Atomic-scale investigation of physical adsorption of water molecules and aggressive ions to ettringite's surfaces

State-of-the-Art Review of Capabilities and Limitations of Polymer and Glass Fibers Used for Fiber-Reinforced Concrete

The concrete industry has long been adding discrete fibers to cementitious materials to compensate for their (relatively) low tensile strengths and control possible cracks. Extensive past studies have identified effective strategies to mix and utilize the discrete fibers, but as the fiber material properties advance, so do the properties of the cementitious composites made with them. Thus, it is critical to have a state-of-the-art understanding of not only the effects of individual fiber types on various properties of concrete, but also how those properties are influenced by changing the fiber type. For this purpose, the current study provides a detailed review of the relevant literature pertaining to different fiber types considered for fiber-reinforced concrete (FRC) applications with a focus on their capabilities, limitations, common uses, and most recent advances. To achieve this goal, the main fiber properties that are influential on the characteristics of cementitious composites in the fresh and hardened states are first investigated. The study is then extended to the stability of the identified fibers in alkaline environments and how they bond with cementitious matrices. The effects of fiber type on the workability, pre- and post-peak mechanical properties, shrinkage, and extreme temperature resistance of the FRC are explored as well. In offering holistic comparisons, the outcome of this study provides a comprehensive guide to properly choose and utilize the benefits of fibers in concrete, facilitating an informed design of various FRC products.

Adsorption Conformation of Comb-Shaped Polycarboxylate Ethers on Ettringite (100) Surface: An Atomic Scales Simulation

The adsorption of five polycarboxylate ethers (PCEs) oligomers with different side-chain number and side-chain length on the ettringite (100) surface in explicit solution is studied by all-atom molecular dynamics (MD) simulations. The adsorption conformations, adsorption energies, the radial distribution functions (RDF) between PCEs and ettringite surface and density distribution profile of water perpendicular to the substrate are analyzed. After dynamic equilibrium, negatively charged carboxylate groups are absorbed on the surface of the disordered ettringite crystal and the side-chains are extended to solution. The influence of the number of side-chain on the adsorption strength of PCEs on the ettringite (100) surface is more significant than that of the length of side-chain. The less number of grafted side-chains, the weaker electrostatic shielding, the electrostatic interaction between PCEs and ettringite (100) surface is stronger, which is favorable for the adsorption. The conformation of adsorbed PCEs is closely related to the length of side chain. The greater the length of side-chains is, the larger will be the coverage area of PCE on surface and the degree of water reduction. The mechanism of adsorption of PCEs on ettringite surface is concluded and the major contribution to the adsorption includes ion pairs, hydrogen bonds and an entropic compensation.

Rapid solidification of Portland cement/polyacrylamide hydrogel (PC/PAM) composites for diverse wastewater treatments

Cementitious solidification is an effective but time-consuming method for waste disposal, and the incorporation of polyacrylamide hydrogel in Portland cement paste is a simple way to enhance the time-efficiency of cementitious solidification. In this study, a series of Portland cement/polyacrylamide hydrogel (PC/PAM) composites suitable for the wastewater treatment were prepared by a one-pot method and their time-dependent reaction processes, mechanical properties and microstructures were tested. Based on the gelation time method, PC/PAM composites showed great solidification efficiency when treating simulated radioactive liquids, organic dye waste and solutions with strong alkalinity and acidity. At temperatures ranging from 5 °C to 40 °C, it took only a few minutes for these composites to solidify wastes. Also, PC/PAM composites containing wastes had a compressive strength that is more than 2 MPa after reacting for 3 days and were suitable for landfill or secondary treatments. The rapid gelation and sufficient strength development demonstrated that PC/PAM composites have great potential for application in solidifying multi-component wastes, especially in some emergency circumstances.

Novel and stable PC/PAM composites were designed and could serve as a matrix for rapid solidification of various wastewaters, greatly facilitating the applicability of the cementitious solidification method in emergency circumstances.

Structure, orientation, and dynamics of water-soluble ions adsorbed to basal surfaces of calcium monosulfoaluminate hydrates

Transport of water molecules and chloride ions in nanopores of hydrated cement paste (HCP) is proven to adversely affect the long-term durability of reinforced concrete structures exposed to seawater or deicing salts. The resistance against chloride attack is primarily associated with the chloride binding capacity of the main HCP constituents. Experimental tests revealed that AFm phases of HCP play a central role in binding the chloride ions. However, many aspects of AFm-solution interactions were largely unknown, especially at their interfaces. This was the motivation of the current study, in which the atomistic processes underlying the transport of water-soluble ions are investigated in detail using the classical molecular dynamics (MD) method. To this end, an aqueous layer containing various concentrations of sodium chloride solution is sandwiched between two basal surfaces of calcium monosulfoaluminate hydrate, which is the most abundant phase of AFm. The adsorption mechanisms of water molecules and diffusing ions are then characterized for inner- and outer-sphere distance ranges from the basal surfaces of monosulfoaluminate. It is found that the self-diffusion coefficient of the chloride and sodium ions present in the outer-sphere range are 83% and 47% larger than those residing in the inner-sphere range. With increasing the distance from the solid surface, an increase in the self-diffusion coefficient is captured. This increase in mobility is larger for chloride ions than sodium ions. This can be understood based on the observation that the inner- and outer-sphere complex formation are the governing adsorption mechanisms for the chloride and sodium ions, respectively.

Molecular dynamics study on ions and water confined in the nanometer channel of Friedel's salt: structure, dynamics and interfacial interaction

As a promising layered double hydroxide, Friedel's salt has gained popularity.