With the growing interest in Mg-based cements, the early-age hydration mechanisms of Mg-carbonate binders, such as MgO/Nq, remain insufficiently studied. This study investigates the early hydration of MgO/Nq binder, focusing on the hydration kinetics, carbonate phase formation, and water partitioning within the paste. These processes were followed using isothermal calorimetry, internal relative humidity measurements, XRD, TGA, FTIR, total carbon analysis, 13C NMR, and 1H NMR.The results reveal two distinct hydration regimes. During the first 20–24 h, MgO and Nq react slowly, forming mainly an amorphous Mg-hydroxycarbonate, MHC. After total consumption of Nq, MgO hydration accelerates markedly, and MHC reacts with MgO and water to form hydrous carbonate-containing brucite, HCB. This transition is accompanied by pronounced heat release, increased water consumption, pore refinement, and a sharp decrease in internal relative humidity. The findings highlight the key role of transient amorphous Mg-hydroxycarbonate phase in controlling the early hydration and microstructural development of MgO/Nq binders.
Chloride diffusion through C-S-H, the primary binding phase in cementitious materials, plays a critical role in controlling corrosion initiation. The temperature dependence of the diffusion rate, described by activation energy Ea, is essential for accurate service-life prediction, but remains poorly understood in C-S-H because the complexity of cement paste composite makes it difficult to isolate the role of C-S-H. In this study, C-S-H samples with controlled Ca/Si were prepared from hydrothermal synthesis and tricalcium silicate hydration. The diffusion activation energies were determined from results of chloride diffusion tests in the temperature range of 5–60 °C on chemically pure C-S-H monoliths with well-defined pore structure, where the effects of pore structure, Ca/Si and surface charge reversal were explored. The experimental results demonstrate that a finer pore structure with a higher fraction of nanopores produced by 800 MPa-compaction, results in a higher activation energy, whereas pore tortuosity shows little influence. The effect of the Ca/Si ratio is explained by the relative contributions of interfacial partitioning and intrapore diffusion, as described by the partition-diffusion model, in which chloride transport involves interfacial partitioning followed by diffusion within the pore: for C-S-H with lower Ca/Si, the more negatively charged surface increases the partition energy through stronger electrostatic repulsion at the pore entrance, whereas after surface charge reversal, intrapore ion-surface interactions become dominant. The overall activation energy is therefore determined by the balance between these two processes.
The decarbonization of precast concrete is primarily driven by the reduction of clinker factor and total binder content. However, their implementation is constrained by stringent early-age requirements and rheological properties associated with industrial production cycles. This paper presents a critical review of low-clinker and low-binder strategies for precast concrete, with specific focus on their interaction with hydration kinetics, rheological behavior and process constraints.The literature analysis highlights that clinker reduction and binder optimization affect, respectively, early-age strength development and fresh-state performance, making their combined application particularly challenging. Various technological solutions are discussed, including composite binders, hydration control through chemical activation (e.g. nucleation seeding and tailored accelerating admixtures), and rheological optimization in low-paste systems through advanced superplasticizer design and particle packing strategies.To complement the review, a full-scale industrial validation is presented. Two concrete mixtures based on limestone calcined clay cement (LC3)-type binders, reduced binder content and targeted accelerating admixtures were implemented in the production of prestressed beams and lightweight sandwich panels under standard plant conditions. The results confirm that early-age requirements can be satisfied within typical demolding times, without modifying the production process or compromising element quality, while reducing the cradle-to-gate GWP by 18% for the prestressed beams and 13% for the sandwich panels, with negligible cost variation.The combined analysis demonstrates that low-clinker, reduced-binder strategies are technically viable for precast applications when mix design is adapted to process-driven constraints.
Serpentine is widely used for CO2 sequestration, where Mg ions released during its dissolution react with CO2 to form stable MgCO3. The resulting silica-rich residue is a promising low-carbon supplementary cementitious material because of its amorphous structure and high silica content. However, the mechanisms controlling the pozzolanicity of acid-activated serpentine (AAS) remain insufficiently understood. This research evaluates the pozzolanicity of AAS and elucidates the mechanisms that govern its reactivity. AAS samples were prepared through controlled acid-leaching under different temperatures, reaction durations, acid concentrations, and solid-to-liquid ratios. Crystalline and amorphous phases were characterized through X-ray diffraction and Raman spectroscopy, while pozzolanicity was accessed via the Frattini test. The results reveal that AAS pozzolanicity is governed by two concurrent processes: dissolution and polymerization of silicate species. Dissolution enhances reactivity by collapsing Q3 structures into smaller silica oligomers and exposing additional reactive silanol groups, while polymerization consumes these reactive groups and suppresses pozzolanicity. Based on the evolution of silicate species, we proposed an index for quick assessment of pozzolanicity, which not only aligns well with results of traditional method but also has higher efficiency and clear chemical meaning.
Engineering Cementitious Composites (ECC) exhibit exceptional ductility and deformation-hardening behavior, with performance significantly influenced by loading configuration. While current theoretical studies predominantly focus on ECC behavior under tension, more prevalent bending scenarios encountered in practical engineering applications have not been adequately investigated. To bridge this gap, a comprehensive multi-scale model capable of simulating ECC's bending behavior was developed. The model integrates ECC's microstructural characteristics and accounts for crack propagation criteria as well as stress redistribution between cracks to accurately represent the multi-crack propagation process under bending. For the tested ECC, the simulated results showed good agreement with experimentations, and the model captures the discrete nature of ECC's bending properties by introducing randomness into parameters. Additionally, the disparities in ECC's mechanical responses under bending versus tension were investigated through both theoretical modeling and experimental validation. By enhancing the understanding of ECC's bending behavior, this study should facilitate the practical application of ECC in engineering contexts.
