Printable alkali-Activated fly ash-slag mixtures, which are homogeneous under pressure and achieve buildability in the extrusion-based three-dimensonal (3D) layer printing process, are developed. A baseline mixture of fly ash and slag with a sodium hydroxide activator is modified to achieve extrusion-based printing requirements, including printability, shape retention, and buildability. The role of additional dry constituents such as microsilica and clay in reducing phase separation under pressure for producing printable mixtures is evaluated. Phase separation in the mixture under pressure is sensitive to the particle size distribution. Printable mixtures, which do not segregate under pressure, have a narrower distribution of particle sizes, indicated by the Rosin-Rammler fit. The link between the rheological behavior of the mixture and its performance in printing is evaluated. The constant strain rate rheological response of the mixtures is distinguished between the yield-Type and Maxwell-flow behaviors. Mixtures that exhibit a Maxwell-flow type response produce a steadily continuing deformation and are not buildable. The distinction between Maxwell-flow and yield-Type behaviors is essential for identifying buildable mixtures. Alkali-Activated mixtures exhibit a viscoelastic response with both elastic and viscous components. The proportion of the storage to the loss modulus from rheological measurements provides an index of buildability. Achieving buildability with multiple layers depends on an internal structure capable of resisting elastic deformation, which is indicated by the development of the storage modulus with time. The role of additives on specific aspects of the rheological behavior of the mixtures is evaluated. The rheological behavior required for printing is achieved using carboxymethylcellulose (CMC), which produces a yield-Type behavior, and enhances the storage modulus and thixotropy of the alkali-Activated mixture. © 2021 American Concrete Institute. All rights reserved.