3D Printable Earth-Based Alkali-Activated Materials (2023-06)¶

Earthen constructions are regaining popularity in the building and construction industry due to its low embodied carbon and recyclability. Often, inorganic binders, for instance, Portland cement, are used to stabilize earth-based materials to reduce dimensional instability and impart durability. However, with growing emphasis on decarbonization technologies in the construction sector, there is a need to develop low-carbon binders that are compatible with earth-based materials. Furthermore, additive manufacturing (commonly called 3D printing) method in construction industry is gaining popularity due to its potential to reduce material wastage, increase production rate and improve quality. While 3D printing of clay has been demonstrated, there is a dearth of investigation into printing of earth-based alkali-activated materials. The research herein aims to develop two classes of 3D printable earth-based alkali-activated slag-fly ash concrete, where (i) 8 molar (M) NaOH solution, and (ii) 4M NaOH solution are used as activators at varying solution to binder ratio to achieve flowability for 3D printing depending on the clay content in the mix. The used earth contained 48–50% clay, primarily Kaolinite with some traces of Montmorillonite. In each of these mixes, the ratio of (slag + fly ash) to aggregate were 1:1, 1:2 and 1:3 by mass where, the clay content varied from 12% to 36% by mass of total fine aggregate. Through several trials, it was found that flowability of 40–50%, determined according to ASTM C1437, offered the desired consistency and rheological properties for extrusion and buildability. Experimental results show that 8M 1:2 mix offered highest buildability of about 460 mm with first layer settlement of only 0.21%. Buildability was found to reduce with reduction of molarity, evident from highest buildability of 377 mm for 4M 1:1 mix. This was attributed to the reduced viscosity of the solution and lower activator concentration, resulting in prolonged setting by 40–60% than 8M 1:1 and 1:2 mixes. On-site ultrasonic pulse velocity (UPV) measurement on the printable material showed 7.50–12% lower velocity through earth-based 4M mixes than the counterparts prepared with 8M, suggesting lower rate of densification and stiffening due to reduced molarity. Introduction of clay plays an important part in 3D printing by improving the thixotropic behaviour and reducing inter-particle friction otherwise existing between manufactured sand (M-sand) particles. While mixes containing combinations of clay-rich soil and M-sand could be extruded at flow values of 40–50%, the mixes with only M-sand (no soil) either could not be unextruded except for 8M 1:2 or offered poor printing quality mix even at a much higher flow value of 88–90%. The designed mixes offered compressive strength of 19–34 MPa and 17–28 MPa at 28-day age for 8M and 4M of activator solutions respectively. Although a reduction in strength compared to control (only sand and no soil) was observed with higher reduction with increase in clay content, the designed materials satisfy the strength requirement as per Indian standard, IS 1725 for soil-based block constructions.