Hydration re-initiation of borated CSA systems with a two-stage mixing process: An application in extrusion-based concrete 3D printing

Abstract

Enhanced structural build-up and strength development are crucial for various construction processes, especially in extrusion-based concrete 3D printing. However, controlling the stiffening process could be quite challenging as there are conflicting requirements during the pumping and deposition phase. We present a study to enhance the structural build-up of a calcium sulfoaluminate (CSA) cement-based system using a two-stage mixing method. A borated CSA cement mixture having a long open time (> two hours) was intermixed near the nozzle/print head of the 3D printer with a mixture of limestone and calcium hydroxide having an indefinite open time. The hydration and the early-age mechanical behaviour of the different combinations of these two mixtures were investigated by isothermal calorimetry, XRD analysis, unconfined compression tests, and ultrasonic pulse wave velocity tests. The results indicate that in the presence of calcium hydroxide, the borated phase (ulexite) covering the ye'elimite and being responsible for the long open time, rapidly destabilizes and the hydration reaction re-initiates. The hydration progresses rapidly, resulting in enhanced pore segmentation and a rapid increase in mechanical properties (compressive strength and elastic modulus), significantly faster than what is reported for conventional 3D printable concrete. Finally, a 1.5 m tall column was printed with this two-stage mixing method in a time period of <10 min – demonstrating the feasibility of the developed mixture for 3D printing applications.