The construction industry is a primary contributor to global CO₂ emissions, making the development of novel low-carbon building materials a critical scientific and engineering challenge. A promising strategy involves the valorization of waste concrete fines, which are generated in vast quantities. The thermal activation of these fines at 600–800 ℃ transforms the hydrated cement paste into a recycled cement rich in dicalcium silicate (i.e., β-C₂S), a clinker phase with a high intrinsic carbonation reactivity. This makes thermally activated recycled cement an ideal precursor for carbon-sequestering materials where CO₂ is utilized as a curing agent to enhance mechanical properties. However, its practical application is hindered by a key technical bottleneck, i.e., relatively low reaction kinetics and insufficient mechanical strength development. Therefore, identifying an effective modifier to stimulate its carbonation potential and optimize the final microstructure is of significant urgency. Metakaolin (MK) as a highly reactive pozzolan is a promising candidate due to its known ability to promote hydration and serve as a nucleation site. This study was to systematically investigate the role of MK as a synergistic component to overcome the performance limitations of carbonated recycled cement, providing a scientific basis for the development of high-performance, green building materials.

The raw materials used were a waste concrete powder (WCP) and a commercial metakaolin (MK). A thermally activated recycled cement (TARCP) was produced via first pressing a mixture of WCP and MK (0%, 10%, and 20% by mass, denoted as TARCP, MK10, and MK20, respectively) and then calcining it in a muffle furnace at 800 ℃ for 1 h. After cooling and grinding, the resulting powders were mixed with water at a water-to-binder ratio of 0.55 to prepare paste specimens. The pastes were cast and cured for 24 h before being demolded and subjected to one of two curing regimes for 7 d and 28 d, i.e., (1) water curing (25 ℃, 75% RH) and (2) accelerated carbonation curing (25 ℃, 75% RH, 20% CO₂).

The compressive strength of 20 mm cubic specimens was tested at each age. The phase evolution was comprehensively characterized by X-ray diffraction (XRD) with Rietveld quantitative analysis, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The pore structure distribution was analyzed by low-field nuclear magnetic resonance spectroscopy (NMR). The microstructure and morphology of the reaction products were determined by scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX).

The experimental results demonstrate that the incorporation of an optimal amount of MK significantly enhances the performance of the carbonated recycled cement. The addition of 10% MK (MK10) yields the optimum results, with the 7-d compressive strength under carbonationof 37.7 MPa, which is increased by 119.2%, compared to the carbonated control sample (TARCP). This superior performance is attributed to a combination of physical and chemical effects induced by MK.

The phase analysis reveals that MK accelerates the consumption of the primary reactive phase like β-C₂S. After 7-d carbonation, the β-C₂S content in the sample MK10 is reduced to just 1.2%, compared to 29.8% in the TARCP control. This accelerated reaction leads to a much higher degree of carbonation. The 7-d carbonation degree for the sample MK10 is 50.79%, which is improved by 70%, compared to the sample TARCP (i.e., 29.87%). Furthermore, the MK promotes the transformation of metastable vaterite to the thermodynamically stable calcite. At 7 days, the sample MK10 is overwhelmingly composed of calcite (i.e., 72.7%), with only a negligible amount of vaterite (1.6%), whereas the sample TARCP contains a significant amount of vaterite (i.e., 8.5%). The FTIR analysis indicates the formation of a highly polymerized amorphous silica-alumina gel in the carbonated MK-modified systems, which is evidenced by the shift of the main Si—O vibration band to a higher wavenumber (~1080 cm^–1), indicating a fundamental restructuring of the silicate network.

The microstructural analysis provides the strengthening mechanism. The NMR results show that the addition of 10% MK significantly refines the pore structure. After 7-d carbonation, the proportion of large capillary pores (i.e., >50 nm) in the MK10 sample decreases from 49.88% (in the sample TARCP) to 36.56%, while the proportion of fine gel pores (i.e., <10 nm) nearly increases from 8.98% to 17.81%. This pore refinement, which is driven by the uniform deposition of carbonation products, results in a much denser matrix, which is the primary reason for the observed strength enhancement. However, the effect of MK is non-linear. An excessive dosage of 20% MK (the sample MK20), while further refining the pore structure (the capillary of pores reduces to 29.75% at 7 d), resulting in lower compressive strength than the sample MK10 at all ages. This is accompanied by a slightly lower final degree of carbonation at 28 d (51.72% for the sample MK20 vs. 52.88% for the sample MK10). This indicates that the excessive pore refinement can create a dense, impermeable microstructure that impedes the diffusion of CO₂ gas into the material core, thereby limiting the overall reaction rate and extent in the later stages. Moreover, the anomaly of lower strength despite a denser pore structure in the sample MK20 increases that the intrinsic properties of the solid phases play a critical role. It is hypothesized that the higher Al/Si ratio in the sample MK20 leads to the formation of an amorphous alumina-silica gel with a lower degree of polymerization, possessing the inferior intrinsic mechanical properties, compared to the gel formed at an optimal Al/Si ratio.

This study demonstratesd that incorporating an optimal amount of metakaolin (10% by mass) could be a highly effective strategy for enhancing the carbonation reactivity and mechanical performance of thermally activated recycled cement. The improvements could be attributed to the synergistic effects of MK, i.e., accelerating the consumption of β-C₂S, promoting the formation of stable calcite, and refining the pore structure via creating a denser, more uniform microstructure. However, an excessive dosage of MK (20%) could be detrimental. It led to a further pore densification, this could inhibit a later-stage CO₂ diffusion, limiting the final carbonation degree. Furthermore, the higher Al/Si ratio could negatively affect the intrinsic strength of the amorphous binder phase. These findings could highlight the non-linear effect of metakaolin and provide a scientific basis for optimizing the design of high-performance, carbon-sequestering building materials derived from recycled cement.