Slag-modified metakaolin-based 3D printed geopolymer: Mechanical characterisation, microstructural properties, and nitrogen physisorption pore analysis

Abstract

This study aims to enhance metakaolin (MK)-based binders with slag up to 30% at 10% intervals and increase the aggregate-to-binder ratio (a/b) from 1.6 in a previous study to 2 for extrusion-based 3D printed geopolymer concrete (3DPGPC). Sodium phosphate (3% by mass of binders) was used to tailor the fresh properties of the slag-modified MK-based 3DPGPC mixtures to achieve sufficient open time for constructability. After 28 days of curing age, cylindrical 3DPGPC specimens cored perpendicular to the printing direction (D1) exhibit an average compressive strength of 23.7–31.52 MPa and the specimens cored parallel to the printing direction (D3) exhibit an average compressive strength of 25.25–33.13 MPa. Also, an increase in a/b from 1.6 to 2 increases the average compressive strength by 18% in D1 and 25% in D3. The average flexural strength of specimens in D1 attained 5.48–7.29 MPa, while the average interlayer bond strength of 3DPGPC beam specimens attained 5.40–6.90 MPa. An increase in a/b from 1.6 to 2 enhanced average flexural strength by 6%. Furthermore, the 28-day splitting tensile strength of cored cylindrical 3DPGPC specimens attained 1.79–2.16 MPa in D1 and 2.04–2.43 MPa in D3. Microstructural characterisation revealed the formation of C-S-H, N-A-S-H and C-A-S-H gel with increasing slag inclusion due to the presence of Ca2+ion. Nitrogen physisorption analysis shows that the adsorption and desorption isotherm curves and hysteresis loops were not altered but more nitrogen by volume was absorbed due to the formation of a significant quantity of C-S-H, N-A-S-H and C-A-S-H gel. The adsorption and desorption isotherms fall under the mesopores structure of 2–50 nm using the IUPAC technical report classification. The Brunauer-Emmett-Teller (BET) surface area increases by 67, 69, and 78% as slag content increases by 10, 20 and 30% due to the polymerisation of the reactive phases, which densify the matrices. The enhanced mechanical performance, microstructural properties, and pore structure system of slag-modified MK-based 3DPGPC ascertained that it is suitable for construction applications.