ABSTRACT
3D printing (3DP) of cementitious materials shows several advantages compared to conventional construction methods, but it requires specific fresh-state properties. Nanomaterials have been used in cement-based materials to achieve specific fresh and hardened properties, being potential candidates for 3DP applications. However, there are no reports on using TiO2 nanoparticles (nano-TiO2) in 3DP cementitious composites. Thus, the current work aims to assess the effect of nano-TiO2 on the fresh performance of 3DP cementitious materials. For this purpose, nano-TiO2 was incorporated in pastes and mortars from 0 to 1.5 wt.%. Time-resolved hydration (in situ XRD) and rheological and printing-related properties (buildability and printability) were evaluated. Results showed that nano-TiO2 particles enhanced the cement hydration kinetics, leading to further ettringite formation up to 140 min compared to plain cement paste. Rheological measurements showed that the nano-TiO2 incorporation progressively increased the static and dynamic stress, viscosity, and structuration rate of pastes. Furthermore, nano-TiO2 improved the buildability of the composites, progressively increasing the maximum number of successive layers printed before failure from 11 (0 wt.% TiO2) to 64 (1.5 wt.% TiO2). By contrast, the nano-TiO2 addition reduced the printability (i.e., the printable period during which the sample was able to be molded by the 3D-printing process) from 140 min (0% TiO2) to 90 min (1.5% TiO2). Thus, incorporating "high" nano-TiO2 contents (e.g., >1 wt.%) was beneficial for buildability but would require a quicker 3DP process. The adoption of nano-TiO2 contents of around 0.75−1.00% may be an interesting choice since it reduced the printability of paste by 30 min compared with the control mix but allowed for printing 24 layers (118% higher than plain mortar).
ABSTRACT
Wastewater (sewage) treatment plants generate a high volume of sludge that, although it is classified as class II-A (not inert) non-hazardous waste, is commonly disposed of in sanitary landfills. Hence the environmental urge to assess its valorization possibilities. The present study describes the use of a sewage sludge as raw material in the production of red ceramic bricks by extrusion, focusing on the technological changes brought about by the presence of the sludge during the plastic forming process, in terms of the plasticity of the mixture and its subsequent extrudability. To quantitatively identify the best moisture conditions for the extrusion of high-quality products, shear strain amplitude sweep (torsional) tests were conducted on green (moist) prismatic samples produced with different moisture and sludge contents. For sewage sludge contents up to 10 wt%, the optimal moisture content was identified at 31-33 wt%. Higher water demand was identified for 15 wt% of sludge, for which optimal extrusion results required 35 wt% moisture. The total linear shrinkage after drying and firing, as well as the water absorption, were within the limits required for ceramic bricks for all the sludge contents. The results for compressive strength of the fired bricks also demonstrate that the incorporation of up to 15 wt% sewage sludge into the clay mixture is highly feasible for the production of extruded ceramic bricks.
