The behavior of lightweight cement-based and alkali-activated mortars after exposure to temperatures of 500, 750, and 1000
C was compared. In cement-based mortars, cement was partially replaced (40 vol%) by several refractory fillers (coal fly-ash, biomass fly-ash, cocciopesto, metakaolin). In alkali-activated mortars, metakaolin was partially substituted (0, 25, 50 wt%) by coal fly-ash. The substitution of cement by 40% metakaolin and the substitution of metakaolin by 50% fly ash enhances the resistance to high temperatures of cement-based and alkali-activated mortars, respectively. After exposure to 1000
C, the high residual compressive strength of cement-based mortars with metakaolin is due to the formation of new crystalline species, whereas in alkali-activated mortars is due to their high densification.

Introduction

In order to overcome these problems, the use of refractory fillers or aggregates, such as fly ash, silica fume, blast furnace slag [3–۵], or lightweight aggregates [6], is suggested. The addition of refractory particles can reduce shrinkage stresses and enhance the mechanical properties of the material after exposure to high temperature [7–۹]. The use of lightweight aggregates, such as vermiculite, perlite, pumice, expanded glass, etc., reduces the apparent density of composites and consequently the dead loads of structures [10,11], ensuring at the same time higher thermal insulation [12], improved acoustic insulation [13], and higher refractoriness, and, therefore, higher fire resistance. Lightweight aggregates can have natural (pumice or vermiculite) or synthetic origins. Synthetic aggregates can be obtained from natural raw materials (e.g., expanded perlite or expanded clay) or from waste materials (e.g., expanded glass). The use of wastes is recommended to avoid the depletion of natural resources and to increase the sustainability of buildings [14].