The escalating global energy demand underscores the critical need for advanced solutions for energy‐efficient buildings. Passive thermal energy storage systems using microencapsulated phase change materials (PCMs) offer promise but face integration challenges in cementitious materials due to weakening mechanical strength, which arises from poor shell strength and weak interfacial bonding with cementitious phases. This study introduces a novel approach for synthesizing functionalized microencapsulated PCMs from fly ash‐based cenospheres addressing interfacial compatibility. Cenospheres are perforated for PCM encapsulation and sealed using two different materials: 1) melamine‐formaldehyde (MF), a standard polymeric shell; and 2) silica, selected for its chemical compatibility with cementitious phases. Experimental results show that the silica sealing improved mechanical strength by 50% over those of MF, corroborated by molecular dynamic simulations showing silica's binding energy with calcium silicate hydrate exceeded threefold, with more than twice the uniaxial tensile strength. Thermal analyses confirmed the preservation of PCM in both sealing approaches. This work establishes a transformative pathway for advancing PCM‐based thermal energy storage in building materials.
