Sahar Iftikhar, Konstruktionsteknik

The cement and concrete industry is actively pursuing decarbonization strategies, with supplementary cementitious materials (SCMs) offering an effective route to reduce clinker content and associated CO₂ emissions. Among these, clay-based SCMs, particularly low-kaolinite mixed-layer clays, are widely available and represent a promising pathway for sustainable binder development. To date, however, the behavior of more abundant low-kaolinite clays remains insufficiently understood. This is while, current mechanistic understanding stems largely from kaolinite‑rich systems such as LC³, where ample AFm formation and strong calcite–aluminate synergy help stabilize hydrates during carbonation. In contrast, low‑kaolinite clays form hydrate assemblages with limited AFm and increased incorporation of aluminum into C-A-S-H. Because aluminum uptake into C-A-S-H is comparatively slow, the partitioning of Al between C-A-S-H and AFm phases evolves significantly with curing. This time‑dependent aluminate chemistry weakens known carbonation‑buffering mechanisms and introduces uncertainty regarding how these binders carbonate, providing the motivation for the present study.

This work investigates the carbonation behavior of binders with incorporation of low-kaolinite activated clays as SCM, in comparison with ordinary Portland cement, focusing on the evolution of hydrate phases, C-A-S-H structure, pore network, transport properties, and local micro-mechanical properties. Carbonation experiments were conducted under controlled conditions (3% CO₂ and 57% RH). Thermogravimetric analysis was used to quantify phase evolution, while ²⁹Si and ²⁷Al MAS NMR provided insight into structural changes in the C-A-S-H. Mercury intrusion porosimetry and X-ray computed tomography were used to characterize pore structure and connectivity, and electrical conductivity together with micro-Vickers hardness measurements were used to assess transport and micro-mechanical properties.

The results show that carbonation in low‑kaolinite clay systems is governed by the coupled evolution of hydrate chemistry and pore structure rather than by portlandite depletion alone. In OPC system, carbonation is initially buffered by portlandite, and proceeds through CH dissolution and CaCO₃ precipitation, resulting in progressive pore refinement and the formation of a relatively dense but still connected pore network. In contrast, the clay-blended systems exhibit earlier CH depletion, leading to an earlier involvement of AFt and AFm destabilization and subsequent C-A-S-H decalcification. The extent and initiation of these processes are influenced by curing: with 28 days of curing in standard carbonation tests, rapid phase destabilization results in pronounced C-A-S-H decalcification and pore coarsening at the meso-scale, whereas prolonged curing promotes a more stable hydrate assemblage, delaying severe C-A-S-H decalcification. Despite these differences in phase evolution, carbonation in the clay-blended systems is accompanied by strong fragmentation of the coarse pore network. µXCT and conductivity measurements reveal the formation of a relatively tortuous carbonation front, characterized by reduced macropore connectivity and increased transport resistance. As a result, carbonation becomes increasingly transport‑limited at later stages, yielding overall carbonation depths comparable to OPC despite reduced chemical buffering capacity.

These findings demonstrate that carbonation resistance in low-kaolinite clay binders is governed by the interplay between phase assemblage evolution and progressive transport restrictions of CO₂ and cannot be interpreted solely in terms of chemical buffering capacity.