The influence of microencapsulated phase change material (PCM) characteristics on the microstructure and strength of cementitious composites: Experiments and finite element simulations

Abstract

Phase Change Materials (PCMs) incorporated into cementitious systems have been well-studied with respect to energy efficiency of building envelopes. New applications of PCMs in infrastructural concrete, e.g., for mitigating early-age cracking and freeze-and-thaw induced damage, have been proposed. Hence this paper develops a detailed understanding of the characteristics of cementitious systems containing two different microencapsulated PCMs. The PCMs are evaluated using thermal analysis, vibrational (FTIR) spectroscopy, and electron microscopy, and their dispersion in cement pastes is quantified using X-ray Computed Microtomography (μCT). The influences of PCMs on cement hydration and pore structure are evaluated. The compressive strength of mortars containing PCMs is noted to be strongly dependent on the encapsulation properties. Finite element simulations carried out on cementitious microstructures are used to assess the influence of interface properties and inter-inclusion interactions. The outcomes provide insights on methods to tailor the component phase properties and PCM volume fraction so as to achieve desirable performance.

Introduction

Among the multitude of methods to enhance the energy efficiency of building materials and indoor thermal comfort, one method that has gained prominence is the use of thermal energy storage (TES) materials. Phase change materials (PCMs) are combined sensible-and-latent thermal energy storage materials that can be used to store and dissipate energy in the form of heat [1], [2], [3], [4], [5]. A large number of recent studies have focused on the use of PCMs in building materials to achieve this objective [6], [7], [8], [9], [10]. Several methods have been advanced to incorporate PCMs in concrete, including direct incorporation and in the form of microencapsulated particles [11], [12], [13]. Microencapsulated PCMs, that are commercially available in a powder form, can be directly added during the mixing process of concrete [14], [15], [16], [17], [18]. The other common approach is to employ macro encapsulation, which refers to the impregnation of PCMs as a liquid into the pores of lightweight aggregates, which are then used as inclusions in concrete [19], [20], [21].

A recent work has outlined the other advantages that PCMs can offer to structural concretes by virtue of their capacity to store and release heat [18]. The influence of microencapsulated PCMs on semi-adiabatic temperature rise and cool-down rates in hydrating cementitious systems, the development of restrained thermal stresses and strains that result in thermal cracking, and on the fracture properties have been elucidated in the aforementioned study. PCMs having suitable phase change enthalpy and phase transition temperatures can also: (i) restrict the magnitude of diurnal-or-seasonal temperature variations and deformations of restrained concrete elements over long time scales to limit damage due to thermal fatigue, and (ii) help limit the number and/or intensity of freeze-thaw cycles experienced by exposed concrete structures [20], [22].

The applications described above require concretes to demonstrate adequate thermo-physical and mechanical properties over extended time periods. In general, incorporation of soft inclusions such as PCMs (either in the micro- or macro-encapsulated forms) decreases the mechanical properties of the composite including its strength and stiffness [17], [18]. The property reductions are substantial in the case of cement pastes; however the presence of stiffer phases such as aggregates can reduce the severity of property loss. It is also possible to mitigate some of the property loss through appropriate material design including matrix strengthening methods [18], [23]. Previous studies also have noted degradation of microcapsules in cementitious systems [17], [23]. The ability to store and release heat also depends on the pore volume, sizes, and their distribution in the cementitious system, which could be impacted by the incorporation of PCMs [24], as well as the size and dispersion of PCM particles. All of these necessitate a fundamental characterization of the PCMs and their effects on cementitious systems in order to design: (i) mechanical performance-equivalent systems containing adequate amounts of PCMs as needed to satisfy thermal requirements, and/or (ii) microencapsulated PCMs capable of enduring mechanical and chemical stresses produced during mixing and placing concrete, and induced due to the high pH cementitious environment.

This paper examines the influence of two microencapsulated PCMs having different particle sizes, and that are encapsulated in shells that have different physico-chemical and mechanical properties, on the microstructure and properties of the resultant cementitious system. A fundamental characterization of the microencapsulated PCMs is carried out, as are studies on microstructure and mechanical response of the resulting composites. Relevant insights on the influence of the PCM encapsulation and geometry on the resulting properties, and pointers to appropriate material design of PCM-cement composites are obtained through a combination of advanced experimental tools and finite element simulations.