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.

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Recycling aluminosilicate-based solid wastes is imperative to realize the sustainable development of constructions. By using alkali activation technology, aluminosilicate-based solid wastes, such as furnace slag, fly ash, red mud, and most of the bio-ashes, can be turned into alternative binder materials to Portland cement to reduce the carbon footprint of the construction and maintenance activities of concrete structures. In this paper, the chemistry involved in the formation of alkali-activated materials (AAMs) and the influential factors of their properties are briefly reviewed. The commonly used methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TG), nuclear magnetic resonance spectroscopy (NMR), and X-ray pair distribution function technology, to characterize the microstructure of AAMs are introduced. Typical characterization results of AAMs are shown and the limitations of each method are discussed. The main challenges, such as shrinkage, creep, efflorescence, carbonation, alkali–silica reaction, and chloride ingress, to conquer for a wider application of AAMs are reviewed. It is shown that several performances of AAMs under certain circumstances seem to be less satisfactory than traditional portland cement systems. Existing strategies to improve these performances are reviewed, and recommendations for future studies are given.@en

This study aims to experimentally investigate the autogenous deformation and the stress evolution in restrained high-volume ground granulated blast furnace slag (GGBFS) concrete. The Temperature Stress Testing Machine (TSTM) and Autogenous Deformation Testing Machine (ADTM) were used to study the macro-scale autogenous deformation and stress evolution of high-volume GGBFS concrete with w/b ratios of 0.35, 0.42, and 0.50. The early-age cracking (EAC) risk (quantified by stress-strength ratio) and stress relaxation were analyzed extensively based on ADTM and TSTM results. Furthermore, Environmental Scanning Electron Microscopy (ESEM), X-ray Diffraction (XRD), and Mercury Intrusion Porosimetry (MIP) were conducted to explore the micro-scale origin of the autogenous deformation of high-volume GGBFS concrete, which supports the observations on the macroscale measurement of TSTM/ ADTM tests. This study finds that the ettringite formation in the first two days results in autogenous expansion, which can delay the appearance of tensile stress. The magnitude of autogenous expansion depends on the compatibility of ettringite content and pore size. The w/b ratio of 0.42 turns out to be optimal because it produces the highest amount of ettringite and results in the highest autogenous expansion. In comparison, the w/b ratio of 0.35 introduces significant autogenous shrinkage after the expansion peak and therefore corresponds to a high early-age cracking risk.

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In 3D concrete printing, fast structuration is a prerequisite for ideal buildability. This paper aims to study the impact of inorganic additives, i.e., CaCl₂ and gypsum, on structural build-up and very early-age hydration of limestone-calcined clay-cement (LC3) pastes within the first 70–80 min. Results show that, increasing the dosage of CaCl₂ or gypsum can accelerate storage modulus G' and static yield stress evolution with time, as well as increase chemically bound water (H) content and total specific surface area (SSA_total). Furthermore, good correlations were found between G' and H content, as well as static yield stress and the ratio of free water content to SSA_total. The acceleration by CaCl₂ can be attributed to stimulating C₃S and C₃A hydration and promoting crystal formation, i.e., ettringite, portlandite, and Friedel's salt. Additionally, the increase in gypsum percentage led to a large amount of unreacted gypsum in the system, resulting in an increase in SSA_total.

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Formulation of quaternary blended system containing ordinary Portland cement or clinker, slag, limestone and calcined clay (LC2) appeared to be a viable approach to developing low-clinker cements without severely sacrificing mechanical performance at later ages. This paper investigates the effect of two material parameters, i.e., LC2-to-slag ratio and gypsum content, on fresh properties, hydration, and compressive strength of quaternary blended cement pastes (about 65 wt% of LC2 and slag in the binder). Results show that the increase in LC2 proportion decreased flowability and increased water retention capacity, yield stress, and plastic viscosity, as well as accelerated the evolution of stiffness with time (G′ growth). On the other hand, the addition of 2–4 wt% gypsum had little effect on most of the fresh properties. A new metric, the free water indicator, was proposed to describe the effect of free water content and the total specific surface area of binding materials. It correlated strongly with the growth of structural build-up metrics. Finally, adding gypsum delayed the aluminate peak and enhanced compressive strength only at 3 days, whereas increasing slag content reduced accumulated heat of hydration (7 days) but improved 28-day compressive strength. Therefore, adjusting LC2-to-slag ratio of the quaternary blended cement is a feasible way to meet requirements for fresh properties and compressive strength.@en

Autogenous shrinkage may be a critical issue concerning the use of limestone-calcined clay-cement (LC3) in high-performance concrete and 3D printable cementitious materials, which have relatively low water to binder (W/B) ratio. Adding an internal curing agent, i.e., superabsorbent polymer (SAP), could be a viable solution in this context. However, employing SAP (without adding additional water) may also influence the fresh properties of LC3 composites by increasing yield stress and viscosity, which may be beneficial for 3D printability. Therefore, this study attempts to use SAP as a rheology modifying admixture with the aim of investigating the impact of SAP on flow behavior, structural build-up, hydration kinetics, compressive strength, and autogenous shrinkage of LC3 pastes with a fixed W/B (0.3). In addition, hydroxypropyl methylcellulose (a typical rheology/viscosity modifier in 3D printable cementitious materials) was also employed in two mixtures to compare their effects. Results show that adding SAP increases the dynamic yield stress and the apparent viscosity, as well as structural build-up and hydration, but decreases the compressive strength at 3, 7 and 28 days. Furthermore, using SAP (especially 0.2 wt% SAP) not only promotes the early-age expansion but also effectively mitigates the autogenous shrinkage of LC3 pastes for up to 7 days. Overall, the obtained results indicated that SAP could act as a promising rheology modifier for the development of 3D printable cementitious materials.

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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.

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Additively manufactured vascular networks have great potential for use in autonomous self-healing of cementitious composites as they potentially allow multiple healing events to take place. However, the existence of a vascular tube wall may impede with the healing efficiency if it does not rupture timely to release the healing agent. The issue of vascular material design has therefore been a major topic of research. To overcome this, dissolvable Polyvinyl Alcohol (PVA) filament is adopted in this study to fabricate the vascular networks. Fabricated networks are coated with wax, placed in cementitious mortar and removed upon hardening, thereby leaving a network of hollow channels. Different printing directions were expected to affect the dissolvability of printed structures and were therefore fabricated and tested. Different shapes (i.e., 2D and 3D) of vascular networks were printed and embedded in the cementitious mortar. Four-point bending tests and permeability tests were performed to investigate the healing efficiency. Multiple healing cycles were applied in the cracked specimens. The results show that the vertically printed PVA tubes with wax coating have good dissolution behaviour. As expected, the existence of vascular networks decreases the initial flexural strength of the specimens. In terms of healing efficiency, excellent mechanical and water tightness recovery were achieved when using epoxy resin as the healing agent. The mechanical recovery after the first healing process is higher than the following healing process. The watertightness of the cracked samples keeps decreasing with the increase of healing cycles. Specimens embedded with 3D vascular networks have higher healing potential than those utilizing 2D vascular networks.

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For a better understanding of the hydrotalcite-like phase with SEM-EDS microanalysis, the present research paid special attention to the data acquisition and interpretation of this technique. A lower Mg/Al ratio was obtained when using a higher accelerating voltage, and a beam energy of 10 kV was more appropriate than 15 kV for investigation when the slag rim was thin, to compromise to meet the requirements of obtaining an adequate overvoltage ratio and minimizing the interference. Additionally, it was noted that the Mg/Al ratio decreased from zones rich in hydrotalcite-like phase to zones rich in the C−S−H gel phase, and indiscriminately fitting scatter points selected from the slag rim would bias the Mg/Al ratio of the hydrotalcite-like phase. According to the standard-based microanalysis, it was concluded that the analysis total of the hydrates within the slag rim was in the range of 30–40%, lower than that located in the cement matrix. Besides the water chemically bound in the C−S−H gel phase, the hydrotalcite-like phase also contained a certain amount of hydroxide ions and chemically bound water.@en

This paper presents the influence of P₂O₅ incorporated in slag on the hydration characteristics of cement-slag system. It was found that the gradual addition of phosphorus oxide in slag did not change overall mineralogy of the hydration products. Except hydration retardation in the dormant stage, chemically bound water and portlandite contents, hydration degree of slag, and pore structure at all investigated ages were similar among cement-slag pastes with different P₂O₅ percentages. Furthermore, significantly higher amount of monosulfate was observed as the P₂O₅ content in slag increased. In addition, a higher Al/Si atomic ratio was measured in the C-S(A)-H gel phase formed in the cement matrix. However, similar Ca/Si atomic ratio of C-S(A)-H gel phase and Mg/Al atomic ratio of hydrotalcite-like phase were determined in all slag pastes, irrespective of the addition of P₂O_5. In contrast to magnesium ion which was retained within the original slag boundary, phosphorus ions could migrate into cement matrix. Therefore, P/Si atomic ratio of the C-S-H gel phase increased with the increasing phosphorus oxide content in slag, reaching up to ∼0.08.

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Through the integration of SEM-BSE and TEM, we gained a comprehensive 3-dimensional understanding of different distribution patterns of inner hydration products of slag. For fully hydrated small slag grains, two distinct sub-zones were formed in the rims. Lath-like, well-crystalline hydrotalcite-like crystals were found to precipitate, grow, and accumulate near the boundary, forming a layer with a thickness slightly exceeding 0.5 μm. In the center, entrapped calcium and silicon played roles in the formation of a homogeneous and fibrous C−(A)–S–H gel phase. The concentration equilibrium between cement matrix and grain core led to the establishment of a similar grey pixel value and Ca/Si atomic ratio of gel phase at ~1.10. As the size of slag grains increased, three sub-zones became visible. Hydrotalcite-like phase was enriched near the boundary, followed by a sandwiched area abundant in C–(A)–S–H gel phase. Due to the low mobility and increased migration distance, newly released magnesium from reaction front accumulated locally to form a new Mg-rich region.

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Limestone-calcined clay-cement (LC3), as one of the most promising sustainable cements, has been under development over the past decade. However, many uncertainties remain regarding its rheological behaviors, such as the metakaolin content of calcined clay. This study aims to investigate the effect of increasing the content of fine-grained metakaolin in calcined clay on the rheology of LC3 pastes. Rheological behaviors and early-age hydration of studied mixtures were characterized using flow curve, constant shear rate, small amplitude oscillatory shear and isothermal calorimetry tests. Results show that increasing the content of fine-grained metakaolin decreased flowability but promoted structural build-up and early-age hydration. These phenomena can be attributed to the decrease of mean interparticle distance caused by the increased amount of fine-grained metakaolin, which may enhance colloidal interactions, C-S-H nucleation and direct contact between particles. Overall, modifying the fine-grained metakaolin content is a feasible approach to control the rheology of LC3 pastes.

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To understand the influence of slag chemistry on the carbonation resistance of slag-rich cement, this paper explored the carbonation characteristics of blended cement systems with different Al₂O₃ contents in slag through accelerated carbonation test. Irrespective of slag chemistry, three main CO₂ binding phases were identified during accelerated carbonation test, i.e. carbonated Ca-Al AFm phases (amorphous or nano-crystalline), carbonated hydrotalcite-like phase, and calcium carbonate (amorphous calcium carbonate, vaterite, and calcite). Additionally, it was noted that the classification employed for slag reactivity (based on slag chemistry) cannot be extended to predict carbonation resistance of slag-rich cement directly. The main challenge occurred for slag with high alumina content. The experimental results showed that Al₂O₃-rich slag exhibited a high reactivity and can be considered as a reactive component in the blended mixture; however, it did not contribute to carbonation resistance of the mixture. Especially for CO₂ binding capacity, it was similar for systems with varied alumina content in slag (from 3.69 to 18.19 wt.%) in the completely carbonated area.

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This study investigated the evolution process of high-volume slag cement (HVSC) paste from a chemo-mechanical standpoint. HVSC specimens with a 70 w.t. % slag replacement rate were studied at various ages. Evolution of phase assemblage, microstructure development, and micromechanical properties were analyzed using TGA/XRD/MIP/SEM-EDS and nano-/micro-indentation techniques. A two-scale micromechanical model was built to predict the effective elastic modulus based on the nanoindentation results. Key findings include: 1) Between 7 and 28 days, the formation of calcium silicate hydrate (C-S-H) gel phase improves the effective elastic modulus by filling capillary pores; 2) From 28 to 90 days, the phase assemblage and microstructure remain stable, with a transition from low-density to high-density C-S-H; 3) Between 90 days and 2 years, slag rims produced by slag grains result in increased elastic modulus; 4) The two-scale micromechanical model, combined with nanoindentation data, accurately predicts the effective modulus of HVSC composites, although the unhydrated slag grains-hydrated cement matrix interface may cause an overestimation at an early age. With longer curing time, this interface disappears owing to the continuous hydration of large slag particles and therefore a good match is found between the modelling and experimental results.

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Slag rim mainly consists of secondary precipitations such as C–S–H gel phase and hydrotalcite-like phase, which originate from the hydration of slag. In this paper, the micro-mechanical properties of slag rim were characterized by nanoindentation in combination with SEM. It was found that, compared to the C–S–H gel phase, slag rim showed about a 15 GPa higher modulus of elasticity. At the early age, slag cement paste mainly consisted of low-density C–S–H gel phase, high-density C–S–H gel phase, and unhydrated slag particles, as well as calcium hydroxide based on the results of nanoindentation; at the later age, the system contained C–S–H gel phase, slag rim (a mixture of C–S–H gel phase and hydrotalcite-like phase), and unhydrated slag particles.@en

This paper studies chemical composition of partially and fully hydrated slag grains in a (nearly) 40-year-old field concrete from the Netherlands. The concrete samples were assumed to be sufficiently aged to contain fully hydrated slag grains as well as partially hydrated large slag particles with thick rims. Our analysis showed that three different elemental zoning could be identified depending on the original slag grain size. Upon full hydration of a small slag grain (i.e., <8 μm), two distinct regions were identified corresponding to a hydrotalcite-like phase in the outer rim and a C–S–H gel phase in the core, respectively. As for medium (8–17 μm) and large (>15 μm) slag grains, three distinct regions were clearly visible. Hydrotalcite-like phase was mainly observed in the outer rim and the core. C–S–H gel phase was found to be precipitated in the region between the outer rim and the core.

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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).

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This paper reports the carbonation characteristics of a cement-slag system exposed to accelerated carbonation testing, and its improved carbonation resistance with the increasing MgO content in blast furnace slag, in which hydrotalcite-like phase plays a key role. Our research showed that the hydrotalcite-like phase started to carbonate upon contacting with the carbonate ions and bound more than 15 wt% CO₂⁻³ in the mildly carbonated and transition areas. This value was positively associated with the magnesia content of slag. Additionally, the proportion shared by hydrotalcite-like phase decreased in the fully carbonated area, and more CO₂ was fixed in the form of calcium carbonate. Consistent with the thermodynamic modelling, the ratio of CO₂ bound in carbonated hydrotalcite-like phase to the total CaCO₃ continued to decrease as the CO₂ ingress progressed. On the other hand, the reaction between hydrotalcite-like phase and CO₂ was found to be volumetrically stable due to binding CO₂ in the interlayer space, and Mg was still distributed within the original slag grain region. Mg/Al atomic ratio of hydrotalcite-like phase remained nearly the same before and after carbonation. Results of this study quantitatively emphasized the favorable effect of hydrotalcite-like phase to improve the carbonation resistance of slag-rich cementitious systems.

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A deeper insight into SO₃/Al₂O₃ ratio including the contribution of alumina in slag at early age is required to ensure a properly sulfated slag cement. In this paper, to investigate the effect of gypsum and alumina of slag, emphasis was laid on the hydration characteristics of C₃S-gypsum-slag system during the early age, of which slag was synthesized in the laboratory with varying Al₂O₃ contents from 3.69 to 18.19 wt%. The duration of dormant period during the hydration of C₃S depended on Al₂O₃ content of slag significantly; however, the amount of silicate reaction before the onset of aluminate reaction was independent of slag chemistry and gypsum content added. The rate of aluminate reaction was controlled by the availability of reactants, SO₄²⁻ and Al³⁺ ions in particular, which were sourced from gypsum and slag, respectively. Calcium monosulfoaluminate only occurred in mixture when slag contained a high amount of Al₂O₃ (18.19 wt% in this study) at early age, and its formation proceeded continuously at the expense of ettringite. Sulfur rich species incorporated in slag started to participate into aluminate reaction after the main hydration peak of C₃S, and it played a similar role to gypsum.

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The effect of slag of different origins (synthetic slag produced in the laboratory and commercial slag collected from different steel factories) with comparable chemical composition, amorphous content and particle size distribution, on the hydration characteristics of slag cement was investigated. In order to study the effect of sulfur in slag, a model cement paste of a C₃S-slag blend was also produced. It was found that origin of slag has very little impact on the hydration process of cement-slag system. Synthetic slag shows a comparable compressive strength gain to commercial slag at 1 day, and from then on, a lower compressive strength is reached by it compared with that of commercial slag until 28 days. The sulfur in slag starts to participate in reaction after 1 day and it dominates the rate of heat release in calorimetric measurement, consistent with the result of compressive strength test. It significantly affects the elemental composition of the cementitious matrix at 7 days, and higher Al/Si and S/Ca ratios can be detected in cement-commercial slag blend at 7 days. The sulfur is involved in the formation of AFm-phase, such as calcium monosulfoaluminate, and the thermodynamic modelling shows that upon the gradual incorporation of sulfur in slag, calcium monosulfoaluminate precipitates continuously with the consumption of strätlingite and portlandite. The conclusion obtained in the paper provides a basis to understand the role of sulfur in slag on the hydration process of slag cement

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