The effect of hydration space on cement hydration is essential. After a few days, space restriction affects the hydration kinetics which dominate the expansion, shrinkage and creep of cement materials. The influence of space restriction on the hydration products of tricalcium silicate was studied in this paper. The microstructure, morphology and composition of calcium silicate hydrate (C-S-H) were explored from the perspective of a specific single micropore. A combination of Raman spectra, Fourier transform infrared spectra, scanning electron microscopy and energy dispersive X-ray spectroscopy were employed. The results show that space restriction affects the structure of the hydration products. The C-S-H formed in the micropores was mainly composed of Q³ silicate tetrahedra with a high degree of polymerization. The C-S-H formed under standard conditions with a water to cement ratio of 0.5 mostly existed as Q² units. Space restriction during hydration is conducive to the formation of C-S-H with silica tetrahedra of a high polymerization degree, while the amount of water filling the micropore plays no obvious role on the polymeric structure of C-S-H during hydration.

Identifying the phase evolution and revealing the chemical bonds of hydrated cements accurately is crucial to regulate the performance of cementitious materials. In this paper, a coupled Raman/BSE-EDS analysis was proposed to determine the chemical bonds of tricalcium silicate hydrates and the interface transition zone (ITZ) between inner C-S-H and anhydrates. The results show that the Raman/BSE-EDS method can accurately identify the chemical bonds of inner C-S-H and inner ITZ regions, which confirms the mixed structure of inner C-S-H and nano calcium hydroxide (CH). The inner ITZ shows a lattice change region with a thickness of 700–1000 nm, which can be attributed to the pre-disassembly process of C₃S crystal. The successful application of coupled Raman/BSE-EDS provides new insight into the hydration process and multi-structure features of traditional cementitious materials.

Exploring and modifying the C-S-H structure at a micro–nano level is an effective solution to improve the performance of Portland cement. Compared with organics inserting C-S-H, the research on the performance of a polymer-binding C-S-H structure from nanoscale to macroscale is limited. In this work, the mechanical properties of a modified C-S-H, using hydroxyl-terminated polydimethylsiloxane (PDMS) as the binders, are evaluated. The PDMS-modified C-S-H structures are introduced into macro-defect-free cement to obtain stress–strain curves changes at a macro scale. The AFM–FM was adopted to measure the morphology and elastic modulus of C-S-H at a nano scale. The molecular dynamics (MD) simulation was performed to assess the toughness, tensile properties, and failure mechanism. The results show that the PDMS-modified C-S-H powders change the break process and enhance ductility of MDF cement. The elastic modulus of PDMS-modified C-S-H is lower than pure C-S-H. When PDMS molecules are located between the stacking crystal units, it can enhance the toughness of C-S-H aggregates. The PDMS-modified C-S-H stacking structure has better plasticity, and its tensile strains are higher than the pure C-S-H. PDMS molecules hinder the initial crack expansion, leading to the branching of the initial crack. In addition, the measurement of AFM–FM can identify and obtain the mechanical properties of basic units of C-S-H. This paper enhances the understanding of cement strength sources and modification methods.

Calcium aluminate cement (CAC) is advantageous in marine engineering, and using materials directly from the sea when mixing CAC can reduce both construction costs and ecological pollution to the sea. However, using seawater as the mixing water in CAC is difficult due to the strength reduction. In this work, CACs were prepared with siliceous minerals (SMs) (diatomite and silica fume (SF)) and sodium chloride solution (SCS) as the mixing water. For comparison, other samples were immersed in SCS rather than mixed with SCS. The strength, mass and length changes of CAC mortars cured at 20°C and 40°C were determined. The phase assemblages and morphology were assessed using X-ray diffraction, thermogravimetric-differential scanning calorimetry and scanning electron microscopy. Using SCS as the mixing water was beneficial to the later strength development of CAC. It promoted the formation of C₂ASH₈ in CAC pastes and showed no strength loss even when cured at 40°C. The strengths of the SM-modified CAC mortars mixed with SCS were higher than those immersed in SCS. The combination of diatomite and SCS significantly increased the CAC strength, especially at 20°C, while SF contributed to better stability in mass and length.