## 1 Introduction
[22] also highlight the high stability at elevated temperatures of silica aerogels; however, in this case, for temperatures ranging from 30 \({}^{\circ}\)C and 1150 \({}^{\circ}\)C no thermal decomposition or mass changes were registered. Silica aerogel is also presented as a fire resistant material, in opposition to organic fuel foams that release toxic gases when burning [14; 22]. The incorporation of silica aerogel in mortars is growing, but there are still some uncertainties concerning the mortar components (and respective content) required to obtain high thermal performance with acceptable mechanical performance [23; 24]. It has been shown that the incorporation of air entrainers and rheological agents at adequate ratios allows improving the performance of thermal mortars. The air entrainers increase porosity, and the rheological agents improve the bond of the components and may also contribute to a decrease of the thermal conductivity [25; 26]. It is also important that thermal mortars can maintain their properties and fulfil their initial functions during or after exposure to elevated temperatures. Several situations can cause such exposure, namely some in-service conditions in industrial applications (e.g., power stations) and/or accidental actions (e.g., fire or blast explosion) [27; 28; 29]. In those cases, it is relevant to understand the residual performance of the mortars after exposure to elevated temperatures. Several studies addressed the effects of exposure to elevated temperatures on conventional mortars--the most relevant for the present study are highlighted next. According to [PERSON] [30], the exposure of cement mortars to elevated temperatures (up to 1000 \({}^{\circ}\)C in steps of 100 \({}^{\circ}\)C in a hot-airoven/muffe furnace for a soaking period of 5 h) causes material deterioration and changes in several properties, including the loss of the cement paste binding capacity, reduction of durability, increased shrinkage on drying, structural cracking and discoloration of aggregates. [PERSON] et al. [31] studied the effects of elevated temperatures on the mechanical properties of high strength mortars. A mortar of Portland cement and sand with graphite addition (refractory material) was produced and exposed to temperatures of 300 \({}^{\circ}\)C, 600 \({}^{\circ}\)C and 900 \({}^{\circ}\)C without mechanical loading; the heating rate was 2 and 8 \({}^{\circ}\)C/min, the duration of exposure to maximum temperature was 1 and 10 h, and the cooling rate of the furnace was about 0.4 \({}^{\circ}\)C /min. After cooling, the mortars were subjected to compressive tests: strength reductions of about 29% and 65% were reported after exposure to temperatures of respectively 300 \({}^{\circ}\)C and 600 \({}^{\circ}\)C; for 900 \({}^{\circ}\)C exposure, the compressive strength was negligible. In terms of flexural strength, the mortars exhibited performance reductions of 36% and 87% after exposure to 300 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, respectively. With the increase of exposure temperature, the mortars also presented increasing reductions in the static modulus of elasticity: after exposure to 300 \({}^{\circ}\)C the reduction was 44%. According to the authors, the main factors that influence the mechanical resistance of the mortars are (i) the thermal incompatibility between their constituents (sand has a thermal expansion coefficient of \(11\times 10^{-6}\)/\({}^{\circ}\)C while that of the cement paste is \(16\times 10^{-6}\)/\({}^{\circ}\)C), and (ii) the decomposition of the calcium hydroxide within the cement paste beyond 300 \({}^{\circ}\)C, which causes the occurrence of internal microcracks. [PERSON] et al. [32] studied the effect of exposure to elevated temperatures on the properties of a conventional cement-based mortar (Portland cement CEM II B/L 32,5N) incorporating sand aggregate, namely on its porosity and gas permeability.