Post-processing Methods to Improve Strength of Particle-Bed 3D Printed Geopolymer for Digital Construction Applications

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 slag-based 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 a 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 alkali modulus (SiO 2 /M 2 O where M = Na or K), and cured at two different curing temperatures (ambient temperature (23 • C) vs. 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 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.

Upcycling of Wastewater Sludge Incineration Ash as a 3D Printing Technology Resource

“…Figure 5 shows the 3D printed cubic specimens produced using the composite powder materials, which have been prepared using URHC and different ratios of WSIA by weight with and without post-treatment in different solutions (Experiments 1-5). Figure 6 shows the comparison among Experiment 1 [the control experiment without post-treatment referred to as the "green" sample in previous studies (Nematollahi et al, 2019)], Experiment 2 (post-treatment with the immersion liquid of water), Experiment 3 (using alkali solution used as a commercially available surface hardener), Experiment 4 (employing an alkaline solution of Na 2 SiO 3 and NaOH), and Experiment 5 (utilizing PDMS solution). The results show an increase in volumetric densities in Experiments 2-5.…”

“…It is particularly suitable for applications using powder-based inorganic materials by employing the methods of material extrusion, including fused deposition modeling, and binder jetting (BJ) among seven 3D printing methods (ASTM International, 2015). Ordinary Portland cement (OPC) is traditionally used in the construction industry; however, in recent years, several sustainable BJ 3D printing alternatives have been studied to replace OPC, which exhibits a high energy demand and produces carbon dioxide Sanjayan, 2016, 2018;Nematollahi et al, 2017Nematollahi et al, , 2019Park et al, 2018;Xia et al, 2019). Alternatives include an inorganic polymer or geopolymer, which acts as the binding agent in concrete via geopolymerization reactions (Davidovits, 1989).…”

“…Studies also show that additional post-treatment for 3D printed products results in a compressive strength that is ∼18 times greater than that observed before post-treatment (Xia and Sanjayan, 2016). Moreover, to improve geopolymer strength, diverse processing conditions, such as curing temperature, medium, and time during post-processing were assessed along with different mixing proportions of geopolymer precursors (Park et al, 2018;Xia and Sanjayan, 2018;Nematollahi et al, 2019;Xia et al, 2019). The development of sustainable 3D printed alternatives to OPC is still in the early stages of commercialization or application in the construction industry.…”

Particle-Bed Binding by Selective Paste Intrusion—Strength and Durability of Printed Fine-Grain Concrete Members

“…Another group of researchers of the SCA method are using geopolymer binders [ 15 , 21 , 22 , 23 , 24 ]. In those investigations, the layer direction as well as the post processing showed also an important effect on strength.…”

Influence of the Composition of the Activator on Mechanical Characteristics of a Geopolymer

“…Also interesting are recent studies on some mechanical properties of geopolymer composites reinforced by ultra-high-molecular-weight polyethylene fibers and on the strength of powder-based three dimensional (3D) printed geopolymer samples [21,22].…”