Rapid rheology control and stiffening of 3D-printed cement mortar via CO2 flash mixing in a 2 K printing system

Carbon dioxide (CO 2 ) has been increasingly applied to modify the fresh and rheological properties of cement mortars and concrete, enhancing the mortars’ mechanical properties through CO 2 capture. For 3D printing, most research has adopted the 1   K (one-component) printing system for CO 2 -integrated cement mortar prepared by batch mixing. In contrast, limited work has been conducted on mortars subjected to CO 2 flash mixing in the 2   K (two-component) system, where pumped fresh mortar was mixed with continuously injected CO 2 within a timeframe of seconds during secondary mixing. In this paper, we report the development of a novel low-carbon cement mortar mixture consisting of ordinary Portland cement (OPC), ground granulated blast furnace slag (GGBS) and calcium hydroxide (CH) in binders that exhibited instant change in rheological properties and rapid stiffening when subjected to CO 2 flash mixing. The rheological properties of the CO 2 -mixed mortar improved with increasing proportions of GGBS and CH in the mortar mix. In-situ chord length measurements suggested that the improved rheological properties of the mortar after CO 2 flash mixing were related to the rapid growth of fine CaCO 3 crystals, driving subsequent particle flocculation. The instant flocculation was primarily attributed to electrostatic attraction between particles with opposite surface charges in the OPC-GGBS-CH system induced by CO 2 flash mixing.