High-resolution quality control is essential to produce reliable concrete products that are optimized towards material minimization. For this purpose, in-line measurements have a high potential since they measure the material that is later used in the product at a high frequency. The current paper assesses how well various in-line measurements correlate with the bulk yield strength at deposition. Accordingly, a novel methodology is presented that allows for the correction of the contribution of variation in flow rate to the in-line readings, thereby improving the predictions. An experimental program is performed on a 3D concrete printing system with various in-line sensors. The power of the pumping motor is found to have the weakest correlation with the yield stress at deposition, with an adjusted R-squared of 0.28, explained by the low efficiency of the pump, which causes noise in the measurement. The pressure difference over a straight cylindrical and the torsional force that is required to provide an oscillatory moment to this straight steel pipe are found to have a moderate correlation with an adjusted R-squared of 0.53 and 0.46, respectively, which is explained since yielding of the bulk concrete did not occur. Lastly, the pressure difference over the nozzle is found to have a good correlation with the bulk yield strength at deposition, with an adjusted R-squared value of 0.76, which is explained since yielding of the bulk concrete occurred. It is thus recommended to measure the pressure difference over the nozzle for in-line quality control of the bulk yield stress.
Water strongly influences the durability of cementitious materials in civil infrastructure. While physically plausible models exist to describe the long-term water absorption behavior of concrete, their underlying mechanisms remain unverified under relevant conditions. Here, using X-ray computed tomography, we track the gas-liquid-solid configuration within thousands of overcapillary-sized voids during 9 months of water absorption. Results reveal that long-term water uptake is linked to progressive macro-void saturation, evidenced by the gradual shrinkage of trapped gas bubbles. Their behavior is governed by buoyancy, surface wettability, and capillary forces acting at their curved interfaces and driving their dissolution. Contrary to prevailing models, void size has limited influence; instead, gas bubble dissolution is controlled by diffusion of dissolved gases and thus transport distance. Detailed analysis of the evolving air-water-void interface configuration enables critical evaluation of existing concepts and refines the mechanistic understanding of water in mortar and concrete – a topic of increasing importance with eco-friendly binders.
Hydrogrossular is the solid solution between Katoite (Ca3Al2(OH)12) and Grossular (Ca3Al2(SiO4)3). The hydrogrossular series are major hydration products of calcium (sulfo)aluminate cement and are also present in traditional cement systems hydrated at elevated temperature. Hydrogrossular is also important to the Earth's hydrosphere under different hydrothermal conditions. However, the formation kinetics and possible existence of a miscibility gap in hydrogrossular remains poorly understood. In this paper, we hydrothermally synthesized hydrogrossular under different Si concentrations, temperatures and durations. We were able to synthesize hydrogrossular with continuous chemical composition within the miscibility gap proposed in literature. We hypothesized a kinetic-driven multi-step crystallization process to explain the coexistence behavior rather than equilibrium immiscibility proposed in previous studies. Under our investigated hydrothermal synthesis conditions, the maximum Si4+ replacement to 4H+ in hydrogrossular showed a linear increasement trend with synthesis temperature, data from previous literature with different synthesis conditions are broadly consistent with this empirical correlation. These findings provide updated insights into the (thermo)chemistry of the hydrogrossular series and its formation in cement systems.
Low-carbon cements with reduced clinker factors increasingly rely on high-specific-surface-area supplementary cementitious materials (SCMs), notably calcined clays and emerging recycled fines. These binders often exhibit increased water demand, faster workability loss, and reduced early strength relative to ordinary Portland cement (OPC) and blended cements with fly ash or slag, thereby challenging the robustness of conventional admixture formulations. This review synthesizes current mechanistic understanding of how superplasticizers and hydration-controlling admixtures govern hydration kinetics and fresh-state performance in composite cements, with an emphasis on calcined-clay systems. For polycarboxylate ether (PCE) superplasticizers, we discuss structure–activity concepts in terms of adsorption, evolving surface area during early ettringite formation, and the effects on C₃S dissolution and C-S-H/CH nucleation. For accelerators and strength-enhancing admixtures, we evaluate calcium salts, alkanolamines, and C-S-H seeding, highlighting their impact on the hydration of SCM-rich binders. For retarders, we compare dissolution-, nucleation-, and growth-controlled mechanisms and explain how the presence of SCMs alters them. Finally, we connect the hydration processes of low-carbon cements to rheology through surface coverage, time-dependent specific surface area, particle packing, and the percolation threshold, and identify key knowledge gaps for predictive admixture design in low-carbon concretes.
Volume stability of C-S-H affects drying shrinkage of cementitious materials, which is closely related to its water retention. However, how Ca/Si modulates the C-S-H water retention and its influence on drying shrinkage remained unclear. We investigated the C-S-H water retention with varying Ca/Si and found this performance first increased but then attenuated with increasing Ca/Si. At low Ca/Si, C-S-H displayed high coordination degree of silicate chains, reducing available silanol sites for hydrogen bonding, thus weakening interaction with water molecules. Simultaneously, longer silicate chains packed loosely, forming larger gel pores. As Ca/Si increased, C-S-H showed lower coordination degree of silicate chains, generating more non-bridging oxygens for hydrogen bonding. Concomitantly, shorter silicate chains packed densely, yielding smaller gel pores. At very high Ca/Si (C1.7-S-H), excess Ca2+ compromised the hydrogen bonding interaction with water and led to Ca (OH)2 formation, which also enlarged gel pores. Results indicated that drying shrinkage was not governed solely by capillary pore structure, but also by water retention of C-S-H, advocating an efficient strategy for mitigating drying shrinkage by enhancing gel-water stability.
Natural siderite (FeCO3) was recently introduced as a novel SCM in OPC systems. Past investigations looked into the reaction mechanisms of these systems, as well as their durability on both a lab scale and real-scale. This study aims to evaluate the effects of both siderite and ankerite-rich (CaFe(CO3)2) mining residue in accelerated cementitious systems, like shotcrete. The systems were analyzed in terms of compressive strength, microstructure, carbonation resistance, and Ca2+-leaching (sintering potential), two durability parameters that can normally be inversely affected by SCMs. Siderite was tested (i) as fine aggregate (0-2 mm) and (ii) as part of the binder in binary and ternary systems. Ankerite-rich mining residue was used as part of the binder in binary systems, both in fine and ultra-fine fractions. Durability tests were complemented with mineralogical and chemical analyses. Iron carbonate-OPC systems reduced the Ca2+-leaching of mortars while simultaneously not worsening carbonation resistance. Despite lowering the buffer capacity through consumption of portlandite, siderite proved (i) capable of densifying the ITZ and (ii) stabilizing C-S-H during carbonation, possibly by Fe3+ incorporation into C-S-H. These effects also helped increase final compressive strength. Ankerite-rich mining residue, especially in the ultra-fine fraction, had reactions similar to siderite, albeit weaker due to its lower amount of reactive Fe. This study strengthens past investigations aiming for the usage of siderite as a novel SCM and opens new possibilities for investigations of Fe-rich mining residues in concrete production.
Power-laws have long been used to describe creep in cementitious systems, from C–S–H to concrete, yet their mechanistic origin remains unclear. Diffusion, layer sliding, and permeation are often invoked as microscopic drivers of creep in C–S–H, but it is uncertain whether these represent distinct mechanisms or different expressions of a single underlying process. This work explores the interrelation of these processes and their connection to creep kinetics using theoretical considerations informed by molecular simulations. C–S–H systems with slit pores spanning interlayer and gel pores are simulated via equilibrium molecular dynamics to get self-diffusion coefficients of confined water, and non-equilibrium Couette-like simulations to probe shear behavior. The theory provided shows that power-law creep may emerge naturally from subdifusive dynamics. Power-law exponents and creep shear modulus can be obtained from (subdiffusive) mean squared displacements obtained from molecular simulations. Subdiffusion in C–S–H interlayers is shown to be effectively asymptotic rather than transient. Confinement induces an exponential increase in water viscosity and enhanced subdiffusion, suggesting a feature that might be shared by other nanolayered adsorbing materials. Water relaxation emerges as the fundamental mechanism, having dielectric relaxation as an associated manifestation. Diffusion, shear behavior, viscosity, dielectric relaxation, and creep thus appear as macroscopic expressions of the same structural relaxation processes. Finally, it is shown that the mechanistic picture provided might explain not only power-law creep but also logarithmic creep: at short to long times, subdiffusion-controlled motion of interlayer water drives power-law creep, whereas at very long times, activated rearrangements (after exhaustion of low activation-energy barrier processes) dominate logarithmic creep.
This paper presents a novel methodology for analyzing power consumption curves during the concrete mixing process, with a particular focus on their relationship to the granulometric composition of mixtures. The proposed methodology enables the interpretation of power consumption curves as indicators of changes in the granular composition of concrete recipes, thereby facilitating the implementation of inline measurement systems for real-time quality control during mixing. In this way, the concrete mixer can be interpreted not only as a processing device, but also as a sensing system capable of capturing characteristic signatures of mixture composition during production. In its fresh state, concrete exhibits complex physical phenomena that span multiple disciplines, from fluid dynamics, which is relevant to flow behavior and pumping, to statistical mechanics, which addresses stochastic variations in grain size distribution and packing density. A deeper analysis of power consumption curves can significantly enhance quality control and optimize concrete production and industrial applications where this methodology is implemented.
Metakaolin-based Na-geopolymers can ceramicize during heating, but the extent to which ambient-temperature reaction progress predetermines crystallization remains unclear. In this study, geopolymers were prepared using a 5–10 mol/L activator-composition series. Calorimetry, proton relaxometry, selective dissolution, infrared spectroscopy, X-ray diffraction, thermal analysis, phase quantification, and electron microscopy showed that increasing NaOH molar concentration was associated with greater reaction extent and a shift in the high-temperature response from broad amorphous relaxation to a distinct crystallization exotherm.; the QXRD estimate of nepheline increased from 6.2% to 79.9% after heating at 1000 °C. Additionally, increasing silicate modulus from 1.2 to 1.4 lowered the apparent Kissinger activation energy from 350.9 to 178.5 kJ/mol and reduced the measured image-analysis mean nepheline grain diameter from 1.18 to 0.80 μm. This study establishes an association-based framework linking ambient-age reaction development and gel chemistry with the subsequent fired phase assemblage, apparent crystallization kinetics, and grain size.
Twin-pipe 3D concrete printing enables a rapid transition of concrete from a pumpable state to a buildable state through inline material mixing. However, optimization of the static mixer remains challenging due to the complex rheology and opaque nature of cement-based materials. In this study, magnetite was used as a density-contrasted tracer by replacing quartz aggregate in one constituent stream, enabling post-mixing visualization using X-ray computed tomography (CT). A geometry-consistent image analysis method based on equal-area segmentation of circular cross-sections was developed to quantify mixing homogeneity. Five static mixer geometries were experimentally evaluated using this CT-based approach. To complement the experiments, computational fluid dynamics (CFD) simulations with particle tracing were performed to investigate flow development and mixing evolution. The CFD predictions were validated against CT-derived mixing indices. Based on this combined CT-CFD framework, the five mixer types were comparatively assessed in terms of mixing performance, pressure drop, energy dissipation, and velocity distribution. The results reveal clear trade-offs between mixing efficiency and hydraulic resistance among different mixer geometries. The proposed methodology provides a quantitative framework for linking mixer geometry to flow and mixing behavior in twin-pipe 3D concrete printing and offers practical guidance for static mixer design.
Iron sulfide–bearing aggregates are a primary cause of internal sulfate attack in concrete, yet the intrinsic oxidation behavior of pyrite and pyrrhotite under alkaline conditions relevant to cementitious systems remains unclear. This study provides a quantitative, mechanistic comparison of their oxidation pathways through controlled batch experiments with elevated dissolved oxygen at pH 10–13 across a range of particle sizes, complemented by ion chromatography sulfur speciation and X-ray photoelectron spectroscopy (XPS).XPS analyses revealed that laboratory-synthesized monoclinic pyrrhotite retained an oxidized Fe(III) surface layer and a deeper S-rich altered region even after acid washing, indicating a persistent passivating surface structure that limits access to intrinsic dissolution behavior. However, pyrite formed thinner, more easily removed surface layers. Measured pyrite oxidation rates were consistent with reported intrinsic kinetic values, whereas pyrrhotite rates were nearly two orders of magnitude lower than those reported in recent high-pH studies which may be attributed to the passivation layer, experimental conditions, and type of pyrrhotite. Aqueous speciation revealed distinct pathways: pyrrhotite generated thiosulfate-dominated sulfur pools, while pyrite produced mainly sulfite and thiosulfate.Together, these results demonstrate that pyrrhotite's damaging role in concrete foundations cannot be attributed to inherently faster high-pH oxidation kinetics. Instead, deterioration likely reflects the interaction between pyrrhotite's persistent multilayer surface structure, oxygen-limited microenvironments within concrete, and the delayed release of partially oxidized sulfur species that ultimately generate sulfate. These mechanistic insights provide a basis for improving aggregate evaluation protocols and interpreting regional differences in deterioration timelines observed in pyrrhotite- and pyrite-bearing concrete foundations.
Seawater-mediated carbonation recycling of hydrated cement provides a promising pathway for enhancing concrete sustainability throughout its life cycle, yet the phase transformation during successive leaching and carbonation remains insufficiently understood, despite their critical roles in governing microstructural evolution and final material performance. Herein, experimental investigations combined with thermodynamic modeling were conducted to systematically examine the compositional evolution of both liquid and solid phases. Results show that prolonged leaching significantly alters hydrate assemblages and enriches dissolved Ca ions via ion-exchange reactions, thereby governing subsequent carbonation pathways. Chloride solutions accelerate hydrate decomposition and promote the reorganization of Cl ions into alumino-ferrite-mono (AFm) phases, particularly Kuzel's and Friedel's salts in CaCl2 solution, while Mg ions further destabilize Ca-rich hydrates and induce Mg-rich phases (MH, Ht and M-S-H). Subsequent carbonation transforms both liquid and solid compositions, with solution ions facilitating early CaCO3 and alumina-silica (A-S) gel precipitation and further modifying their structures through ion incorporation. Thermodynamic modeling corroborates the experimental observations and reveals a stepwise conservation of water-stable hydrates into CO2-stable products, primarily A-S gel, calcite, dolomite and natrolite under specific solution conditions, accompanied by corresponding changes in solution pH and ionic concentrations. Mechanistic analysis further demonstrates that leaching and carbonation are governed by coupled dissolution-precipitation-driven element rearrangements within cement hydrates. Specifically, Na ions sustain alkaline conditions and modulate aluminosilicate reorganization, Ca ions mediate decalcification repair and carbonate/AFm evolution with available Cl ions and carbonate species, and Mg ions promote Mg-rich hydrates while suppressing calcite precipitation, collectively dictating CO2 mineralization pathways in saline environments. These findings provide fundamental insights into seawater-mediated CO2 mineralization and support the sustainable design and recycling of cement-based materials.
Surface moisture critically influences interlayer bond behaviour in 3D-printed cement-based materials. This paper introduces a non-destructive approach using near-infrared (NIR) spectroscopy to quantify surface moisture and integrates real-time NIR sensing with a controlled investigation of surface moisture and interlayer bond strength. We first develop, calibrate, and validate an automated NIR-based system. Using this system, we investigate how surface moisture variations—induced by wetting or drying under diverse environmental conditions—affect interlayer performance. Our findings highlight the pivotal role of surface moisture in governing local cement hydration, local porosity, and ultimate interlayer bond strength. Notably, we identify a unique relationship between surface moisture and bond strength for a given printable material, independent of the moisture history (wetting and/or drying) investigated in this paper. Leveraging these insights, we propose an active surface moisture control strategy capable of precisely targeting and regulating bond strength across varying environmental conditions. This approach offers a robust framework for optimising or tuning the structural properties of 3D-printed cementitious materials.
Evaluating the rheological performance of modern concrete requires large datasets due to the diversity of mixture compositions and the inherent variability of cementitious materials. Our previous study introduced automated experimentation for high-throughput rheological measurement of cement-based materials. In this study, the system is extended to enable reliable estimation of Bingham parameters by integrating domain adaptation. A total of 100 mortar samples with systematically varied mix proportions were tested using the proposed framework. The resulting large, high-quality dataset, combined with machine learning, enables accurate estimation of Bingham parameters that are consistent with the measured torque response. Shapley additive explanations further confirm the proposed approach provides physically reasonable Bingham parameter estimates. The proposed system demonstrates its applicability to PCE performance evaluation, detection of subtle variations in rheological behavior, and quantitative assessment toward rheological standardization. Through this work, an automated experimentation platform for rheological characterization of cement-based materials has been completed, establishing a methodological transition toward data-centric concrete rheology.
Existing rheological testing methods for cementitious materials are either straightforward but information-limited or complex and time-consuming, making them inadequate for intelligent construction. This study proposes an energy-transfer analysis approach based on gravity-wave characteristics, enabling rapid rheological identification via non-contact optical sensing. Gravity waves were generated by impacting a rod on cement paste at ten hydration ages (15–150 min), and the resulting surface height variations were precisely measured using a laser profiler. Energy transfer efficiency (ETE) and wave slope (WS) were extracted to quantify wave energy conversion and calibrated against rotational rheometer measurements using the Modified Bingham (MB) and Herschel-Bulkley (HB) models. ETE and WS showed strong correlations with rheological parameters and captured their hydration-dependent evolution. The established identification model achieved accuracies of 94.3 ± 0.7% and 82.0 ± 4.1% for MB and HB models, respectively. Uncertainty propagation further demonstrated the greater robustness of MB-based identification, with a much narrower 95% bootstrap CI than HB-based identification. Energy dissipation physics were found to be consistent with the exponential attenuation of macroscopic waves. This sampling-free method provides theoretical support for online real-time rheological monitoring of cementitious materials.
Alkali-free accelerators (AFAs) are essential components in modern shotcrete, enabling rapid setting and early strength development while complying with increasingly stringent environmental and safety requirements. However, the growing complexity of AFA formulations and their interactions with cementitious systems has hindered the development of a unified understanding of their mechanisms and performance. This paper presents a comprehensive and systematic review of AFAs for shotcrete applications. The compositional design of AFAs is first summarized, highlighting their multi-component nature and the specific roles of individual constituents in accelerating setting and hardening. The effects of AFAs on cement hydration are then critically examined, with particular focus on sulfate balance in regulating aluminate reactions and the evolving understanding of silicate hydration. Subsequently, the influence of AFAs on key properties, including setting characteristics, strength development, and durability-related performance, is discussed. Compatibility issues with different cement types, supplementary cementitious materials, chemical admixtures, and environmental conditions are also evaluated. In addition, current international standards and guidelines for AFAs are compared. Overall, this review provides a structured framework for understanding AFA mechanisms and performance, offering guidance for the rational design of AFAs and their reliable application in engineering practice.
