Alkali-activated concrete (AAC) is a sustainable alternative to ordinary Portland cement concrete, but its large-scale structural performance remains insufficiently understood, particularly in terms of long-term durability. To ensure safe application, continuous monitoring of AAC structures is essential. This paper develops and validates ultrasonic-based damage indicators (DIs) intended to support future lifetime monitoring of precast AAC bridge members. Full-scale laboratory tests were performed on two prestressed AAC beams and a solid slab consisting of three beams with embedded piezoelectric sensors. Active ultrasonic measurements collected throughout loading were processed to derive two DIs: (1) reduction in waveform coherency using direct wave interferometry to indicate crack initiation, and (2) relative wave velocity obtained from an arrival-time picker to track crack propagation. The waveform coherency-based DI consistently identified the onset of cracking at or even before the first visible cracks appeared in digital image correlation (DIC) images, while the velocity-based DI provided a qualitative measure of crack propagation and orientation. Both indicators responded sensitively once degradation developed, enabling early warning of structural deterioration. The validated DIs are intended to inform the development of a lifetime monitoring scheme on a pilot precast AAC bridge on a Dutch national road. This study also provides a practical pathway toward risk-informed operation and broader adoption of AAC in bridge applications.

The use of fibers to address tensile cracking in alkali-activated concrete (AAC) is a topic of ongoing research. The addition of fibers enhances the tensile and flexural characteristics of all types of concrete including AAC. However, the mixing process, the setting time, and the workability are compromised due to the presence of fibers in the hardened matrix. There are different material categories of fibers such as steel, synthetic, carbon, and organic fibers that can impart different characteristics depending on the intended usage of concrete. The present chapter reviews the different types, orientations, dosages, and geometric properties of fibers included in AAC. along with the effects of fiber addition on the mix design, mixing and curing procedure of concrete, as well as the fresh and hardened characteristics of fiber-reinforced alkali-activated concrete (FRAAC). The use of statistical models to predict the mechanical characteristics of FRAAC is also discussed. Finally, the chapter presents the advantages, disadvantages, and safety precautions for this material followed by recommendations for practical usage.

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.

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.

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.

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.

The use of alkaline activator in alkali-activated materials (AAMs) may pose risk of alkali-silica reaction (ASR), and the variations in the mixture design could have great influence on the performance of AAMs system. In this case, this paper investigated the effects of slag fineness (3000–8000 cm²/g) and water-to-binder (w/b) ratio (0.5–0.8) on ASR behavior of alkali-activated slag (AAS) mortars under accelerated mortar testing conditions as specified in ASTM C1260. The length change, mass gain, microstructure and formation of ASR products were examined to evaluate the degradation caused by ASR. It was found for the first time that slag fineness induces a “pessimum effect” in the ASR expansion of AAS mortars. On the other hand, there is a “pessimum effect” in the influence of w/b ratio on ASR expansion in the early-stage (≤14d), and the induced expansion increased with an increase in w/b ratio in the late-stage (>14d). The mechanism governing the effect of slag fineness and w/b ratio is complicated and cannot be explained solely by the properties of ASR products. This work contributes to the understanding of ASR in AAMs system and could provide a basis for the mixture optimization of AAMs.

The shortage of high-quality fine aggregate as an essential component of concrete has become an emerging worldwide concern for the construction industry. Concrete typically comprises up to 30% fine aggregate, which largely influence the strength and durability of the final product. Therefore, finding suitable substitutes for natural fine aggregate has become an important aspect of current concrete research. In this study, we investigated the suitability of using remediated thermal-treated soil and tar-containing asphalt as secondary raw materials in self-compacting concrete (SCC) mixture. The remediated materials were used as both (1) fine aggregate replacement to replace all the river sand, and (2) partial filler/supplementary cementitious material (SCM) replacement. The modified Andreasen and Andersen (A&A) particle packing model was used to determine the optimal replacement level. Based on the optimal mixture design, the impact of the replacement on the fresh and mechanical properties of SCC was evaluated. Additionally, the pozzolanic reactivity of the fine fraction (<125 µm) within the secondary sand was assessed and compared to that of limestone powder. Our findings confirm that using remediated thermal-treated soil and tar-containing asphalt can produce a more circular, sustainable SCC by replacing high-quality natural sand and limestone filler and reducing the environmental impact of conventional SCC. This study contributes to finding viable alternatives to natural fine aggregate and promotes the use of recycled materials in construction.

Alkali-activated concrete (AAC) is regarded as a promising alternative construction material to reduce the CO₂ emission induced by Portland cement (PC) concrete. Due to the diversity in raw materials and complexity of reaction mechanisms, a commonly applied design code is still absent to date. This study attempts to directly correlate the AAC mix design parameters to their performances through an artificial intelligence approach. To be specific, 145 fresh property data and 193 mechanical strength data were collected from laboratory tests on 52 AAC mixtures, which were used as inputs for the machine learning algorithm. Five independent random forest (RF) models were established, which are able to predict fresh and hardened properties (in terms of compressive strength, slump values, static/dynamic yield stress, and plastic viscosity) of AAC with equivalent accuracy reported in the literature. Moreover, an inverse optimization was performed on the RF model obtained to reduce the sodium silicate dosages, which may further mitigate the environmental impact of producing AAC. The present RF model gives practical information on AAC mix design cases.

In this paper, the authors characterized two types of zonation within slag rims in aged alkali-activated slag (AAS) systems through SEM and TEM technology. These two elemental zonation were closely related to the pore structure of AAS pastes, thus providing strong implication for the transport- and durability-related performance of AAS systems. The first type of zonation occurred in the rims of AAS specimens under sealed curing. It was found that lath-like hydrotalcite-like phase accumulated near the boundary while a generally homogeneous and foil-like C-(N-)A-S-H gel phases precipitated in the following sub-zone. When slag rims were thick enough, a new Mg-rich region occurred. The second type of zonation was noticed in the carbonated AAS pastes. For this kind of distribution pattern, C-(N-)A-S-H gel phases were observed near the boundary. Following, the accumulation of Mg and Ca occurred alternatively. Additionally, transformation mechanism between these two types of zonation was also proposed.

This work evaluated the reactivity and leaching potential of municipal solid waste incineration (MSWI) bottom ash as supplementary cementitious material (SCM) and precursor for alkali-activated materials (AAM). The chemical composition of the amorphous phase in MSWI bottom ash was found to be in the same range as that of Class F coal fly ash. The reactivity of MSWI bottom ash as SCM and AAM precursor was tested to be much lower than that of blast furnace slag, but similar to that of Class F coal fly ash. The method of thermodynamic modeling was found useful in providing references for the mix design of MSWI bottom ash-based AAM. Grinding MSWI bottom ash into powder for the application of SCM and AAM precursor increased its leaching potential. Based on the findings of this study, recommendations were provided on how to use MSWI bottom ash to prepare blended cement pastes and AAM.

The results of phase 1 of an interlaboratory test, coordinated by the RILEM TC 267-TRM “Tests for Reactivity of Supplementary Cementitious Materials” showed that the R³ (rapid, relevant, reliable) test method, by measurement of heat release or bound water, provided the most reliable and relevant determination of the chemical reactivity of supplementary cementitious materials (SCMs), compared to other test methods. The phase 2 work, described in this paper aimed to improve the robustness of the test procedure and to develop precision statements for the consolidated test procedure. The effect of the pre-mixing and mixing conditions, and the impact of the mix design on the test method robustness were assessed and fixed for optimal conditions to carry out the R³ heat release test. The effect of the drying step was evaluated to define the R³ bound water test procedure in more detail. Finally, the robustness of the consolidated final test methods was determined by an interlaboratory study to define the precision statements.

In this paper, both synthetic slag and commercial slag covering the common composition range were employed to estimate the correlation between slag chemistry and reactivity through hydraulicity and dissolution tests. It was found that slag reactivity was favorably affected by increasing Al₂O₃ and MgO contents, while the adverse effect of decreasing CaO/SiO₂ ratio could be compensated by higher amounts of Al₂O₃ and/or MgO. When calorimetric measurement was used to assess the reactivity of slag, the effect of sulfur species incorporated in commercial slag should be taken into consideration as a small quantity of it could lead to a major difference of cumulative heat release due to the formation of ettringite. Moreover, a novel graphical method was proposed to estimate the reactivity of slag considering its chemical composition from a new perspective, i.e. a cartesian coordinate system based on (CaO/SiO₂)−(MgO + Al₂O₃).

This study aims to interpret the early-stage rheology of alkali-activated slag (AAS) paste from microstructure perspectives. The microstructures visualized by cryogenic scanning electron microscopy (cryo-SEM) revealed the essential distinction between hydroxide and silicate-activated slag pastes. The hydroxide-based mixture showed typical suspension features, where slag particles were dispersed in the hydroxide activators. In the hydroxide media, even at very early ages (5 min), the solid grains were attached to each other through rigid connections of reaction products, which resulted in high yield stress. As for the silicate-based mixtures, an emulsion phase has been observed between slag particles, which consists of discontinuous water droplets and continuous silicate gels. Fine emulsions with smaller water droplets were observed as the silicate modulus of activators increased, which dispersed the slag particles but on the other hand improved the viscosity of the paste. With increasing water to binder ratio, both yield stress and viscosity of AAS pastes significantly reduced.

Strain hardening cementitious composite (SHCC) is a special class of ultra-ductile material which has autogenous self-healing capability due to its intrinsic tight crack widths. To further improve its healing ability, healing agent (HA) can be incorporated in SHCC, enabling it also the autonomous self-healing mechanism. In this study, the effects of adding bacteria-embedded polylactic acid (PLA) capsules on the mechanical properties of SHCC with different amounts of HA (i.e., 1.25%, 2.5%, 5% by weight to binder) were investigated. Experiments were conducted to examine the composite performance, matrix properties and single fiber pullout behavior of the SHCCs, followed by microscopy characterization of the fiber/matrix interface microstructure. Results show that the inclusion of the PLA-HA up to 5% by weight to binder influenced the tensile performance (i.e., tensile strength and ductility) of SHCC only to a very small extent but significantly reduced the average residual crack widths. The inclusion of HA at a high dosage (5%) increased the crack tip toughness (J_tip) of the matrix by lowering elastic modulus and increasing fracture toughness. Single fiber pullout results show that the fiber/matrix bond properties were enhanced by the addition of the HA, which can be attributed to the formation of a denser interfacial transition zone (ITZ) with less calcium hydroxide crystals as revealed by the scanning electron microscope (SEM) micrographs. The improved bond properties led to higher fiber bridging complementary energy and thus partially sustained the tensile strain capacity as verified by the micromechanical model.