The macroscopic mechanical properties and frost resistance durability of concrete are closely related to the changes in the internal pore structure. In this study, the two-dimensional and three-dimensional ICT (Industrial Computerized Tomography) pore characteristics of C30 concrete specimens before and after freezing and thawing in clean water, 5 wt.% NaCl, 5 wt.% CaCl₂, and 5 wt.% CH₃COOK solution environments are obtained through concrete frost resistance durability test and ICT scanning technology. The effects of pore structure changes on concrete frost resistance, durability, and compressive strength mechanical properties after freezing and thawing cycles in different salt solution environments are analyzed. This paper provides new means and ideas for the study of concrete pores. The results show that with the increase in the freezing and thawing times, the concrete porosity, two-dimensional pore area, three-dimensional pore volume, and pore number generally increase in any solution environment, resulting in the loss of concrete compressive strength, mortar spalling, and the decrease in the relative dynamic elastic modulus. Among them, the CH₃COOK solution has the least influence on the concrete pore changes; the NaCl solution has the greatest influence on the change in the concrete internal porosity. The damage of CaCl₂ solution to concrete is second only to the NaCl solution, followed by clean water. The increase in the concrete internal porosity from high to low is NaCl, CaCl₂, clean water, and CH₃COOK. The change in the pore volume of 0.1 to 1 mm³ after the freeze–thaw cycle is the main factor for reducing concrete strength. The test results have certain guiding value for the selection of deicing salt in engineering.

Molecular dynamics (MD) is an important method for studying the molecular and atomic scale of cement (geopolymer)-based composites which provides an effective method for the optimal design of cementitious materials. In this paper, the research progress of MD simulation in Portland cement and geopolymer-based materials is discussed in detail, including molecular structure models of calcium silicate hydrate, calcium aluminosilicate hydrate, sodium aluminum silicate hydrate gel, and auxiliary experimental techniques. The basic mechanical properties of calcium silicate hydrate, calcium aluminosilicate hydrate and sodium aluminum silicate hydrate in Portland cement-based materials (CBM) and geopolymer-based materials are reviewed. In addition, the dynamic simulation of the interface between CBM and reinforcement materials such as rebar, synthetic fibers, plant fibers and nanoparticles is also discussed. Through the macroscopic experimental results of cement (geopolymer)-based materials and the performance analysis of an MD microscopic model, MD helps to better explain the macroscopic properties of materials, and can quickly and conveniently analyze the mechanical properties, transport properties and interface properties of composite materials, so as to improve the fine design of cement (geopolymer)-based materials. Existing structural models and force fields are affected by environment and time, and MD simulation shows great differences in application range and characterization ability. It is necessary to further study and reveal the internal mechanism for improving concrete performance through a large number of experiments and MD simulation, and lay a theoretical foundation for preparing the next generation of (super) high-performance concrete.

Spontaneous combustion coal gangue (SCCG) is considered to be an aluminosilicate-based solid waste containing various toxic ions. The alkali-activation method for this material can not only fully use its potential hydration activity but also solidify the hazardous components to some extent. Through introducing additional Pb²⁺, the solidification behavior of heavy metal Pb²⁺ for an SCCG-based geopolymer was studied in the present paper. The solidification efficiencies were evaluated by Pb²⁺ leaching rates under neutral and acidic conditions, while its mechanism was explained by the methods of XRD, TG, FT-IR, SEM, and MIP. The results show that the Pb²⁺ solidification efficiency increases along with the curing age, and acidic rather than neutral conditions lead to a more intensive solidification capacity. Judging by the permissive maximum value of 5 mg/L, the Pb²⁺ original concentrations under neutral and acidic circumstances should be lower at 2.0 wt.% and 3.0 wt.%, respectively. The Pb²⁺ absorption is dominated by the physical process, due to the formation of no new hydration products. However, the Pb²⁺ addition would interrupt the reconstruction of the Si-Al network structure, slowing the accumulation of N-A-S-H gel and the densifying of the matrix. When the Pb²⁺ concentration grows, the sizes of hydration productions shrink continuously, more defects appear in the microstructure of the geopolymer, and the pore structure deteriorates rapidly, all of which accelerate the diffusion of toxic ions to the external condition.