Cement and Concrete Research最新文献

The Clausius-Clapeyron equation is an effective tool for describing the effect of temperature on water vapour adsorption isotherms in cementitious materials. The key information is the isosteric energy. This can currently be characterised experimentally using two approaches: (1) the isotherms method, which involves experimentally acquiring the desorption isotherm for two (or more) different temperatures, and (2) the hygrometric method, which involves monitoring the increase in vapour pressure at equilibrium with a small sample subjected to increasing temperature steps. It turns out that although each method has a fairly high uncertainty, the results obtained are similar. Finally, the results seem to suggest that the isosteric energy of Portland-based cementitious materials could be considered unique.

Calcium-silicate-hydrate (C-S-H) is a disordered, nanocrystalline material, acting as a primary binding phase in Portland cement. C-S-H and C-A-S-H (an Al-bearing substitute present in low-CO₂ cement) contain thin films of water on solid surfaces and inside nanopores. Water controls multiple chemical and mechanical properties of C-S-H, including drying shrinkage, ion transport, creep, and thermal behavior. Therefore, obtaining a fundamental understanding of its properties is essential. We applied a combination of inelastic incoherent neutron scattering and molecular dynamics simulations to unravel water dynamics in synthetic C-(A)-S-H conditioned at five hydration states (from drier to more hydrated) and with three Ca/Si ratios (0.9, 1, and 1.3). Our results converge towards a picture where the evolution from thin layers of interfacial water to bulk-like capillary water is dampened by the structure of C-(A)-S-H. In particular, the hydrophilic Ca²⁺ sites organize the distribution of interfacial C-(A)-S-H water.

Due to the physical and chemical effect of polycarboxylate (PCE) ether superplasticizers on the simultaneous hydration of aluminate phase and silicate phase, the structural build-up of cement paste with PCE remains a much-complicated process. In order to reveal the underlying mechanism, this study reports the thixotropic structural build-up of C₃S paste with PCE in the early stage (stage I before initial setting, within 1500 s). It should be subdivided into stage I′ (rapid non-linear increase) and stage I″ (slow linear development), since PCE significantly prolongs the duration of stage I′ from ∼10 s to ∼1000 s. Although PCE does not alter the origins of thixotropy (CSH/C₃S cohesive forces and colloidal interactions), it can change the magnitude of driving forces, greatly depending on its adsorption in the pseudo-contact region. Consequently, the dominant driving force in stage I′ is C₃S cohesive force, while it is colloidal interactions in stage I″. The quantitative models of colloidal percolation characteristic time (t_perc) and thixotropic structural build-up rate (G_thix) are developed, both of which are determined by the surface coverage and initial solid volume fraction. Increasing PCE dosage augments t_perc and diminishes G_thix, until reaching the maximum adsorption threshold (not full surface coverage), beyond which further PCE increase has a minimal effect on t_perc and G_thix.

Understanding the kinetics and mechanisms involved in early-age hydration of tricalcium aluminate (C₃A) in chloride solutions holds promise for implementing seawater-mixed concrete in the marine environment, as C₃A remains the most reactive component of Portland cement (PC), affecting both PC and concrete's early-age hardening and long-term durability. Herein, we conducted a series of meticulously designed ex-situ and in-situ experiments to elucidate the intricate hydration behaviors of C₃A in various chloride solutions. The results reveal that C₃A exhibits distinct hydration kinetics and structural evolution processes in different solutions. The rapid precipitation of alumino-ferrite-mono (AFm) and C₃AH₆ phases contributes to the swift development of hydration heat and storage modulus in water and NaCl solutions, with a slight acceleration observed in the later one. Conversely, the formation of C₃AH₆ is delayed in CaCl₂ and MgCl₂ solutions before 20 min, with the subsequent precipitation of Cl-AFm enhancing its later production, particularly in CaCl₂ solutions. Ab-initio calculations further elucidate that the acceleration effect of Cl ions originates from the ionization and structurization of hydrated surface Ca ions. However, this positive effect is significantly offset by Cl pairing with counterions, resulting in a dramatic adverse effect of solution Ca ions originated from the negative entropy effect of structuralized water molecules and electrostatic repulsion with like-charged surface Ca ions and solvent dipoles. Our findings provide valuable insights for sustainable and durable designs on cement-based materials mixed with seawater.

Almost all properties of hydrated cementitious materials depend strongly on their pore structure. Many methods to quantify porosity have been applied to cementitious materials, but there is a huge confusion in the literature about the utility of these methods. By comparing the results between different methods, including MIP, ¹H NMR and nitrogen adsorption, we highlight that semi-quantitative information can reliably be obtained from these methods. In particular, we demonstrate that they are consistent in their relative range of validity. This range of validity is explained in terms of microstructure features. We also show how the results are linked to macroscopic observables such as microstructure development (degree of hydration) and transport (conductivity and apparent chloride diffusion).

This study enhanced the hydration of steel slag (SS) by the synergistic chemical activation of triisopropanolamine (TIPA) and sulfite/sulfate. Adding 0.2% TIPA into a cementitious material consisting of 62.5% SS and 37.5% desulfurization ash (DA) increased the 3-days and 120-days compressive strength by 9.59 times and 1.78 times, respectively. This enhancement was due to the accelerated dissolution of C₂S and C₂F facilitated by DA and TIPA. The complexation between TIPA and Fe accelerated the consumption of Ca²⁺ and SO₄²⁻, thereby promoted the generation of Fe-containing hydrates such as C₂(F, A)₃S₃H₂ and FeSO₄·7H₂O, significantly refining the pore structure. The TIPA-Fe complex also contributed to C-(F)-S-H gels formation, shifting hydration exothermal peak from 4.8 h to 1.1 h, with the peak value rising from 0.90 J/(g·h) to 57.07 J/(g·h). The Fe evolution even induced a colour change in the matrix. These insights contribute valuable perspectives for utilizing Fe-rich solid-wastes in cementitious materials.

The hydration and pore structure of hardened cement pastes containing various amounts of alkali and hexylene glycol -a shrinkage-reducing admixture (SRA)- are studied. Until three months, SRA retards cement paste hydration regardless of alkali content; after seven days at most, this retardation diminishes with time. Alkalis increase the hydration degree at early ages for all pastes. The pore structure coarsens with the SRA: both the specific surface area and the volume of pores with a 5 nm entry size decrease in the presence of the SRA. The magnitude by which the SRA impacts the pore structure reduces with alkali.

In alkali media, the C-S-H gel uptakes alkalis and aluminum; modifying the C-S-H structure, the gel pore volume, and the interlayer space. The SRA depletes the alkalis from the solution and may increase the C-S-H alkali uptake, which could lead to changes in the gel pore volume and specific surface area.

Hydration of a commercial white calcium aluminate cement (CAC) at 23 °C was modified by silica fume (SF) addition in varying amounts. The process was followed by heat flow calorimetry, quantitative in-situ XRD analysis and Gillmore needle experiments supplemented by pore solution analysis, thermodynamic modelling, and ¹H-TD-NMR measurements. Lower SF/cement ratios accelerate the hydration of CAC. Higher ratios trigger an intermediate heat flow event, which is correlated to increased setting rates. This intermediate event (IE) initiates an induction period of constant duration, which appears later with increasing SF/cement ratios. Results show the IE is caused by an initially hindered CA dissolution, in which dissolved silicon provided by SF plays a crucial role. Increasing the [Si] concentration in the pore solution leads to a further retardation of the IE and eventually prevents the entire hydration reaction if a critical amount is reached. A detailed model explaining the observed behavior is proposed.

A binder mix consisting of CAC, calcite and H₂O was investigated to clarify the influence of the addition of low amounts of calcium sulfate in the form of gypsum, hemihydrate or anhydrite on the hydration at 23 °C. Based on experimental data, a model for the hydration of CAC + calcite in the presence of sulfate ions could be developed. Using heat flow calorimetry at 23 °C, it was shown that hydration is accelerated depending on the solubility rates of the different sulfates. In addition, early hydration is characterized by a sequential reaction in which the calcium sulfate must be consumed completely from the pore solution by the precipitation of ettringite before the already known course of CAC + Cc hydration can occur. Monocarbonate and AH₃ precipitate as the stable dominant hydrate phases during the main reaction, but conversion-sensitive hydrate phases such as CAH₁₀ are not stable.

The influence of triethanolamine (TEA) on the hydration of cement-based materials is closely linked to its adsorption behavior in Ordinary Portland Cement (OPC). This study investigated the adsorption process and mechanism of TEA on OPC and pure mineral phases—alite, gypsum, aluminate+gypsum, and calcium hydroxide (CH)—in the first 8 h. The results revealed that in all single phases, TEA adsorption was associated with alite and CH. The crystal structure of CH did not change during adsorption, likely due to the physical adsorption of TEA. During OPC hydration, the adsorption of TEA was primarily associated with the hydration of alite. In the initial hydration stage, only CH served as the main adsorption receptor, which was supported by adsorption energy simulations using molecular dynamics. As alite hydration progresses, the role of the receptor may shift toward calcium silicate hydrate (C-S-H), as indicated by the calculated adsorption limit of CH. Furthermore, density functional theory (DFT) demonstrated that TEACa²⁺ has the lowest complexation energy when the ligand-to-metal ratio is 1:1 and becomes even more stable when the ligand-to-metal ratio is 2:1.