The present paper reports the preliminary results obtained and analyzed in the framework of a joint Spanish-Japanese project on the behavior of blends of alkali-activated metakaolin (MK) and calcium aluminate cement (CAC). In these experiments, the materials were activated with an 8M solution of NaOH, poured into molds, and subjected to brief thermal curing (2 or 20 h at 85 degrees C). The hardened pastes were tested for mechanical strength and characterized for mineralogy and microstructure by a number of techniques (XRD, FTIR, BSEM, and MAS-NMR). The results showed that under the above conditions, the Al and Ca in the CAC were taken up into the aluminosilicate formed as the main product of the alkali activation of MK. None of the CAC hydration compounds (CAH(10); C(2)AH(8); C(3)AH(6); AH(3)) normally formed were detected in any of the cases studied.