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.@en

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.@en

RILEM TC 267 TRM– “Tests for Reactivity of Supplementary Cementitious Materials” recommends the Rapid Reliable Relevant (R3) test as a method for determining the chemical reactivity of supplementary cementitious materials (SCMs) in Portland cement blends. In this paper, the R3 test was applied to 52 materials from a wide range of conventional and alternative SCMs with the aim to validate such test. An excellent correlation was found between the cumulative heat release and the bound water determined following the R3 test method. Comparison of the R3 test results to mortar compressive strength development showed that all conventional SCMs (e.g. blast furnace slag and fly ashes) followed the same trend, with the notable exception of very reactive calcined kaolinitic clays. It is discussed, through an in-depth statistical regression analysis of the R3 reactivity test results and the 28 days relative compressive strengths, how reactivity threshold values for classification of the chemical reactivity of SCMs could be proposed based on the R3 test results.@en

The uptake mechanism of heavy metal ions in layered double hydroxides (LDHs) is investigated in this paper via solid-solution exchange experiments and first principle study. The uptake capacities of C-LDHs for heavy metal ions from solutions are experimentally investigated and the structures of LDHs doped with various heavy metal ions are revealed. The doped structures of LDHs are further re-established with first principle calculations. The results show that Cu2+ or Cr3+ ions are immobilized in the form of isomorphic substitution for Mg2+ and Al3+ in the plate of the layered structure, respectively, during reconstruction of calcined LDHs, forming a non-stoichiometric structure. The structure of the Cu2+ doped LDHs is identified as [Mg2+ (1−x)(1−z)Cu2+ (1−x)zAl3+ x(OH)2] An− x/n·yH2O, where z is the molar ratio of Cu2+ to Mg2+. The structure of Cr3+ doped LDHs is identified as [Mg2+ 1−x Cr3+ xzAl3+ x(1−z)(OH)2] An− (x+z)/n·yH2O, where z is the molar ratio of Cr3+ to Al3+. The Cu2+ or Cr3+ ions in the hardened cement paste modified with calcined Mg-Al LDHs as immobilizing admixture can be efficiently removed from the pore solution and chemically stabilized in the structure of LDHs.@en

Alkali-activated Materials (AAMs), including those classified as geopolymer, are obtained through the reaction between a solid precursor and an alkaline solution. Compared with ordinary Portland cement (OPC) binders, these materials maintain comparable mechanical properties but have the advantage of reducing greenhouse gas emissions and utilization of industrial by-products and residuals which helps to meet sustainability goals. AAMs are thus considered an environment-friendly construction material with great potential for next-generation concrete. AAMs are inherently brittle. The low ductility of AAMs makes them prone to cracking and corresponding performance degradation, which is detrimental to their durability. Based on the concept of strain-hardening cementitious composite (SHCC), one solution relates to a family of fiber-reinforced composites that have high tensile ductility and multiple-cracking characteristics, i.e., strain-hardening geopolymer composite (SHGC). While much effort has been taken to develop conventional SHCC, scientific and technical knowledge of SHGC is still in the very early stage of development. This PhD project deals with the development of a cement-free strain-hardening geopolymer composite (SHGC) as a high-performance construction material using industrial wastes and by-products through alkaline activation technology: The fracture properties and other mechanical properties of the alkali-activated slag/fly ash (AASF) paste as the matrix for SHGC were experimentally tested. The microstructure and chemistry of the reaction products were investigated to understand the fracture mechanism. It was found that the fracture properties of pastes are strongly related to the chemical composition (Ca/Si ratio) of the main reaction product, i.e., C-(N-)A-S-H gel. The fracture properties were also found to be dominated by a cohesion/adhesion-based mechanism. Furthermore, the compressive strength of AASF paste is primarily determined by its capillary porosity. The fiber/matrix properties, including chemical bonding energy, initial frictional bond, and slip-hardening behavior of fiber during the pullout process were also experimentally studied. The chemistry and microstructure of the reaction product in the fiber/matrix interfacial transition zone (ITZ) were characterized. Their influence on the interface bonding properties was also investigated. It is found that the chemical bonding between PVA fiber and AASF matrix increases with increasing Ca/Si and Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Hence, changing the slag content and the alkali activator Ms appears to be an effective way to modify chemical bonding. Unlike the formation of portlandite near the PVA fiber surface in conventional SHCC, a high-Ca C-(N-)A-S-H phase was formed in the fiber-matrix ITZ of SHGC. This explains the higher chemical bonding energy found in SHGC compared to that in conventional SHCCs. Furthermore, the adhesion mechanism of the PVA molecule in reaction products was studied using MD simulation. The study suggests that the adhesion between PVA fiber and C-(N-)A-S-H gel is primarily due to electrostatic interactions rather than van der Waals interactions. Based on the result of fracture properties of the matrix and fiber/matrix interface properties, the SHGC is then systematically developed following a micromechanics-based design approach. The experimentally-attained matrix and interface properties served as input for the numerical micromechanics model to simulate the crack bridging behavior. Through the micromechanical modeling, the optimal fiber length and volume were selected and the behavior of mixtures with different fiber/matrix combinations was predicted. With this approach, researchers and materials engineers can design and tailor future SHGC more efficiently than by using the commonly used trial-and-error method. Finally, the environmental impact of the SHGC with the most promising performance was also evaluated. This evaluation was conducted using a cradle-to-gate life-cycle assessment (LCA) of SHGC compared to that of conventional SHCC materials. The developed SHGC demonstrates a very promising environmental profile. It has a significant reduction of the global warming potential (GWP) and a lower or similar total environmental impact compared to conventional SHCC materials. In addition, the results also provide recommendations for further improvements in mixture design for the future development of SHGC. This study successfully developed a sustainable slag/fly ash-based SHGC with a lower carbon footprint than conventional SHCC. It is considered a good example to utilize industrial by-products as secondary resources and at the same time contribute to a circular economy. Furthermore, this study helps to understand the fracture properties of AAMs. It also clarifies the adhesion mechanism of PVA fiber in AAMs. All of these give promising guidance for researchers and engineers to design fiber-reinforced AAMs with required fracture properties and interface bonding properties. In particular, it contributes to the design and tailoring strategies for high-performance composite, for instance, SHGC, through proper mixture design. @en

A primary aim of RILEM TC 267-TRM: “Tests for Reactivity of Supplementary Cementitious Materials (SCMs)” is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well.@en

A primary aim of RILEM TC 267-TRM: “Tests for Reactivity of Supplementary Cementitious Materials (SCMs)” is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well.@en

The uptake mechanism of heavy metal ions in Layered double hydroxides (LDHs) is investigated in this paper via solid-solution exchange experiments and first principle study. The structures of heavy metal doped LDHs are revealed with XRD, TEM and solid state NMR analysis combined with first principle calculations. The experimental results show that Cu2+ or Cr3+ ions are immobilized in the form of isomorphic substitution during reconstruction of calcined LDHs, forming a non-stoichiometric structure. The Cu2+ or Cr3+ ions are immobilized via isomorphic substitution for Mg2+ and Al3+ in the plate of the layered structure, respectively. The structure of Cu2+ doped LDHs is identified as as [Mg2+ (1-x)(1- z)Cu2+ (1-x)zAl3+ x(OH)2] Anx/ n·yH2O, where z is the Cu2+/Mg2+ molar ratio. The structure of Cr3+ doped LDHs is identified as [Mg2+ 1-x Cr3+ xzAl3+ x(1-z)(OH)2] An- (x+z)/n·yH2O, where z is the Cr3+/Al3+ molar ratio. The Cu2+ or Cr3+ ions from mixing water can be efficiently immobilized with Mg-Al LDHs in cement.@en

In Nederland wordt veel aandacht besteed aan geopolymeerbeton als een van de mogelijkheden om de CO₂-voetafdruk van beton te verlagen. Hoewel dit materiaal op laboratoriumschaal uitgebreid is onderzocht, zijn praktische toepassingen en ervaringen nog maar beperkt beschikbaar. Bovendien bestaan er voor een brede en grootschalige constructieve toepassing een aantal uitdagingen op technologisch en technisch gebied. Er is een onderzoeksproject gestart waarin een zelfverdichtend geopolymeerbeton (ZGB) is ontwikkeld voor toepassing in een prefab voorgespannen verkeersbrug. Dit onderzoek is uitgevoerd door het Microlab en de sectie Betonconstructies van de TU Delft, Haitsma Beton en de Universiteit Gent en en wordt door de provincie Fyslân ondersteund als launching customer.@en

In recent years, the rapid industrialization and urbanization led to the explosive growth of municipal solid waste incineration (MSWI) bottom ashes (BA) production. However, most of them are directly landfilled, which not only brings environmental burden but also results in loss of potential resources. Present researches have proved that MSWI BA could be utilized as a replacement in Portland cement concrete. However, several drawbacks such as volume expansion, leaching behaviour, and relatively lower strength have been reported. In this study, as-received BA was pretreated to remove the metallic aluminium which is responsible for the hydrogen-induced expansion when blended in OPC concretes. Subsequently, the treated BA samples were used as a substitution for cement at the replacement level of 10%. Micronized sand (M300) was selected as reference materials to investigate the role of treated BA in blended cement system, either as filler or binder material. In the experimental program, the hydration process of different mixtures was monitored by isothermal calorimeter and hydration products were determined by X-ray diffraction (XRD) and Thermalgravimetric analysis (TGA). Results showed that the pretreatment effectively removed the metallic aluminum in BA and no severe expansion or strength decrement were detected. The treated BA showed limited reactivity comparing with Portland cement, however, it still worked better than micronized sand as a filler substitution.@en

Hoewel geopolymeerbeton op laboratoriumschaal uitgebreid is onderzocht, zijn praktische toepassingen en ervaringen nog maar beperkt beschikbaar. Bovendien bestaan er voor een brede en grootschalige constructieve toepassing een aantal uitdagingen. Er is een onderzoeksproject gestart waarin een zelfverdichtend geopolymeerbeton (ZGB) is ontwikkeld voor toepassing in een brug. In de tweede fase van het onderzoek zijn de mogelijkheden voor productie op industriële schaal onderzocht.@en

This paper investigates the influence of a plasticizing admixture on the pore structure refinement of alkali-activated concrete and paste mixtures and the consequently enhanced performance. Alkali-activated fly ash-slag concrete and paste are designed using a polycarboxylate-based admixture with different dosages. The pore structure and porosity are analyzed using mercury intrusion porosimetry (MIP). The workability, compressive strength, chloride migration resistance and electrical resistivity of alkali-activated fly ash-slag concrete and paste are determined. The results show that significantly improved workability and strength development are obtained at an increased admixture content. The admixture improves the gel polymerization product layer most likely around the GGBS particles, densifying the matrix. The 28-day Cl-migration coefficient of admixture (1–2 kg/m3) modified concrete is equal to the reference mixture, while at the highest admixture content the Cl-ingress is increased. At the later ages (91-days), the Cl-migration coefficients of all concretes, non- and admixture-containing samples, are comparable and low (about 2.6 × 10−12 m2/s). The MIP analyses show a significant decrease of the total and effective capillary porosity over time at an increased admixture content. The relationships between the porosity and other properties are discussed, at varying admixture contents.@en

The feasibility of a waste glass powder residue (GP) from glass recycling as partial mineral precursor to produce alkali-activated materials is investigated. GP served as powder coal fly ash (PCFA) replacement within a reference system composed of 50% PCFA and 50% ground granulated blast furnace slag (GGBS). Compared with PCFA, GP was better involved in the alkali activation process by having a higher silica and Ca dissolution. Furthermore, increasing GP replacement up to 30% prolonged the induction period, facilitated the gel formation and yielded a 35% higher 28-day compressive strength. These observations are similar to the effect of using both sodium hydroxide and sodium silicate as alkali activator in alkali-activated slag/fly ash systems. A higher polymerization of the gel network was also observed. Microstructure analysis indicated that the main reaction product is a calcium silicate hydrate type gel substituted with Al and Na (C-(N)-A-S-H type gel). This work largely contributes to the understanding of the reactivity and potential of GP and promotes its practical utilization as a mineral precursor in the production of alkaline cements.@en

This paper aims to study the adhesion mechanism of polyvinyl alcohol (PVA) fiber within alkali-activated slag/fly ash (AASF) matrix using molecular dynamics (MD) simulation in combination with systematic experimental characterization. The adhesion of PVA to C-(N-)A-S-H gel with different Ca/(Si+Al) and Al/Si ratios was modeled using MD simulation, with the related adsorption enthalpy calculated and the adhesion mechanism explored. The experimentally attained chemical bonding energy of PVA fiber in AASF coincides well with the simulation results. In both cases, the adhesion enhances primarily with increasing Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Additionally, MD simulation indicates preferential element distributions of Ca around PVA molecule, which was confirmed experimentally by the detection of the Ca-rich C-(N-)A-S-H gel in the interfacial transition zone (ITZ). This study provides further insights into the adhesion mechanism of PVA fiber to C-(N-)A-S-H gel formed in AASF, which is particularly valuable for the future development of PVA-based high-performance alkali-activated composites.@en

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 (Jtip) 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.@en

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).@en

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.@en

This study presents a comprehensive experimental investigation on the fracture properties of hardened alkali-activated slag/fly ash (AASF) pastes in relation to the microstructure formation and reaction product composition. The main reaction product in AASF is C-(N-)A-S-H gel along with minor hydrotalcite phase, with the polymerization of C-(N-)A-S-H gel substantially governed by its Ca/Si ratio. Strong positive correlations are identified between the Ca/Si ratios of C-(N-)A-S-H gel and the fracture properties KIc (Jtip), whereas, the compressive strength of AASF pastes is primarily determined by its capillary porosity (>0.01 μm). The disagreements between the Ca/Si ratios and corresponding intrinsic mechanical properties of C-(N-)A-S-H gel as proof by contradiction indicate that the fracture properties KIc (Jtip) of AASF pastes could be dominated by a cohesion/adhesion-based mechanism. These findings provide promising guidance for fine-tuning the fracture properties of AASF and also advise on the tailoring strategies for high-performance composite such as strain-hardening geopolymer composite.@en

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.@en

Alkali activated concrete (AAC) has not received broader industry acceptance, one reason of which lies in the uncertainties in the durability against shrinkage and potential cracking. Many studies reported that AAC exhibit larger autogenous shrinkage than OPC concrete. However, it is unable to deduce that AAC should show higher cracking potential than OPC concrete only based on the higher autogenous shrinkage of AAC. The cracking potential of concrete is determined by multiple factors including autogenous shrinkage, creep/relaxation, elastic modulus, and tensile properties of the concrete. However, very few studies have considered these parameters. Furthermore, the influence of precursors (e.g. slag or fly ash) on the cracking potential of AAC induced by autogenous shrinkage is also rarely studied. The aim of this study, therefore, is to investigate the autogenous shrinkage-induced cracking potential of slag and fly ash-based AAC. The free autogenous shrinkage of the specimens is measured by Autogenous Deformation Testing Machine (ADTM). The autogenous shrinkage-induced stress and cracking of the concrete under restraint condition is tracked by Thermal Stress Testing Machine (TSTM). Additionally, the influence of precursors on the autogenous shrinkage induced cracking potential is discussed.@en