Tobermorite and Al-substituted tobermorite with varying Al/(Si + Al) molar ratios were synthesized via the static hydrothermal method. The effect of different Al substitution ratios on the structure of tobermorite was investigated by XRD, SEM–EDS, TG–DSC, FT–IR, ²⁹Si, and ²⁷Al MAS–NMR. The interplanar crystal spacing of (002) increased with the increment of the Al substitution ratio. The maximum ratio of the Al substitution for Si in the structure of tobermorite was about 15 at %, which was limited by the compositions of tobermorite and the electrostatic repulsive forces caused by Al substitution. The DSC curve of the tobermorite with an Al/(Al + Si) molar ratio of 0.15 showed a larger exothermic peak area at 850 °C, indicating that the Al-O-Si bond was formed in the structure of tobermorite and the Al participated in the recrystallization. The ²⁹Si and ²⁷Al MAS–NMR results confirmed that tetrahedral Al was introduced into the structure of the tobermorite. New silicon sites, Q²(1Al) and Q³(1Al), formed by the Al substitution for Si in bridging and non-bridging sites, were detected. Compared to the tobermorite without Al substitution, the adsorption capacity of Al-substituted tobermorite with Al/(Al + Si) = 0.15 to Na⁺ was improved by 44%.

Immersion tests with 5 wt.% Na₂SO₄ solution were carried out to investigate the deterioration of calcium silicate hydrate (C-S-H) gel in a sulfate environment. C-S-H gels with different Ca/(Si + Al) molar ratios were used for comparison. Particular attention was paid to the changes in element composition and Si-O-Si chain structure of the C-S-H gel. The results show that the C-S-H gels with a low Ca/(Si + Al) ratio (0.8–1.0) and appropriate Si-O-Si chain length (14.8) presented better stability in a severe sulfate environment. The C-S-H gels with a higher Ca/(Si + Al) ratio (>2.0) were more sensitive to the attack of sulfate ions. Calcium ions dissolved rapidly during the immersion process, causing the loss of cohesive strength of the C-S-H gel, and then decomposed into fine particles. The C-S-H gel with a lower Ca/(Si + Al) ratio (about 1.1) rarely leached out calcium ions and maintained good micromorphology. The ²⁹Si MAS-NMR results indicate that the Si-O-Si chains with too short or too long chain lengths will break and recombine under the attack of sulfate ions. The Si-O-Si chains with an appropriate chain length (14.8) maintained the stability of the structure of the C-S-H gel in a sulfate environment. These changes are closely related to the asymmetric layered structure of amorphous C-S-H gel. Partial calcium ions between the layers of the main chain structure of Si-O-Si are easily taken away by sulfate ions, leading to the structural instability of the C-S-H gel.

Nanomaterials have been widely used in cement-based materials. Graphene has excellent properties for improving the durability of cement-based materials. Given its high production budget, it has limited its wide potential for application in the field of engineering. Hence, it is very meaningful to obtain low cost nanoplatelets from natural materials that can replace graphene nanoplatelets (GNPs) The purpose of this paper is to improve the resistance to chloride ion penetration by optimizing the pore structure of cement-based materials, and another point is to reduce investment costs. The results illustrated that low cost CaCO₃ nanoplatelets (CCNPs) were successfully obtained under alkali treatment of seashell powder, and the chloride ion permeability of cement-based materials significantly decreased by 15.7% compared to that of the control samples when CCNPs were incorporated. Furthermore, the compressive strength of cement pastes at the age of 28 days increased by 37.9% than that of the plain sample. Improvement of performance of cement-based materials can be partly attributed to the refinement of the pore structure. In addition, AFM was employed to characterize the nanoplatelet thickness of CCNPs and the pore structures of the cement-based composites were analyzed by MIP, respectively. CCNPs composite cement best performance could lay the foundation for further study of the durability of cement-based materials and the application of decontaminated seashells.