*4.3. E*ff*ect of Colloidal Silica on CO2 Curing*

Colloidal silica reportedly contributes to a low diffusivity in a hardened cement paste and a low degree of carbonation because it has the filling/nucleating effect [20,31]. However, the CO2 curing produced an opposite effect. Incorporating the colloidal silica in the mortar samples subjected to the 3 bar CO2 curing increased the CO2 uptake approximately 56% for Mortar (W/C = 0.35) and 19% for Mortar (W/C = 0.5), as documented in Table 4. Incorporating non-reactive nanoparticles reportedly result in a slight increase in the rate of cement hydration because they provide supplementary nucleation sites [32–34]. Colloidal silica provides nucleation sites for the carbonation as well as the hydration of cement. Colloidal silica is more effective in the high-rate carbonation during which cement hydration is dormant. As a result, carbonation by 3 bar CO2 curing (at the age of 3 h) is accelerated.

Conversely, as shown in Figure 4, the strength of the mortar incorporating colloidal silica was not enhanced, but even weakened by the 3 bar CO2 curing. The carbonation products, mostly calcite, are not helpful for improving strength. Calcites are crystallized, and they do not provide a binding force among aggregates. Their inclusion in a paste matrix even cuts a binding link of the main hydrates (C–S–H). The strength of the mortar is consequently less developed.

When the samples incorporating colloidal silica were continuously subjected to 20% CO2 curing, their strengths increased up to 15% at 28 days. Colloidal silica certainly accelerates carbonation with 20% CO2 curing. Here, as opposed to the short-period 3 bar CO2 curing, carbonation continues concurrently with cement hydration in 20% CO2 curing. Calcite crystals produced by the carbonation put C–S–H on themselves at nano-scale, and the C–S–H layer is consequently reinforced by the distributed calcite [35–37]. The continuous 20% CO2 curing consequently improves the strength of the mortar sample.

#### **5. Conclusions**

Curing by CO2 can accelerate and improve the strength of cement-based materials via cement carbonation. In this study, 3 bar CO2 curing was applied to premature cement paste and mortar for 3 h, and then successive conventional curing followed for cement hydration. As a result, the strength of a

paste compact (W/C = 0.15) increased a lot, providing a high CO2 uptake. That of a Paste (W/C = 0.4) consolidated in a mold also displayed a meaningful increase. However, despite cement carbonation, 3 bar CO2 curing resulted in an adverse effect in terms of the strength of a mortar compact (W/C = 0.35), while that of a mortar (W/C = 0.5) consolidated in a mold was unchanged. In contrast, continuous 20% CO2 curing increased the strengths of all the cement paste and mortar samples. Partial carbonation inside a specimen affects the size effect on the strength of the cement mortar. Incorporating colloidal silica provides more nucleation sites for cement carbonation, with the result that the effect of 20% CO2 curing is slightly improved.

**Author Contributions:** Conceptualization, S.H.H.; Methodology, J.H.K; Validation, J.H.K.; Formal Analysis, S.H.H.; Investigation, S.H.H. and T.Y.S.; Data Curation, Y.J.; Writing—Original Draft Preparation, S.H.H; Writing—Review & Editing, Y.J. and J.H.K.; Visualization, S.H.H.; Supervision, J.H.K.; Funding Acquisition, J.H.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MOTIE) (No. 20188550000580).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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