Criticality of microstructural evolution at an early age on the buildability of an accelerated 3D printable concrete

The buildability of a 3D printed concrete structure is influenced by the layer compression during printing, which in turn is governed by the rheological and early age mechanical properties of the mix. The study aims at understanding the effect of alkali-free aluminium sulphate-based accelerator on the microstructural evolution, which affects the macrostructural behaviour governing buildability of the structure. The effect of accelerator on hydration, phase composition, location and arrangement of ettringite, and porosity evolution of two paste systems with and without viscosity modifying admixture and superplasticizer is observed at initial hours. Further, fresh properties, yield stress evolution, and mechanical strength at early and later ages of the printable mix is evaluated. The squeeze flow test, which simulates the physical process of printing, is performed to understand the rheological characteristics of the mix under compressive loading in the initial hours. It is concluded that the precipitation of ettringite in large quantities at the initial hours controls the rheological and mechanical behaviour of the mix. The total ettringite formed increases for accelerated paste system at 30 min compared to the paste without accelerator. The ettringite is located at surfaces of clinker and interstitial spaces, with the crystals interlocked physically with each other. The squeeze flow test further suggests that the flow hardening increases tremendously on increasing the accelerator dosage due to an increase in solid volume fraction of ettringite and the resultant physical interlocking. Finally, a theoretical model is proposed considering the significance of ettringite interlocking and microstructure densification under compression on the plastic flow hardening for accelerated mixes.