresh properties, mechanical properties, durability, leaching risks, and environmental impacts of MSWI bottom ash-blended cement pastes, mortars, and concretes. The analysis aims to assess the influence of the pretreatment and physicochemical properties of bottom ash¹ on the microstructure and performance of blended cementitious materials.² The Ash Impact Strength Index (AISI) is introduced to quantify the effects of various factors on compressive strength, enabling direct comparison across different studies. Based on the statistical analysis of the 28-day AISI, the key quality requirements for MSWI bottom ash as an SCM are proposed, along with the optimal mix design. This work provides valuable insights and practical guidance to support the integration of bottom ash into the cement industry.

The growing incineration of municipal solid waste results in hazardous byproducts, particularly municipal solid waste incineration (MSWI) fly ash and air pollution control (APC) residues. The high toxicity of these residues limits their potential for recycling, leading to their direct disposal in landfills. This landfilling poses a significant environmental risk and presents a major challenge in countries with limited availability of land, such as the Netherlands. In this study, the physicochemical properties of Dutch MSWI fly ash and APC residues were evaluated, including, for the first time, an assessment of trace metal concentrations. High concentrations of heavy metals such as Zn, Pb, Cu, and Cd were identified in most MSWI fly ash and APC residues, along with notable concentrations of trace metals like Bi, suggesting new opportunities for resource recovery. The most hazardous residues were characterized by high contents of chloride, sulfate, alkali oxides, or carbonates, along with low calcium content in their chemical composition. These findings provide valuable insights for the targeted treatment and potential recycling of hazardous MSWI fly ash and APC residues currently being landfilled in the Netherlands.

Cementitious materials can achieve desirable strength development and reduced cracking potential under moist or immersed conditions. However, in this work, we found that alkali-activated slag (AAS) pastes can crack underwater, with a higher silicate modulus showing more pronounced cracking. Chemically, the C-(N-)A-S-H gel in the paste with a higher silicate modulus showed a higher Na/Si ratio and a higher leaching loss of Na, which led to more significant structural changes and gel deterioration underwater. This triggered the propagation of cracks initially present in the material. Physically, the paste with a higher silicate modulus featured a denser microstructure, lower water permeability and higher pore pressure, which resulted in a steeper gradient of pore pressure in the matrix. Consequently, the concentration of tensile stress was simulated at the centre and the corner of the cross-section of the sample. As this simulated concentrated stress exceeded the flexural strength of AAS pastes, significant fractures at the centre and spalling at the corner occurred, consistent with the experimental observation. This work not only elucidated the cracking mechanisms of AAS materials underwater but also provided new insights into mixture designs for these materials under high-humidity conditions.

Non-hydraulic lime (also known as air lime) is an ancient carbonatable binder that has regained attention due to its carbon sink potential. Besides lower CO2 emissions during production, air lime also absorbs carbon dioxide during its hardening process. Yet, the challenge with non-hydraulic lime (and alternative binders in general) lies in the absence of data, standards, and replicable studies. For instance, air lime is often used in masonry mortars, where volume stability (shrinkage and creep) is required to ensure structural safety, but research on this issue remains scarce. Therefore, the objective of this study was to apply existing frameworks to experimentally measure the total creep (EN 12390–17:2019) of non-hydraulic lime-containing mortars. Four groups were analyzed, with non-hydraulic lime contents of 100 %, 67 %, 50 %, and 0 % (binder volume). Specimens were subjected to three different curing conditions and then loaded and monitored for up to 240 days. The results showed that (i) unlike Portland cement, non-hydraulic lime mortars needed more time to develop strength under natural environmental conditions; (ii) the phenolphthalein test did not accurately monitor the carbonation depth of air lime-rich mortars; (iii) air lime-rich systems showed less shrinkage, possibly due to carbonation-induced expansion; and (iv) air lime-rich groups exhibited greater creep strains, confirming their high deformability. These findings demonstrate that air lime-containing mortars exhibit a distinct time-dependent behavior, highlighting the need to adapt existing standards for more accurate and reproducible long-term performance assessments.

This research investigated the use of wood biomass fly ash (WBFA) as a key component in developing low-carbon cementitious materials. WBFA was first subjected to water pretreatment and grinding to remove metallic aluminum and free lime, reducing expansion and cracking risks. Characterization of WBFA showed its high calcium and alkali-bearing phases but limited aluminosilicates. Dissolution test showed WBFA had strong alkalinity, suggesting its role as an activator for aluminosilicate-bearing minerals. A novel cement- and chemical-free binary binder was developed using 50 % treated WBFA and 50 % blast furnace slag (BFS). Paste with a water-to-binder ratio of 0.4 achieved 40 MPa compressive strength at 60 days. The use of superplasticizer significantly improved flowability, allowing the water-to-binder ratio to be reduced to 0.25, which resulted in compressive strength up to 58 MPa at 60 days. Calcium aluminate silicate hydrates (C-A-S-H) gels and ettringite were identified as the main reaction products in the pastes.

Under the directives of the RILEM Technical Committee 294-MPA, this publication reports on the findings of an interlaboratory study that tested fiber-reinforced GGBFS-based alkali-activated concrete (FRAAC), with participants from Belgium, India and Slovenia. The research also elaborates prediction models for the tensile splitting strength of GGBFS-based FRAAC. This research endeavoured between 2020 and 2024 to find a globally reproducible FRAAC mix that could attain the required mechanical strength and workability criteria. The primary goal of the interlaboratory study was to generate FRAAC without the use of superplasticizers in order to maintain an S4 class consistency slump and achieve the desired 28-days cube compressive strength of 40 MPa. Steel and PVA fibers were determined to be incorporated to the GGBFS-based AAC mix at 0.3 and 0.1% volume fractions, respectively, through iterative interlaboratory investigations. Experimental program was conducted to examine the compressive and tensile splitting strength of these FRAAC combinations at different curing ages, ranging from 1 to 720 days. The findings indicate that while there were a few interlaboratory variations in the mechanical properties, the FRAAC produced was uniform across all participants. The desired compressive strength of 40 MPa was attained by GGBFS-based FRAAC with both steel and PVA fibers at 28 days. Although FRAAC containing steel fibers exhibited the higher early compressive strength, FRAAC prepared with steel and FRAAC prepared with PVA both demonstrated a 720-days compressive strength of about 61 MPa. The FRAAC mixes with steel fiber additions exhibited a tensile splitting strength that was approximately 30% higher than the mix with PVA fibers. Nonetheless, at all ages, the tensile splitting strength of both FRAAC mixes was clearly higher than 2 MPa. These results support reliable and consistent experimental findings, which allude towards FRAAC as a sustainable substitute for conventional Portland cement concrete.

This study focuses on the numerical modeling of the reaction and microstructure development of a one-part granite-based geopolymer, which is often used for carbon capture and storage (CCS) applications. This work extends the capabilities of GeoMicro3D to model one-part geopolymers containing different precursors and activators (solid and in solution). The model considers the particle size distribution of different solids and the real shape of particles to prepare the initial simulation domain. Further, the dissolution rates of different solids estimated from the experiments were used to model the dissolution of different elements in the pore solution. Subsequently, the model utilizes classical nucleation probability modeling coupled with thermodynamic modeling to estimate the precipitation of products in the microstructure. Experiments were performed to study the pore solution, reaction degree, and amount of products in the microstructure, which were further compared with the simulation results to check the rationality of the model.

This report presents a meticulous synthesis of collaborative interlaboratory research conducted within the purview of the RILEM Technical Committee 294-MPA, with two expert groups named RRT1 and RRT2, and encompassing ten participants from Belgium, China, Finland, India, Italy, Japan, the Netherlands, and the United Kingdom. The RRT1 expert group mainly focused on the ground granulated blast furnace slag-based alkali-activated concrete (GGBFS-based AAC) mix design and mechanical properties. In turn, the RRT2 expert group focused on the fresh properties of GGBFS-based AAC. The investigation, conducted between 2020 and 2024, aimed to establish globally reproducible mix design and mixing protocols for GGBFS-based AAC. Developed by the RRT1 and RRT2 expert groups, these protocols have emerged through iterative experiments followed by a comprehensive interlaboratory study. The outcomes highlight the reliable production of GGBFS-based AAC across participants, with minor deviations in fresh and mechanical properties that are largely consistent with those observed in Portland cement concrete (PCC). The primary objective of the developed GGBFS-based AAC mix design was to achieve a defined consistence class S4, while targeting a compressive strength threshold of approximately 50 MPa at 28 days. This objective was effectively realized, with the average compressive strength values reaching 56 MPa at 28 days and 64 MPa at 720 days. While the average splitting tensile strength stabilized at 3.2 MPa over the 720 day period. These findings underscore the growing importance of AAC within the construction sector, particularly due to its reproducible and reliable experimental results, as the industry increasingly shifts toward more sustainable alternatives to traditional cement-based materials.

Efflorescence is a significant aesthetic and structural issue for alkali-activated materials (AAMs). This study addressed this issue at the aggregate level for the first time. The results indicated that substituting sand with aluminosilicate-based lightweight fine aggregate (LWFA) by 20 % or 50 % in volume reduced the efflorescence of alkali-activated slag (AAS) mortars by 14.6 % or 43 %, respectively. The mitigation mechanisms of LWFA were proposed in terms of gels, pore solution and microstructure. Specifically, the pozzolanic reaction of LWFA provided gels with additional Si and Al, which contributed to binding Na⁺ in the pore solution, thereby reducing the available Na for efflorescence formation. Combined with its internal curing effect, LWFA densified the surrounding pastes, which hindered the transport of ions and water, thus limiting the formation of efflorescence products. Furthermore, the incorporation of LWFA enhanced the flexural strength of mortars without significantly compromising compressive strength.

With the development of waste recovery techniques, previous research has revealed that coarse fractions of municipal solid waste incineration (MSWI) bottom ash (BA) after proper treatment could be applied in the construction sector, while the fines are seldom recovered in practice and normally landfilled. This study explores the potential application of fine MSWI BA (0–2 mm) as a supplementary cementitious material (SCM) in Portland cement (PC) mixtures. Mechanical and chemical pre-treatment approaches have been designed with various conditions to optimize the treating process. The chemical and mineralogical compositions, as well as the metallic Al content in BA were characterized before and after the pre-treatment. It was found that both methods are effective in removing the metallic Al content in BA, Moreover, BA derived from mechanical treatment exhibited more contribution to the hydration reaction in PC mixtures, as revealed by the amount of reaction products and mineral phases formed in hardened trial mixtures. BA obtained was further partially blended in PC mortars to evaluate the performance as compared to SCMs and inert fillers. It was found that treated BA resulted in a slight retarding effect on the reaction kinetics. Treated BA behaved better than the coal fly ash to contribute to the strength development, while the inclusion of BA did not lead to significant influences on the workability.

Carbonation of alkali-activated slag (AAS) materials has been primarily concerned in atmospheres with gaseous CO₂. This study, by contrast, highlights that AAS pastes would also be carbonated under tap water immersion. Calcite is the main CO₂-bear phase in both sodium hydroxide- and sodium silicate-activated AAS pastes, and the paste pre-cured for a longer curing period shows more severe carbonation. Additionally, calcium carbonate can densify the deteriorated microstructure of sodium hydroxide-activated paste caused by long-term leaching. The indentation modulus of pastes subjected to tap water immersion is higher than those under deionized water immersion. The uptake of CO₃^2- by hydrotalcite (Ht) and gels is also detected, resulting in the formation of Ht-CO₃ and decalcification of gels. Due to the synergistic effect of leaching and carbonation, a characteristic layered distribution of pastes close to the exposure front is observed, comprising the carbonated layer, transitional (carbonated + leached) layer, and leached layer, progressing from the outermost to the inner regions. Eventually, the kinetics of underwater carbonation, as well as the discrepancy between dry and underwater carbonation, is revealed.

Efflorescence presents not only as a cosmetic concern but also as a structural issue, which impacts the performance of alkali-activated materials (AAMs). In this study, the mechanisms of efflorescence of alkali-activated slag (AAS) pastes are investigated. First, the efflorescence of AAS pastes with different alkali dosages (3 %, 5 % and 7 %), activator types (sodium hydroxide (NH) and sodium silicate (NS)), exposure atmospheres (ambient, N₂ and 0.2 vol% CO₂), and relative humidities (40 %, 60 % and 80 %) was observed. Subsequently, leaching tests were performed and the impacts of efflorescence on AAS pastes at different heights were studied. It was found that a lower relative humidity facilitated more rapid and severe efflorescence. The positioning of efflorescence products was dependent on the porosity of the matrix. Compared to NH pastes, NS pastes subjected to semi-contact water conditions were more vulnerable to cracking problems, which turned out to be exacerbated by the formation of efflorescence products. A new method to quantify efflorescence was developed and it corresponded well with both efflorescence observations and leaching experiments. Furthermore, a competitive reaction between Ca and Na in the presence of carbonate ions was identified. CaCO₃, a representative product of natural carbonation, was rarely found in the regions where efflorescence products (sodium carbonate) formed. Regarding compressive strength, NS pastes were more adversely affected by efflorescence than NH pastes.