Climate change has become one of the most urgent challenges facing our planet today. The consequences we are gradually experiencing have been driven by human activity. Specifically, the increase in energy demand, met mainly through the combustion of fossil fuels such as coal, oil derivatives and natural gas, has significantly increased greenhouse gas emissions, especially carbon dioxide (CO2), leading to global warming. To address the environmental problems arising from climate change, which we are gradually experiencing, it is clear that the development of the use of renewable energy sources is the key to the transition from fossil fuels to these innovative energy alternatives, in order to achieve zero emissions and contribute to decarbonization. However, the deployment of these clean energies requires the development of systems that guarantee continuous energy production, to overcome interruptions caused by the variability of natural resources like wind, sun, or water. A viable solution to this issue is employing energy storage technologies to correct the mismatch between energy supply and demand. In particular, in the specific case of the use of the sun as a renewable thermal energy source, thermal energy storage (TES) systems are of great interest, since more than half of the energy demanded in industry is thermal energy. Among the different sensible TES media, conventional concrete is emerging as a very attractive option for use as TES due to its low cost, high availability, ease of processing, high specific heat, good mechanical stability at high temperature and excellent operational performance when subjected to thermal cycling. And despite its moderate thermal conductivity, research has shown that incorporating multiple heat exchangers through which the heat transfer fluid (HTF) passes in concrete improves its efficiency, albeit at an increased cost. However, caution should be exercised in the use of concrete as the production of its precursor, Portland cement (PC), is a significant contributor to greenhouse gas emissions, particularly CO2. It is estimated that for every ton of PC produced, approximately one ton of CO2 is released into the atmosphere. For this reason, construction materials must be rethought and one of the lines of research to reduce CO2 emissions is the search for alternative precursors known as supplementary cementitious materials (SCM). SCMs enable the full or partial substitution of PC. Complete replacement of PC leads to the development of alkali-activated materials (AAM), while partial replacements, typically around 70-80%, result in the development of hybrid materials (HM). This Doctoral Thesis involves the fabrication of both alternative cementitious materials, AAM mortars and HM mortars, to investigate their feasibility as TES. Specifically, for both alternatives, the main precursor used as a substitute is blast furnace slag (BFS), an industrial by-product that has proven to be a promising alternative. In the case of the AAM mortar composed of BFS, SLAG, the activation of the precursor is carried out with sodium silicate due to the excellent mechanical properties of the final cementitious material. Nevertheless, the use of solutions makes the workability of these systems difficult, so HM with BFS (HSLAG) are also manufactured, which hydrate in the presence of water. HM mortars are composed of almost 80% BFS, about 20% PC and 5% sodium sulphate to promote the alkaline medium necessary for BFS activation. After verifying through a life cycle analysis (LCA) that alternative mortars offer benefits in terms of carbon footprint and water footprint, as well as continuing to manufacture alternatives focused on PC substitution, the possibility of replacing the natural aggregate with glass waste (GW) is investigated. The substitution of sand is carried out in the three types of mortars (AAM, HM and reference PC) with the aim of reducing water consumption, as sand is the component with the highest water demand. However, only the AAM system, SLAG, allows up to 25% of sand to be replaced by GW (SLAG75), thanks to the high cohesion of its main reaction product, the C-A-S-H gel. When the alternatives are manufactured together with the PC reference mortar, both the compressive mechanical properties and the key thermal properties for a TES, thermal conductivity and specific heat, are evaluated before and after various thermal treatments. After analyzing how the mechanical and thermal properties are affected after thermal treatments −including exposure to constant temperatures and thermal cycling−, it is determined that the alternative systems offer comparable and even superior mechanical stability under temperature exposure than a conventional PC system. In addition, alternative materials, characterized by their thermal conductivity and specific heat, show a superior suitability for TES applications compared to PC. More specifically, the AAM system, SLAG, exhibits operational characteristics superior to PC by reducing heat-up times and increasing its storage capacity, which allows for a reduction in TES volume and a reduction in heat exchanger surface area. While the HM system, HSLAG, does not reach the performance of SLAG, it does offer operational improvements compared to PC. These promising results are attributed to less degradation of the reaction products generated in the alternative mortars and better cohesion between the binder and the aggregate. This last factor had a negative effect on SLAG75, as the weakness in the bond created between the binder and the GW, as well as a greater difference in the coefficients of thermal expansion (CTE), lead to the generation of porosity, and even cracks, which determine both the mechanical and thermal behavior. Thus, when selecting a material such as TES, porosity must be controlled and evaluated as a critical parameter. The results displayed by the PC alternative systems developed in this Doctoral Thesis demonstrate their suitability to be selected as sustainable TES both at low-medium and high temperatures. Consequently, it can be generally concluded that the proposed alternative materials show a promising potential for their application as TES blocks. Thus, further research and development in this field could lead to the widespread adoption of these materials as TES, thus contributing to the transition towards sustainable and renewable energy systems.