This paper presents an investigation on workability and strength of geopolymer concrete made of fly ash (FA), blast furnace slag (BFS) and a multicompound activator of Na2SiO3 and NaOH solutions. The FA/BFS ratios were 100:0, 70:30, 60:40, 50:50, 40:60, 30:70 and 0:100. The workability of geopolymer concretes was performed by using slump test and controlled by using different types of admixtures. Polycarboxilate, melamine, naphthalene-based superplasticizers (SPs) and retarders were used. The results showed that when alkali activating solution to binder ratio is fixed, a good workability was achieved without SPs for FA rich mixtures. However for BFS rich mixtures a combination of SPs and retarder was needed to reach similar workability and setting. The increase of slag content increased the compressive strength but decreased the workability and setting time. In all mixtures, a good workability with slump values between 20-25 cm and high compressive strength (~70MPa at 7d) were achieved when retarder and superplasticizer were added into the mixtures. For slag based geopolymer concretes, the best performances were found by using N-based SPs. The air content measured for all mixtures was between 2.5-3%.