Printable alkali-activated mixtures, which are homogeneous under pressure and achieve buildability in 3D concrete printing by extrusion-based layer deposition are developed. A baseline mixture of alkali-activated fly ash and slag is modified by using dry constituents such as micro-silica and clay. Carboxymethylcellulose (CMC) is used with the alkaline activating solution as a viscosity modifier. The roles of the different elements in influencing the yield stress, viscosity, and thixotropic buildup in the mixture are evaluated. The influence of yield stress and viscosity in influencing shape retention in printable mixtures is evaluated. Buildability is identified with the thixotropic buildup of elastic modulus and yield stress in the mixture. Phase separation in the mixture under pressure is influenced by the particle size distribution and the viscosity of the activating solution. Mixtures which do not exhibit phase separation under pressure have a narrower distribution of particle sizes, indicated by the Rosin-Rammler fit. Clay and CMC enhance the thixotropic buildup in the alkali-activated mixture and also contribute to achieving homogeneous flow under pressure by mitigating phase separation.