Sustainable ultrafine fly ash bricks with PCM microcapsules enhance indoor thermal regulation in hot climates

Fly ash bricks are promising sustainable materials for building envelopes; however, under high solar exposure, their sensible heat storage alone may be insufficient to limit indoor heat gain. As the demand for energy-efficient and sustainable building materials grows, there is a need for solutions that can improve the thermal performance of these materials. This preliminary study investigates the outdoor thermal performance of ultrafine fly ash bricks incorporating a surfactant-free silica-shell microencapsulated phase change material (MePCM), synthesized via Pickering emulsion, under hot climatic conditions. The thermal performance of these MePCM-integrated bricks was compared to conventional fly ash bricks without MePCM using several metrics, including temperature profiles, heat flux variations, attenuation index, thermal buffering capacity, decrement factor, and time lag. The MePCM-integrated brick maintained lower interior temperatures throughout the day, reducing the daily maximum interior temperature by up to 7.0°C and, on average, by 3.6°C. It also moderated heat-flow peaks, with peak heat flux decreasing by 23.1% and a mean daily peak heat flux attenuation index of 0.306. Time lag increased from 6.28–8.88 hr for the conventional brick to 8.58–11.95 hr for the MePCM-integrated brick, indicating stronger thermal inertia and delayed heat transmission. The results highlight the potential of MePCM-incorporated fly ash bricks as an effective passive cooling strategy for energy-efficient building design.