Microstructural design and thermal characterization of composite diatomite-vermiculite paraffin-based form-stable PCM for cementitious mortars

The thermal performance of cementitious mortars can be improved with the use of form-stable phase change materials (PCMs) consisting of a solid-liquid paraffin mix impregnating diatomite in a diatomite-vermiculite mineral system. The present study includes designing, developing, and characterizing a novel PCM based on a combination of diatomite and calcined, expanded vermiculite, and an optimized paraffin system consisting of a mixture of solid paraffin and naphthenic oil. The diatomite is expected to store the paraffin and wrap the vermiculite particles, increasing the amount of stagnant air. X-ray fluorescence is used to assess the chemical composition of the diatomite and vermiculite. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) measurements are used to characterize the paraffin mixes aimed at optimum performance for residential and commercial buildings. PCM samples made from paraffin and diatomite (PCM1), vermiculite (PCM2), and a mix of diatomite and vermiculite (PCM(1 + 2)) are characterized by scanning electron microscopy and FT-IR. Cement mortars are prepared in the volume ratios of 1:4 cement:sand (reference mortar), 1:4:1 cement:sand:diatomite, 1:4:1 cement:sand:vermiculite, and 1:4:1 cements and PCM. The thermal properties of these materials are evaluated. The DSC results of the paraffin mixtures indicate that 20% naphthenic oil is the ideal concentration in the paraffin mixture to optimize the solid-liquid temperature (57.45 degrees C) and latent heat for building applications (159.42 J/g). All PCMs have reduced thermal conductivities compared to the reference mortar (1.47 W/m.K), with the lowest being associated with the PCM (1 + 2) sample (0.82 W/m.K). The PCM(1 + 2) sample also presents an improved combination of diffusivity, volumetric heat capacity, and thermal resistivity. The simultaneous addition of vermiculite and diatomite to the PCM composition improves the thermal properties, while maintaining the basic characteristics such as chemical and structural stability. (C) 2019 Elsevier Ltd. All rights reserved.