Role of chemical admixtures on 3D printed Portland cement: Assessing rheology and buildability

• The buildable zone was found for yield stresses greater than 500 Pa. • Retarders and superplasticizers can extend the open time while maintaining buildability. • Retarders and superplasticizers can interact and lose their effectiveness. • Higher accelerators contents are required when combined with superplasticizers. • The effectiveness of chemical admixtures is influenced by the mixing procedure. Concretes intended for printing need specific rheological properties that are not found in traditional ones. Printable concretes must have as much consistency as possible and, at the same time, still need to be pumpable. Although the use of chemical additives is a known solution to overcome these problems, their effectiveness can be partially or totally compromised when used in combination. For example, a particular superplasticizer may not work in the presence of a setting retarder or may weaken the potency of an accelerator. The lack of literature on the possible interactions of different additives used in co-assembly motivated this work. In this, it was demonstrated how fluid mixtures can be rapidly made buildable, continuously, from pumping to printing with different chemical admixtures. Setting retarders, accelerators and a PCE-based superplasticizer were tested on different contents, singly and in co-assembly, to determine their interactions from a rheological point of view. The main results show that the superplasticizer and setting retarders are efficient in improving the open time maintaining the buildability to a critical concentration. When added in co-assembly, their effectiveness can be nullified if an intercalated stirring is not applied, therefore presenting a strong interaction. The setting accelerators have also shown their role rapidly increasing the structuration rate of cement pastes but needing significantly higher contents when co-assembled with the superplasticizer to compensate for their retarding and repulsive effect.