Print-Head Mixing of Geopolymer and OPC Slurries for Hybrid Alkali-Activated Cement in 3D Concrete Printing (2024-05)¶

Hybrid alkali cement (HAC) is regarded as a sustainable alternative to cementitious materials due to the use of more than 70 % of supplementary cementitious materials while the issues associated with geopolymers are avoided to a great extent. This paper proposes a new method of delivering HAC-based mixtures by a two-part printhead mixing process suited for 3D concrete printing (3DCP). The two-part mixing process addresses the conflicting rheological requirements in 3DCP by facilitating the rapid early-age strength development after placement while showing excellent pumpability prior to the extrusion. The proposed approach is based on introducing the Portland cement in a secondary mixing process to partially replace the fly ash or slag in the printable geopolymer mix. A series of experiments assessing the printability of fresh concrete following the twopart mixing process and the properties of hardened concrete were assessed. The results demonstrate that the replacement of FA with Portland cement (HAC-FA) yielded a more pronounced enhancement than slag-replaced mixes (HAC-S) with the increase in static yield stress (SYS) and modulus of elasticity by 17 times and 3.5 times respectively at 30 min, compared to the control mix. The hardened properties of printable mixes, namely compressive strength and interlayer bond strength at 28 days, revealed an enhancement for the HAC-25FA mix by 20 % and 64 %, respectively, again in comparison to the control mix. The apparent volume of permeable voids (AVPV) and microstructural analysis validated the hardened concrete properties, where the HAC-FA mixes showed the densest microstructure with a reduced apparent porosity, compared to other mixes.