Effects of temperature and CO2 concentration on the early stage nucleation of calcium carbonate by reactive molecular dynamics simulations.

It is significant to investigate the calcium carbonate (CaCO3) precipitation mechanism during the carbon capture process; nevertheless, CaCO3 precipitation is not clearly understood yet. Understanding the carbonation mechanism at the atomic level can contribute to the mineralization capture and utilization of carbon dioxide, as well as the development of new cementitious materials with high-performance. There are many factors, such as temperature and CO2 concentration, that can influence the carbonation reaction. In order to achieve better carbonation efficiency, the reaction conditions of carbonation should be fully verified. Therefore, based on molecular dynamics simulations, this paper investigates the atomic-scale mechanism of carbonation. We investigate the effect of carbonation factors, including temperature and concentration, on the kinetics of carbonation (polymerization rate and activation energy), the early nucleation of calcium carbonate, etc. Then, we analyze the local stresses of atoms to reveal the driving force of early stage carbonate nucleation and the reasons for the evolution of polymerization rate and activation energy. Results show that the higher the calcium concentration or temperature, the higher the polymerization rate of calcium carbonate. In addition, the activation energies of the carbonation reaction increase with the decrease in calcium concentrations.

New insights into the early stage nucleation of calcium carbonate gels by reactive molecular dynamics simulations.

The precipitation of calcium carbonate (CaCO3) is a key mechanism in carbon capture applications relying on mineralization. In that regard, Ca-rich cementitious binders offer a unique opportunity to act as a large-scale carbon sink by immobilizing CO2 as calcium carbonate by mineralization. However, the atomistic mechanism of calcium carbonate formation is still not fully understood. Here, we study the atomic scale nucleation mechanism of an early stage amorphous CaCO3 gel based on reactive molecular dynamics (MD) simulations. We observe that reactive MD offers a notably improved description of this reaction as compared to classical MD, which allows us to reveal new insights into the structure of amorphous calcium carbonate gels and formation kinetics thereof.

Salt frost damage evolution and transport properties of recycled aggregate concrete under sustained compressive loading.

The frost damage behavior of recycled aggregates concrete (RAC) in a cold region is inherently more complex due to the incorporation of recycled coarse aggregate (RCA). In real-world service environments, the combined effects of mechanical loading and environmental conditions further make RAC's damage mechanism more intricate. This study explores the impact of uniaxial compressive loading (at 0.1fc, 0.3fc, and 0.5fc, respectively), freeze-thaw cycles, and chloride penetration on the relative dynamic elastic modulus (RDEM), mass transport properties, and microstructure of RAC with varying RCA replacement ratios. The results indicate that specimens loaded at 0.3fc exhibit enhanced frost resistance, with reduced water absorption and chloride ion content. Additionally, a damage model is developed to quantify the effects of mechanical loading, freeze-thaw cycles, and chloride penetration on RDEM degradation. The investigation using X-ray computed tomography (X-CT), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) techniques reveals that as compressive stress levels increase, the microstructural density and porosity of RAC initially decrease before increasing. Moreover, the RDEM of RAC decreases with decreasing pore sphericity. Compared to the R100-S55 samples, the pore sphericity of R100-S55-0.5fc samples increased by 60.4 % in the range of 0.4-0.5, resulting in a decrease of approximately 17.72 % in the RDEM. Furthermore, the initial sorptivity of frost-damaged RAC exhibits a significant linear relationship with porosity. Overall, this study elucidates the evolving trends of mass transport properties and microstructure in RAC under loading and freeze-thaw conditions, laying a theoretical groundwork for the widespread application of RCA.

Effect of Corrosion on the Bond Behavior of Steel-Reinforced, Alkali-Activated Slag Concrete.

Alkali-activated slag concrete (ASC) is regarded as one of the most promising sustainable construction materials for replacing ordinary Portland cement concrete (OPC) due to its comparable strength and outstanding durability in challenging environments. In this study, the corrosion of steel bars embedded in ASC and OPC was studied by means of an electrically accelerated corrosion test of steel bars in concrete. Meanwhile, the bond performance of the corroded steel bars embedded in ASC was tested and compared with corresponding OPC groups. The results showed that ASC and OPC behaved differently in terms of bond deterioration. The high chemical resistance of ASC decreased the corrosion of steel bars and, thus, increased the residue bond strength and the bond stiffness.

Bond Performance of Steel Bar and Fly Ash-Based Geopolymer Concrete in Beam End Tests.

This paper presents a comprehensive investigation of the bond characteristics of steel bar reinforced geopolymer concrete (GPC). The ASTM A944 beam end tests were conducted on GPC beams reinforced with plain or ribbed bars. The bond-slip curves and the bond strength of GPC beams were obtained. The relationship between the bond stress and relative slip in plain and ribbed bar reinforced GPC has been represented by empirical formulae. The bond testing results were compared with those of corresponding ordinary Portland cement concrete (OPC) using statistical hypothesis tests. The results of hypothesis testing showed that GPC was significantly superior to OPC in terms of bond capability with plain bars and bond stiffness with ribbed bars. The statistical analysis indicated that the bond-slip relations derived for OPC are inapplicable to GPC; thus, new bond-slip relations are suggested to estimate the development of bond stress and relative slip between GPC and steel bars.