There exists an increasing concern about the dangers originated in a building in case of fire. In this work the behaviour of vermiculite as aggregate in fireproof mortars was compared with mortars formulated with two magnesium by-products aggregates which undergo an endothermic decomposition. Fire resistance and mechanical properties were evaluated. An optimal mixture of both magnesium by-products as aggregates allows formulating mortars that improve fire resistance at temperatures greater than 450 °C compared with vermiculite mortars. It would be interesting to obtain mortars formulated with magnesium by products and vermiculite to study possible synergic effects.