To address the global challenges of waste valorization and climate change mitigation, alkali-activated slag (AAS) is used as a sustainable alternative binder to produce low-carbon concrete. However, the low hydraulic reactivity and slow early-age strength development of AAS limit its practical use in marine civil engineering. This research examined the impact of synergy between carbonation curing and marine exposure on the mechanical properties and durability of AAS pastes. The findings indicated a synergistic enhancement and resistance to chloride-ion erosion in carbonated AAS pastes, with a 24-hour carbonated specimen achieving a strength of 42 MPa compared to the 6-hour specimen (12.7 MPa) after 28 days of corrosion. This was attributed to the combined effects of carbonation and chloride-ion penetration, which not only reduced porosity, free chloride ions, and pH but also refined the pore structure. The reduced porosity facilitated the progressive development of calcite in the paste matrix, thereby encapsulating non-carbonated particles within the densified matrix, enhancing the stability of CS-H gels and promoting the formation of Friedel's salt. Overall, carbonated AAS pastes exhibited stable CO2 sequestration capacity, remarkable stability, and corrosion resistance in marine environments, indicating that they are well-suited for marine civil engineering applications.
Magnesium potassium phosphate (MKP) cements are acid-base binders that are attractive for a range of applications. Their interactions with the surrounding environment are largely governed by the chemistry of their pore solution, which remains poorly documented, particularly for near stoichiometric cement pastes (molar ratios Mg/P approximate to 1 and H2O/P approximate to 5), due to the limited volume of residual free water in the pore network. This study investigates the pore solution composition of MKP cement pastes through a combined experimental and thermodynamic approach. Several extraction protocols are compared and allow, through the use of correction factors derived from a statistical analysis, a more reliable determination of the pore solution composition at low water content. Experimental results show that the pH increases rapidly during early hydration, and that the solution is highly concentrated in potassium and phosphorus, while the magnesium concentration remains much lower. Thermodynamic simulations reproduce the general trends, with an increase in pH and a decrease in ion concentrations as the water content increases in the paste. However, the predicted concentrations are approximately one order of magnitude lower than the measured values. This discrepancy indicates that the pore solution remains oversaturated with respect to the identified phase assemblage, even after prolonged curing (up to 6 months). Finally, protocols are proposed to prepare synthetic solutions mimicking the pore solution of stoichiometric MKP cement pastes after 28 days of hydration, or at thermodynamic equilibrium.
To explore the influence of freezing temperature on the freeze-thaw (F-T) damage of cement matrix, two F-T cycling regimes were applied: -15 to 5 degrees C and-20 to 5 degrees C. The mass loss, surface damage, and mechanical performance evolution of cement specimens following F-T cycles were evaluated at the macro-scale. Additionally, scanning electron microscopy (SEM) and micro-indentation experiments were used to examine the microcrack evolution and micro-mechanical responses at the micro-scale. The results demonstrate that freezing temperature has a crucial regulatory effect on the F-T damage of cement matrix. Low-temperature environment accelerates the initiation and propagation of microcracks, promoting the rapid formation of microcrack networks. This subsequently causes mass loss, surface spalling, and amarked degradation in the mechanical performance of cement-based materials. Moreover, F-T damage exhibits a two-stage characteristic. The initiation and propagation of microcracks in the early stages have limited influence on the macro-and micro-mechanical properties of the material. In the later stages, microcracks rapidly coalesce into a large-scale crack network, intensifying surface spalling and causing a sharp decline in mechanical performance. This study elucidates the mechanisms of F-T degradation of cement matrix under freezing temperatures from both micro-and macro-scales. It provides experimental support for the freeze-resistance design of cement-based materials in cold regions.
