Cement and Concrete Research最新文献

The reaction kinetics of silica fume in Portland cement binders directly affect porosity and strength development during the early and intermediate stages of hydration. This study sheds light on silica fume’s dissolution rate and mechanism at 23 °C in a 0.1 mol L^-1 KOH solution as a function of its composition, specific surface area, and saturation state. Conductivity measurements are used to continuously track the reaction progress and correlate it to dissolved concentration. The dependence of the dissolution rate on silica fume surface area and saturation state is described, and a rate equation consistent with standard chemical kinetics is developed to model the rates as a function of the thermodynamic driving force. The inferred rate equation can be used to improve models of microstructure evolution in blended cement binders.

Several experimental studies have been conducted on the thermoelectric properties of cementitious materials, but a detailed inspection of the intrinsic properties of their main ingredients is still missing. This work focuses on the thermoelectric properties of portlandite and tobermorite, two mineral components found in Ordinary Portland Cement pastes. To this end, atomistic simulations were carried out to predict the thermoelectric properties of cement-based materials. The methodology is based on the density functional theory approach together with GW-quasiparticle and Boltzmann transport equation methods. As expected, the undoped minerals have low thermal conductivity. However, both the Seebeck coefficient and the electrical conductivity can be dramatically increased by appropriate carrier doping. In fact, an enhanced figure of merit of Z = 0.6 at 650 K and 0.79 at 600 K is observed for portlandite and tobermorite. Therefore, our results confirm that there are still much promising prospects for enhancing the characteristics of concrete materials for energy harvesting.

In Trois-Rivières, Canada, numerous commercial and residential structures suffered severe concrete degradation due to internal sulfate attack (ISA). The deterioration was attributed to the use of sulfide-bearing aggregates obtained from nearby quarries. The objective of this study was to investigate the mechanisms involving pyrrhotite oxidation and the role of biotite in this process. Detailed analysis, including reflected light optical characterization, mineral chemistry, and EPMA chemical mapping, revealed significant oxidation of pyrrhotite within damaged concrete, resulting in the formation of iron oxyhydroxides as secondary phases. Furthermore, a textural association between biotite mica and pyrrhotite was observed in fresh and damaged aggregate samples. While the chemical composition of biotite remains unchanged, it accelerates the deleterious reactions when in direct contact with pyrrhotite. As oxidation proceeds, naturally occurring biotite crystals near pyrrhotite act as pathways, facilitating the ingress of oxygen and water while simultaneously channeling the release of iron and sulfur compounds.

Solid waste derived clinker design methodology was established based on thermodynamic simulation and experiment validation via the recycling of incinerated sewage sludge ash (ISSA) and recycled concrete fine (RCF). Compared with 1000 °C and 1100 °C, 1200 °C was the optimum temperature for C₂S-rich clinker synthesis due to the highest simulated belite content after sintering. A utilization rate of 95 % was achieved, accomplishing the maximization of solid waste recycling. Due to the CaCO₃ phase formation by the reaction of high-content belite (46.8 %) with CO₂, carbonated 1200ARS eco-cement showed higher compressive strength than OPC and other eco-cements. The lowest porosity (18.1 %) with dense microstructure was also obtained by carbonated 1200ARS. Benefiting from the high CO₂ reactivity of belite, 1200ARS displayed the optimum CO₂ sequestration capacity with a higher carbonation degree (30.1 %) than other batches, providing a promising direction for future development of the low-carbon cement industry to achieve environmental sustainability.

A new Python code for the automated generation of realistic bulk calcium silicate hydrate (C-S-H) structures is introduced. The code was used to generate 400 structures with Ca/Si of 1.3, 1.5, 1.7, and 1.9. The generated structures are in excellent agreement with experimentally measured C-S-H properties (Ca/Si, 2H/Si, MCL, Si-OH/Si, and Ca-OH/Ca). Molecular dynamics was used to simulate the structures, which were then investigated for their structural features and energetic stability. The results indicate very similar short-range ordering and energetic stability between all 400 structures. Finally, it is shown how computational C-S-H models can be used to understand the experimentally measured pair distribution functions. The code, named pyCSH, is available as open source under GPL-2 license at GitHub https://github.com/hegoimanzano/pyCSH.git and https://github.com/akmiitm/pyCSH.git.

The effect of NaOH on dispersing performance of HPEG PCE superplasticizers and early strength was studied in a calcined clay (CC) blended cement (OPC:CC = 70:30 wt./wt.). Activator impact on workability and mechanical properties was investigated on mortar via fluidity and strength tests. The results revealed a time-dependent impact. At early ages (16 h – 3 d), the strength of the NaOH-activated OPC/CC blend exceeds that of OPC, while at longer curing times ($>$ 3 d) it is $\sim$ 30 % below that of OPC. Among the HPEG PCEs tested, the polymer possessing a medium long side chain (n_EO = 50) performed best. Generally, NaOH addition prompted significantly increased PCE dosages. Heat flow calorimetry and in-situ XRD measurements suggest that NaOH promotes initial hydration of both OPC and CC, but later delays OPC reaction. Zeta potential measurements confirmed that NaOH significantly reduces the adsorbed amount of PCE on the binder, thus explaining higher dosage requirement.

This study proposes a two-scale model for tricalcium silicate (C₃S) hydration to examine the microstructure of hydrated C₃S paste. The needle-like morphology of a calcium silicate hydrate (C-S-H) layer, as observed in many studies, was constructed using this model based on the assumption of C-S-H needles. The boundary nucleation and growth pattern of the C-S-H needles were illustrated, and the impingement among C-S-H needles and particles was considered. The pore size distribution was calculated using the rolling sphere method for the hydrated particle system and C-S-H layer. The simulation results were compared with existing experimental data, revealing consistency for the C₃S paste with different hydration times. The shape and growth parameters of the C-S-H needles were studied to model their microstructure. The microstructure constructed using this model was found to be more reasonable and could provide a more accurate description of the C-S-H morphology.

The very early age flocculation of a cement based paste, in which cement is partially substituted by a calcined clay, metakaolin, is studied. We use rheological and ultrasonic reflection to monitor the evolution of the elastic properties of the paste with time, and light scattering to follow the change of its relaxation modes at small scales. We show that, at very early ages, of the order of hundreds of seconds after the paste preparation, a network of flocculated particles establishes, which manifests by a slow down of the dynamics of the relaxation modes of the paste at small scale. Then, this network consolidates and the macroscopic elastic modulus of the paste progressively increases with time, leading eventually to the setting of the paste. These observations show that, whatever the degree of replacement of clinker by metakaolin, a connected network establishes at very early times after the paste preparation, although the kinetics of flocculation slows down with clinker replacement.