**1. Introduction**

Due to some advantageous properties of thermal conductivity, high density, and high mechanical strength of Portland cement concrete (PCC), it is a frequently used concrete for applications utilizing PCMs in especially microencapsulated form [1–3]. However, carbon dioxide emission during the production of PCC causes a negative effect on the environment. Compared to PCC, geopolymer concrete (GPC) has several beneficial properties as PCC, but also higher initial strength [4], superior acid resistance [5], high fire resistance [6], and shorter setting time [7, 8]. These features of GPC make it an alternative binder for preparation of PCM containing cementitious materials to be considered for improving building energy efficiency.

Phase change material (PCM), the thermal energy storage (TES), is one of the promising methods used to reduce the environmental impact and to increase energy efficiency of buildings. Phase change materials (PCM) are latent heat storage materials, which can store and release large amounts of energy during a phase change that can occur according to one of many matter transitions (solid–liquid or solid–gas or liquid–gas or solid–solid). However, the most commercially viable transition is between the liquid and solid phases. When the temperature rises above melting point of PCM, this last one melts and absorbs heat, when the temperature drops below melting point; the PCM solidifies and release heat. Heat can also come from other sources such as non-air-conditioned buildings and industrial machinery. There are three main types of PCM: organic, inorganic and eutectic. The most commercially viable PCM is organic since it is chemically stable, safe and nonreactive, does not loss its effectiveness with cycling, can be microencapsulated and has a wide temperature range [9]. The addition of PCM into building materials leads to store the excess of the outdoor environment heat (During the day) and reduce heat transfer to the indoor side of the concrete wall. While, during the cold periods (or at night) the PCM release the stored heat into the building if the inside temperature is too low, and thus causing an increase comfort level in building through providing heating in the winter and providing cooling in the summer without the use of an air-conditioning system. Thereby reduce the fossil fuels-based energy consumption.

Most of the investigations were focused on the addition of microencapsulated PCM (MPCM) to the standard concrete recipes. The literature survey indicated that the combination of MPCM with cementitious material resulted in low composition fraction due to the fact that more loading is resulted in the final product with low TES capacity and low mechanical strength. Another problem is the leakage of PCM caused by breaking some parts of capsules during mixing and compression processes. Moreover, the combination of MPCM with GP is not cost effective because of the high cost and complex synthesis nature of MPCM.

Moreover, microencapsulated or encapsulated phase change materials are particles consisting of a core material the (PCM) and an outer wall (shell). The shell is an inert, stable polymer or plastic or metallic [10]. It does not melt under normal processing and use conditions. The shell acts as a barrier between the core material and the surrounding matrix and controls the volume change of the PCM during its phase change. Due to the very high cost of PCM metal-based encapsulations other method has been adopted to avoid the high cost of the encapsulation of PCM. This method consists in direct incorporation of non-encapsulated PCM into concrete materials. However, this method leads to the leakage of PCM during its liquid state, as well as the corrosion of the concrete matrix due to the corrosive nature of some PCMs. Not to mention the corrosion of the exposed surfaces of the concrete matrix and the reinforcement bar embedded in concrete causing by the environment (air, humidity, salt water, or other hostile environment), which enhances the porosity and permeability of the materials, thereby reducing its mechanical and structural properties.

Because of its cost-effective and eco-friendly, 3D printing is an excellent method to create a building with an efficient thermal regulation and make the integration of PCM in building more efficient, less expensive and more environment friendly [11]. However, among the obstacles for application of 3D printing in geopolymer-based construction are the low mechanical strength, long setting time, and low tensile strength [12, 13].
