Development of a calcium sulfoaluminate-Portland cement binary system for twin-pipe 3D concrete printing

Abstract

Recently, the twin-pipe pumping (TPP) system has been developed for 3D concrete printing which involves pumping two mixtures, typically a Portland cement-based mixture and a pure limestone powder-based mixture that contains an accelerator. Following this, the two mixtures are allowed to be intermixed in a static mixer placed just prior to the nozzle which causes immediate stiffening and a very high buildability for the printed concrete post-extrusion. However, the flow division pattern produced by the mixing baffles of the static mixer results in striations consisting of unmixed limestone powder resulting in weak zones that lower the mechanical strength of the printed element. In this study, we present a TPP system consisting of Portland cement-based mixtures and a borated calcium sulfoaluminate (CSA) cement-based mixture. The mixtures are capable of reacting independently with water as compared to the pure limestone powder-based mixture. After getting blended in the static mixer right before extrusion, the retardation in the borated CSA cement is destroyed by the alkaline component contained in the other stream, leading to the rapid hardening and a high construction rate post extrusion from the nozzle. The hydration characteristics were studied using a Vicat penetrometer, isothermal calorimetry, and ultrasonic pulse velocimetry, while the early age mechanical behavior was characterized by using uniaxial unconfined compression and slow penetration tests. Results indicate that Portland cement alone cannot produce enough alkalinity to destroy the ulexite phase and reinitiate the hydration of CSA cement, while adding a nominal dosage of calcium hydroxide can provide enough increase in pH to design a system suitable for the stiffening control. However, abundant calcium hydroxide would reduce the hydration rate at later ages due to the dilution effect, as indicated by the P-wave velocity evolution and yield stress evolution. In addition, the hardened behavior of printed elements was studied by performing compression and flexural tests. Results show that the CSA-Portland cement binary binder system avoids the formation of weak zones and hence improves the mechanical behavior.