Abstract
The strength of powder-based 3D printed geopolymer samples immediately after the de-powdering process (‘green’ strength) is inherently very low. Therefore, different post-processing techniques have been explored in the previous study of the authors to enhance the ‘green’ strength of the printed geopolymer. The highest strength of around 30 MPa was achieved for the printed geopolymer sample cured in an alkaline solution for 7 days at 60°C. Although this strength is sufficient for a wide range of construction applications, the necessity for the heat curing procedure, which requires significant amount of energy, can compromise the sustainability credentials of the developed powder-based 3D printed geopolymer and limit its commercial viability and large-scale applications in the construction industry. To tackle this issue, this study aims to develop a new post-processing method which eliminates the necessity for the heat curing. The influences of type of curing medium, duration and temperature of curing, and testing direction on the compressive strength of the printed geopolymer were investigated. The ‘green’ printed geopolymer samples were immersed in four different curing mediums, including two sodium (Na)-based and two potassium (K)-based activators with different alakli modulus (SiO2/M2O where M=Na or K), and cured at two different curing temperatures (ambient temperature (23°C) versus 60°C) for 7 and 28 days. The compressive strength of the ‘post-processed’ printed geopolymer specimens was measured in two different testing directions, namely the binder jetting direction and layer stacking direction. The results showed that the 28-day compressive strength of the ambient temperature cured printed geopolymer sample was comparable to the 7-day compressive strength of the corresponding heat cured sample. Therefore, the feasibility of enhancing the strength of printed geopolymer by curing in an alkaline solution at ambient temperature was established. This newly developed post-processing method based on the ambient temperature curing is more viable and less energy-intensive, yet provides comparable strength, as compared to the previously developed post-processing method based on the heat curing. The results also showed that the strength of the printed geopolymer samples cured in the K-based activators was lower than that of the specimens cured in the Na-based activators.
Highlights
Two common 3D Concrete Printing (3DCP) techniques are used in the current construction industry, namely extrusion-based 3DCP and powder-based 3DCP
The possible reason for the significantly higher compressive strength of the heat cured samples can be explained as follows: Before the post-processing treatment, it is hypothesized that the “green” strength may come from the formation of calcium silicate hydrate due to the reaction among slag, sodium metasilicate powder and the liquid binder
During the post-processing treatment, when the green samples are immersed in the alkaline solutions, it is hypothesized that polymerization occurred within the sodium/potassium silicate solution, resulted in formation and development of geopolymeric products, which in turn led to the densification of porous structures of the “green” samples
Summary
Two common 3DCP techniques are used in the current construction industry, namely extrusion-based 3DCP and powder-based 3DCP ( known as particle-bed 3DCP). The extrusion-based 3DCP is a selective material deposition technique aimed for on-site concrete construction In this technique, the cementitious material is extruded by a digitally controlled extruder to manufacture the component layer-bylayer [e.g., Contour Crafting (Khoshnevis, 2004), Concrete Printing (Le et al, 2012a,b)]. The cementitious material is extruded by a digitally controlled extruder to manufacture the component layer-bylayer [e.g., Contour Crafting (Khoshnevis, 2004), Concrete Printing (Le et al, 2012a,b)] In this process, the material must be fluid enough during the transferring in the pumping system and must have sufficient viscosity and yield stress to keep the shape after being extruded out. Large-scale buildings, such as houses, could be built using this method, and this could be achieved without any formwork. Formwork is the temporary structure and mold for pouring wet concrete, typically built with timber. Formwork represents a source of waste, as all formwork made of timber is discarded sooner or later, contributing to a generally increasing amount of waste in the world (De Schutter et al, 2018; Sanjayan and Nematollahi, 2019)
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