Durability and pore structure of metakaolin-based 3D printed geopolymer concrete

Abstract

An alternative to Portland cement concrete is metakaolin (MK)-based digitally fabricated geopolymer concrete. However, global adoption is sparse due to the prolonged setting time of a two-part geopolymer concrete (GPC) system and the uncertain long-term durability properties. The durability and pore structure of the MK-based 3DPGPC (M1) and slag-modified MK-based 3DPGPC (M2) are examined and juxtaposed with mould-cast specimens. Firstly, the fresh properties, rheological behaviour, compressive and flexural strength of 3DPGPC and cast specimens were characterised. Thereafter, the durability and pore structure are investigated by examining the drying shrinkage, water absorption, capillary and gel porosity, and oxygen permeability index (OPI) and X-ray computed tomography (X-CT) analysis. A 5% slag inclusion reduced workability and final setting time from 17 hours in M1 to 4 hours in M2. Also, slag inclusion increased the initial static and dynamic yield stresses by 0.1 and 2%, respectively, resulting in an increase in buildability from 27 of 42 layers. At the 28-day curing age, the average compressive strength of M2–3DPGPC is 11 and 21% higher than M1–3DPGPC in D1 and D3 specimens, while the flexural strength is 33 and 28% higher, respectively. Drying shrinkage and water absorption are mitigated with slag inclusion, and the OPI compares with OPC concrete. Specimens cored along the printing direction (D3) are less permeable compared to disc specimens’ core in the perpendicular to the printing direction (D1). The M1 and M2–3DPGPC specimens contain lower average CT scan macro pores of 2.98 and 1.81% in comparison to the mould-cast specimens having 4.48 and 4.07%. The porosity is position-dependent in 3DPGPC due to the presence of more voids at the interlayer region. 3DPGPC specimens depicted a more compact pore structure in the range of 0.1–1.7 mm, whereas pores in mould-cast are in the range of 0.1–2.5 mm. The durability index tests indicate that 3DPGPC is a potentially durable material.