The enhancement in the workability of Portland cement concrete when coal fly ashes are added to the mix is well-known. This behaviour has been attributed to the spherical shape and the smooth surface of the fly ash particles. On the other hand, when other combustion residues with pozzolanic properties are added to concrete mix formulations, a loss in workability is observed, and an increase in water content may be necessary for obtaining a satisfactory plastic consistency of the concrete, or alternatively chemical additives may be used (plasticizers and superplasticizers). The effect of the combination of fly ash with other mineral admixtures in respect of the effect on the workability of mortars has been studied. The pozzolanic combustion residues examined were: silica fume, sewage sludge ash, spent fluid catalytic cracking catalyst and rice husk ash. Additionally, and due to the pozzolanic properties of coal fly ash and these residues, the strength developments of Portland cement mortars containing fly ash and the selected admixtures have also been measured.