Hydration and microstructure of calcium sulfoaluminate-Portland cement binder systems for set-on-demand applications

The concept set-on-demand in the context of digital fabrication refers to a long enough open time of fresh concrete during the delivering process but a quick strength build-up after extrusion, which is indispensable to achieve a high construction rate. In this paper, we explore the hydration and microstructure of a combination of two reactive streams including a retarded calcium sulfoaluminate (CSA) mixture and a Portland cement (PC) mixture. Experiments were conducted on both the constituent mixtures and the combined mixtures with different volume ratios of the constituent mixtures. Hydration heat release, hydration products, pore structure, early-age mechanical behavior, and autogenous shrinkage are specifically investigated. It is concluded that the PC mixture can be effectively used to trigger the initial hydration of the retarded CSA cement, whereby the formation of ettringite can be boosted depending on the volume ratio of the CSA mixture to the PC mixture. In addition, it is shown that the difference in the volume ratio affects the pore structure and the autogenous shrinkage behavior, which can be well correlated to the phase assemblage of the CSA-PC system.