In this paper, the thermal and mechanical behaviors of mortars mainly composed of coal fly ash (80 wt%) are studied with the aim of analyzing the influence of the unburned matter of the ash on the fire resistance and mechanical strength characteristics of mortars potentially used for passive protection against fire. All the properties have been evaluated after 28 days of setting time, comparing the properties of mortars containing ashes with different loss on ignition (LOI) contents and studying the behavior of mortars in which fly ash has been submitted to a pre-treatment aimed at removing the unburned matter. The insulating properties of the mortars have been analyzed by means of thermal analysis techniques; also the standard fire resistance test has been reproduced on 200 mm-height, 50 mm-diameter cylindrical test probes. Different mechanical properties such as compressive and flexural strength of the probes (before and after the fire resistance test), superficial hardness and dynamic elasticity modulus have been measured. The results obtained show a slight influence of the ash unburned matter content on the insulating capacity and mechanical properties of the mortars. On the other hand, the thermally-treated ash mortars showed better mechanical and insulating properties, although presumably the cost of the thermal pretreatment does not justify the improvement achieved. (c) 2008 Elsevier Ltd. All rights reserved.