## 1 Introduction
Compressive tests were carried out, and it was observed that the cement/sand mortar had a strength value 99% higher than that of the composition with the smallest amount of vermiculite and 173% higher than that of the composition with the highest amount of aggregate. [PERSON] et al. [17] evaluated the effect of expanded vermiculite on the properties of lightweight mortar. Three different compositions were produced, and it was expected that the ones with a higher amount of aggregate would lead to lower velocities due to the high amount of air inside the voids. The results confirmed this: the composition with the lower content of vermiculite had a velocity around 11% and 21% higher than the other two, respectively. The same authors also assessed the combined effect of silica fume and expanded vermiculite on the properties of lightweight mortars [18], developing twelve compositions, each with different contents of cement to vermiculite. Through ultrasonic pulse velocity tests and flexural and compression tests, it was clear that samples with smaller quantities of vermiculite presented enhanced strengths and homogeneity. [PERSON] et al. [17; 18] confirmed the conclusions of [PERSON] et al. [19]; they studied the replacement of natural sand with expanded vermiculite in mortars, developing compositions with different contents of binder and expanded vermiculite, and submitting the specimens to ultrasonic pulse velocity tests. The compositions without vermiculite added had an ultrasonic pulse velocity of 2800 m/s; the 15% vermiculite by weight composition had a velocity of 2660 m/s, and the 30% vermiculite by weight composition had a velocity of 2330 m/s. [PERSON] et al. [20] assessed the influence of the introduction of expanded perlite and paraffin into cement-based compositions. Nine cubic samples were tested under compression at 28 days of age. It was concluded that the addition of lightweight aggregates leads to a decrease of about 60% in the compression strength of the compositions. [PERSON] et al. [21] studied mortar compositions' mineralogical and mechanical properties. They found that plain gypsum mortar had a mean Young's modulus of 4.006 GPa,flexural strength of 2.48 MPa, and compressive strength of 4.75 MPa. [PERSON] et al. [22] obtained similar results for plain gypsum reference mortar. [PERSON] et al. [23] developed a high-fired plain gypsum mortar with Young's modulus ranging from 2.21 to 4.28 GPa and compressive strength ranging from 3.57 to 4.53 MPa. [PERSON] [24] compared two gypsum panels with moduli of 1.57 and 2.37 GPa. [PERSON] et al. [11] found that adding nano and micro silica to cementitious materials decreases porosity, increases durability, activates pozzolanic reactions, and improves microstructure by reducing portlandite crystal size. Synthesising the above information, (i) the use of vermiculite and/or perlite as lightweight aggregates of fire-proof mortars improves the thermal insulation of gypsum-based mortars; however, it reduces their mechanical properties, and (ii) the effects of the addition of nano and micro silica particles on cement-based mortars are already well known; however, the behaviour of gypsum-based mortars is unknown. In this context, experimental work has been carried out at the University of Coimbra to characterise the mechanical properties of gypsum-based mortars but with the addition of nano and micro silica particles because they improve thermal performance.