Effects of incorporating bentonite, metakaolin, microsilica, and calcium carbonate on the rheological properties of portland cement-based 3D printing inks

Cementitious mixtures for 3D printing require specific rheological properties in the fresh state, which is why it is necessary to incorporate admixtures and/or mineral additions (modifiers) into such mixtures. The objectives of this article were to compare the effects of bentonite (Ben) (1 %–5 %) and mineral additions, such as metakaolin (MK) (5 % and 10 %), microsilica (MS) (5 % and 10 %) and calcium carbonate (Ca) (5 % and 10 %), on the rheological properties in the fresh state and to determine the printability of cementitious mixtures based on ordinary Portland cement (OPC). Among the rheological properties, the static yield stress (τ0), flow behavior, viscosity (η) and thixotropic of the mixtures were evaluated. Complementarily, the properties of materials in the fresh state, such as the settlement (minislump), flow rate (flow table) and buildability, were determined. The printability was evaluated through laboratory-scale additive manufacturing tests by printing from a hollow cylinder (unfilled) 50.8 mm in diameter and 208 mm in height (≈52 layers). The results showed that the incorporation of Ben between 1 % and 5 % had a significant effect on the rheological properties of the mixtures in the fresh state, increasing τ0 to 203.8 % (5 % Ben) and structural regeneration (thixotropy) to 98.5 %. Ben decreased the fluidity of the mixtures while significantly increasing the buildability. Among the mineral additions used, MS and MK increased the rheological properties of the mixtures as a function of the amount incorporated (5 % and 10 %, respectively), while Ca had a fluidifying effect on the mixtures, thereby affecting the buildability during the 3D printing process. This phenomenon was corrected by simultaneous adding 3 % Ben. The results obtained were considered a starting point for the design of OPC-based 3D printing mixtures with these modifiers (Ben, MK, MS and Ca).