Supersulfated cement (SSC) is a low-carbon material made from slag, gypsum, and a small amount of alkali activator. However, the poor carbonation resistance limits its large-scale application. In this study, activated carbon with high CO₂ adsorption capacity, as well as activated carbon modified by calcium hydroxide (CH), were doped into SSC (0.5% and 1.0%, in mass) to investigate the effects on the mechanical properties, carbonation depth, phase composition and microscopic morphology before and after the carbonation of SSC. This research aims at providing a effective methond to increase the carbonation resistance of SSC.

P·I 42.5 Portland cement (conforming to GB 175—2023), S95 powder, gypsum and coconut shell-based activated carbon particles (Jiangsu Hartel Carbon Materials Technology Co., Ltd.) were used. The activated carbon powder particles were ball-milled and passed through a 150 μm sieve. The activated carbon particles were immersed in saturated CH solution for 48 h and dried to obtain CH-modified activated carbon. The water-cement ratio of both paste and mortar specimens was 0.4. The dosage of activated carbon was 0, 0.5%, and 1.0% of the total mass, respectively. The compressive strength of mortar specimens (40 mm×40 mm×160 mm) were tested after curing for standard cured for 3, 28 d and after carbonation for 1, 3, 7, 14 d. The hydration degree of slag in SSC before carbonation was analyzed by EDTA method. The phase composition of hardened SSC before and after carbonation were measured by combination of a thermal gravimetric analyzer (TGA) and X-ray diffraction (XRD), The pore structure and micro-morphology hardened SSC before and after carbonation was tested by low-field nuclear magnetic resonance (LF-NMR, field emission scanning electron microscopy (ZEISS GeminiSEM 560), respectively.

Activated carbon significantly improved the carbonation resistance and mechanical properties of SSC. Similar to nanomaterials such as carbon nanotubes, the active functional groups on the surface of activated carbon can provide nucleation sites for the growth of C-(A)-S-H gels, a phenomenon that has been confirmed by SEM results. Combined with the analysis of TG and XRD, it can be seen that this nucleation effect increases the number of C-(A)-S-H gels in the SSC. Meanwhile, according to the LF-NMR, the activated carbon would decrease the porosity and critical pore size of the SSC, thus enhance the compressive strength before and after carbonation.

After carbonation, the activated carbon group generated more calcite than the control group. This indicates that activated carbon promotes the formation of C-(A)-S-H gel and possibly increases the proportion of Ca²⁺ in the gel. This is related to the strong adsorption capacity of activated carbon for Ca²⁺. Therefore, the mechanism by which activated carbon enhances the carbonation resistance of SSC is as follows: Activated carbon provides nucleation sites for C-(A)-S-H gel, promotes the formation of C-(A)-S-H gel, densifies the pore structure, thereby improving the mechanical properties and carbonation resistance of SSC. Furthermore, the clusters formed by activated carbon and C-(A)-S-H gel contribute to improve the distribution uniformity of the gel within SSC, further enhancing structural stability.

Furthermore, activated carbon has a high CO₂ adsorption capacity. After the carbonation decomposition of the C-(A)-S-H gel encapsulated on the surface of activated carbon, the exposed activated carbon can absorb the invading CO₂. Saturated activated carbon might act as a 'barrier' to CO₂, preventing CO₂ penetration and further enhancing carbonation resistance.

The carbonation resistance effect of CH-modified activated carbon is significantly lower than that of activated carbon. The CH distributed in the activated carbon could have dissolved into the liquid phase of the SSC and mainly act as an alkali activator. This leads to an excessive amount of alkali activator in the SSC system, thus reducing the C-(A)-S-H gel content, increasing the AFt content, and making the overall structure looser and more susceptible to carbonation. Therefore, activated carbon is effective in enhancing SSC's carbonation resistance and can also be used to load other functional components.

This study demonstrates that activated carbon could significantly enhance the performance of SSC, particularly the mechanical properties and carbonation resistance. The results show that activated carbon can provide more nucleation sites for the generation of C-(A)-S-H gels, enhance the Ca/Si of C-(A)-S-H gels, increase the amount of C-(A)-S-H gel, densify the pore structure, and improve the compressive strength by 10.6% at the dosage of 0.5%. However, the enhancement effect diminishes at higher activated carbon content.

In contrast, CH-modified activated carbon would lead to increase in the carbonation depth and porosity of SSC, significantly reducing its mechanical properties and carbonation resistance. This could be due to CH-modified activated carbon fails to effectively adsorb and immobilize CO₂, instead of raising the alkali activator content in SSC. The excess alkali activator results in a decrease in C-(A)-S-H gel content, an increase in AFt content, and a more porous and easily carbonated structure.

This paper demonstrates the significant practical value of activated carbon as a low-cost material. It not only greatly enhances the mechanical properties and carbonation resistance of SSC, but also offers greater functionalization potential due to its porous structure. By loading other components into the pores of activated carbon, activated carbon is expected to provide an effective means for the functionalization of SSCs and even other materials. The multifunctionality of activated carbon not only opens up new paths for optimizing the properties of materials, but also offers the possibility in a wider range of construction materials in the future.