Behaviour of a mortar with cement – sand dosage of 1:3 added with prismatic angular particles of recycled expanded polystyrene (REPS) obtained by manually grating expanded polystyrene blocks is described. Density, absorption, compression, indirect tension, and residual post-fire compression at temperatures of 50°C - 500°C tests were performed on 320 cubes with 5cm side and on 56 cylinders with 5cm diameter and 10cm height added with REPS percentages in volume of 10% - 80%. Results show that while the density, compression, indirect tension, and post-fire residual compression of the mortar reduce up to 23%, 83%, 76% y 19%-54%, respectively, with increments in the volume percentage of REPS; the absorption of the mortar increases up to 308%. A bilinear model that predicts the effect of increasing the percentage in volume of REPS on the physical and mechanical properties of the mortar is discussed.