Magnesium oxysulfate (MOS) cement is a typical eco-friendly cementitious material, which presents excellent performances. In this work, a novel multiscale modeling strategy is proposed to simulate the hydration and pore structure of MOS cement system. This work collected and evaluated the Gibbs free energy of formation for main hydrates and equilibrium constant of main reactions in MOS cement system based on a first principle calculation using Material Studio. Followingly, the equilibrium phase compositions of MOS cement system were simulated through PHREEQC to investigate the molar ratio dependence of equilibrium phase compositions. Results showed that large M (MgO/MgSO₄) was beneficial for the formation of 5Mg(OH)₂·MgSO₄·7H₂O (Phase 517) and large H (H₂O/MgSO₄) tended to decompose MOS cement paste and cause leaching. The microstructure-based method visualized the hydration status of MOS cement systems at initial and ultimate stages via MATLAB and the results showed that large M was significant to reduce porosity, and similar results for the case of small H. Fractal analysis confirms that fractal dimension of pore structure (D_f) was significantly decreased after the hydration of MOS and was positively correlated to the porosity of the paste. In addition, it can be referred that large M and small H were beneficial for modifying the microstructure of MOS paste by decreasing the value of D_f.

Cement-based materials, including cement and concrete, are the most widely used construction materials in the world. In recent years, the investigation and application of fractal theory in cement-based materials have attracted a large amount of attention worldwide. The microstructures of cement-based materials, such as the pore structures, the mesostructures, such as air voids, and the morphological features of powders, as well as the fracture surfaces and cracks, commonly present extremely complex and irregular characteristics that are difficult to describe in terms of geometry but that can be studied by fractal theory. This paper summarizes the latest progress in the investigation and application of fractal theory in cement-based materials. Firstly, this paper summarizes the principles and classification of the seven fractal dimensions commonly used in cement-based materials. These fractal dimensions have different physical meanings since they are obtained from various testing techniques and fractal models. Then, the testing techniques and fractal models for testing and calculating these fractal dimensions are introduced and analyzed individually, such as the mercury intrusion porosimeter (MIP), nitrogen adsorption/desorption (NAD), and Zhang’s model, Neimark’s model, etc. Finally, the applications of these fractal dimensions in investigating the macroproperties of cement-based materials are summarized and discussed. These properties mainly include the mechanical properties, volumetric stability, durability (e.g., permeability, frost and corrosion resistance), fracture mechanics, as well as the evaluation of the pozzolanic reactivity of the mineral materials and the dispersion state of the powders.

Dam concrete suffers from serious abrasion damages in southwestern China, the abrasion resistance of concrete is therefore one of the most important factors determining the reliability even the safety of the dams. In the present work, the effects of fly and/or silica fume on the mechanical properties, drying shrinkage, as well the cracking and abrasion resistance of concrete were investigated, then the pore structures of concrete added with fly ash and silica fume and the pore surface fractal dimensions ([ ]) were determined by the mercury intrusion porosimetry (MIP) method and a fractal model, respectively. Finally, the relationships between the abrasion resistance of concrete and the porosity as well as the [ ] were revealed and discussed. The results indicate that silica fume significantly increases the drying shrinkage especially at early age, while the incorporation of fly ash and silica fume together can decrease the early plastic shrinkage-induced cracking risk and the final shrinkage to some degrees. Besides, the utilization of 5[ ]wt.% silica fume and 20[ ]wt.% fly ash together increases the abrasion resistance and mechanical property by about 4–9% at various ages. In addition, the compressive strength and the abrasion resistance of concrete are linearly correlated with the concrete porosity and the [ ]. Both the fly ash and silica fume could decrease the porosity of concrete and increase the [ ], therefore the concrete containing fly ash and silica fume have desired mechanical property and abrasion resistance. Moreover, [ ] has a more profound effect on the abrasion resistance of concrete than the concrete porosity. The addition of 5[ ]wt.% silica fume and 20[ ]wt.% fly ash together is suitable for the concretes used for wearing surfaces in terms of mechanical property, cracking and abrasion resistance.

The utilization of phosphorus slag (PHS) to replace the fly ash in the construction of hydraulic projects has attracted a growing attention in China. In this study, the influence of PHS fineness and content on cement hydration, mechanical strength, permeability as well as the pore structure and fractal dimension (Ds) of concrete have been discussed. The results indicate that the PHS addition retards the cement hydration and hence decreases the hydration heat within three days. The incorporation of PHS with a Blaine specific surface area of 505m2/kg could participate in the early pozzolanic reaction and consequently offsets the retarding effect to some extent. The incorporation of 20–40wt.% PHS declines the early strength of concrete, but this reduction effect on strength can be eliminated to some degrees by mechanically grinding the PHS. The compressive strengths of concrete added with PHS with a high fineness of 505m2/kg (abbreviated as PHS-H) are about 16.0–20.6% higher at three days and 8.9–11.0% higher at 180 days compared that of the control concrete. The contribution of PHS-H to the pore structure refinement is more significant than that of PHS with a low fineness of 302m2/kg (abbreviated as PHS-L) at various ages because PHS-H is of much higher reactivity and can consume more Ca(OH)2 than PHS-L which leads to a denser microstructure and a lower chloride diffusion coefficient (DRCM). The incorporation of PHS decreases the Ds at three days, whereas the concrete incorporated with PHS has much higher Ds than that of control one at late age. The DRCM value increases with increasing the porosity and the most probable aperture, while Ds has a more significant effect on DRCM than the porosity and the most probable aperture. The concrete added with 20wt.% PHS-H exhibits the highest Ds and the lowest DRCM value at long-term age among the five concrete mixtures in this work.

Low-heat Portland (LHP) cement is a kind of high-belite cement, which has the characteristic of low hydration heat. Currently, it is extensively used in the temperature control of mass concrete. Based on the thermodynamic database of OPC-based materials, the thermodynamic software GEM-Selektor (noted as GEMS) is used for simulating the hydration products of the LHP cement paste. Then, according to the GEMS thermodynamic simulation results, MATLAB is used to visualize the initial and ultimate stages of LHP cement pastes; the effects of curing temperature and water to cement (w/c) ratio on hydration products are addressed; and the porosity, fractal dimension, and tortuosity of different pastes are calculated. It is found that an appropriately high curing temperature is important for reducing porosity, especially in the early hydration stage. Hydration time also has a significant impact on the hydration of LHP cement paste; long hydration time may reduce the impact of temperature on hydration products. The w/c ratio is another important consideration regarding the hydration degree and porosity of LHP paste, and under different curing temperatures, hydration times, and w/c ratios, the porosity varies from 5.91–32.91%. The fractal dimension of this work agrees with the previous findings. From tortuosity analysis, it can be concluded that the high curing temperature may cause significant tortuosity, further affecting the effective diffusivity of LHP cement paste. For cement pastes with low w/c ratio, this high curing temperature effect is mainly reflected in the early hydration stage, for ones with high w/c ratio, it is in turn evident under long-term curing.