Mechanical anisotropy evaluation and bonding properties of 3D-printable construction and demolition waste-based geopolymer mortars

Abstract

This work aims at evaluating the anisotropy (direction-dependency) in terms of mechanical performance and bonding properties of entirely construction and demolition waste (CDW)-based geopolymer mortars fabricated by 3D-additive manufacturing (3D-AM) technique. In the study, a combination of hollow brick (HB), red clay brick (RCB), roof tile (RT), concrete waste (CW) and glass waste (GW) obtained from various demolition sites and different combinations of alkaline activators including sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)2) were used for geopolymerization. CW was also used as fine aggregate in geopolymer mortar production. Specimens were subjected to ambient curing conditions until testing ages. Direction-dependent mechanical performance of printed specimens was evaluated at 7-, 28- and 90-day via compressive strength test in three different loading directions of perpendicular, parallel, and lateral to the printing path and flexural strength test in two different loading directions of perpendicular and lateral to the printing path. Moreover, bond strength between the consecutive printed layers were tested through direct and splitting tensile strength tests at the end of 7-, 28- and 90-day ambient curing and used to compare the directional performance of tested mixtures. In addition, compressive and flexural strength test results of printed specimens were compared with those of conventional mold-casted specimens. Results showed that alkaline activator content affects the mechanical properties considerably. According to compressive and flexural strength test results, 3D-printed geopolymer mortar specimens have anisotropic behavior and the bond performance between consecutive layers is one of the main influencing parameters for the anisotropic behavior of 3D-printed structures. However, perpendicular-loaded 3D-printed specimens showed similar or slightly better performance compared to the mold-casted ones, indicating that the bond zone had little influence on the performance of specimens loaded in perpendicular loading direction. This study pointed out that the anisotropic performance of printed structures can be diminished with the enhanced bond adhesion between consecutive layers and the adhesion can be improved by optimizing the rheological properties and matrix performance of the mixtures.