Mechanical and thermal performance of cementitious composites incorporating SiO2-encapsulated n-octadecane microcapsules and carbon nanomaterials

SiO 2 @n-OD microcapsules showed high latent heat and leakage resistance. • MWCNTs and CNFs improved pore structure and interfacial continuity. • Repeated cycling confirmed stable short-term thermal buffering. • SiO 2 shells and carbon nanomaterials balanced thermal and mechanical performance. Cementitious composites incorporating phase change materials usually suffer from a trade-off between latent-heat storage and mechanical integrity, mainly because organic PCMs behave as soft inclusions and introduce additional thermal resistance. Here, we propose a synergistic strategy that combines SiO 2 -encapsulated n-octadecane microcapsules with one-dimensional carbon nanomaterials, including multi-walled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs), to improve the compatibility between thermal functionality and cement-matrix performance. SiO 2 @n-OD microcapsules were prepared by a sol–gel route, and the optimized M2 microcapsules showed a latent heat of 170.12 J/g, good leakage resistance, and stable phase-change behavior after repeated heating–cooling cycles. The addition of 10 wt% microcapsules reduced the 28 d compressive strength from 37.8 MPa for the reference paste to 20.8 MPa because of the soft inclusion effect, weak interfacial regions, and increased pore defects. Introducing 0.5 wt% carbon nanomaterials markedly compensated for this deterioration. In particular, the composite modified with 0.5 wt% CNFs reached a 28 d compressive strength of 38.5 MPa, corresponding to 104.1% compensation of the strength loss caused by microcapsule incorporation. This recovery was supported by improved hydration response, reduced mercury-accessible pore volume, better interfacial continuity, and possible fiber-bridging effects. In the short-term laboratory-scale heating–cooling cycling test, the optimized composite maintained a stable thermal-buffering effect, with a maximum average temperature difference of 6.38 ± 0.17 °C relative to the reference panel. These results suggest that coupling SiO 2 -encapsulated PCMs with one-dimensional carbon nanomaterials is a feasible strategy for balancing latent-heat storage, mechanical compensation, and thermal regulation in cementitious composites.