## 1 Introduction  
In recent years, the concern with environmental impact and thermal comfort is assuming an increasing relevance in the design and construction of buildings. The increasing demand for thermal comfort in buildings together with the energy and environmental European Directives (Directives 2002/91/EC [1] and 2010/31/EU [2]) has promoted the development of new solutions to improve the thermal performance of buildings and reduce their energy consumption. In different countries, new regulation for buildings (e.g., in Portugal, REH [3], among others) have boosted the use of new technologies for the production of innovative materials and coating systems that are able to ensure high thermal performance, without compromising their mechanical resistance. Among these solutions are thermal mortars [4], obtained through the incorporation of insulating aggregates, which can contribute to the energy efficiency of constructions, due to their low thermal conductivity coefficient and ability to maintain coating functions [5; 6]. According to European standard (EN) 998-1 [7], the maximum thermal conductivity coefficient of thermal mortars is 0.1 W/(m.K) (Class T1) and 0.2 W/(m.K) (Class T2). These mortars are also required to present minimum values of compressive strength for classes CS I (0.4 to 2.5 MPa) and CS II (1.5 to 5 MPa), capillary water absorption coefficient lower than or equal to 0.40 kg/(m\({}^{2}\).min\({}^{0.5}\)) (W1 class) and water vapor permeability coefficient lower than 15. To minimize the environmental impact and energy consumption of buildings, the performance of coating mortars can be improved by incorporating sustainable and innovative insulating [1; 2; 5; 8] and lightweight materials [7; 9; 10; 11; 12; 13], such as recycled aggregates and/or nanostructured materials [6; 14; 15]. The incorporation of insulating aggregates improves the mortars' thermal performance by significantly reducing their bulk density in the hardened state, allowing these aggregates to be classified as lightweight aggregates according to EN 206-1 and EN 13055-1, provided that their density is lower than 1200 kg/m\({}^{3}\)[9; 10]. The present study focused on three types of thermal insulating aggregates: expanded clay, granulated expanded cork and silica aerogel. Expanded clay combines good thermal resistance (up to 1000 \({}^{\circ}\)C), high porosity and low bulk density (300 to 700 kg/m\({}^{3}\)), with high mechanical strength and low cost production, which makes it a valid solution to be used as an insulating material in construction. Expanded clay has high percentage of semi-closed pores (up to 90% of its volume), contributing to its low thermal conductivity (approximately 0.10 W/(m.K)) [16; 17; 18]. Cork is a very low density material (between 100 and 140 kg/m\({}^{3}\)), with low thermal conductivity (0.042 to 0.070 W/(m.K)) and good mechanical properties. It is also an organic and cellular material [11; 13; 15]. It is used as building material because of its high physical, chemical stability and relatively low cost [12; 13]. Silica aerogel is a nanostructured, extremely light and hydrophobic material characterized by an open pore structure with 95% of air content, bulk density of 3 to 500 kg/m\({}^{3}\), and very low thermal conductivity (0.012 to 0.021 W/(m.K)) [19; 20; 21]. The high cost and difficulty of production are presently the main obstacles to its widespread use, which is often limited to high technology applications. Silica aerogel is used not only due to its high performance as a thermal insulator, but also due to its inorganic structure; it is non-flammable and resistant up to 1400 \({}^{\circ}\)C [14]; according to the literature, silica aerogels slowly start to shrink from 500 \({}^{\circ}\)C, with the speed of the shrinking process increasing at higher temperatures. [PERSON] et al.