3D printing of high-volume fly ash mixtures for digital concrete construction

• Main Author: Panda, Biranchi
• Other Authors: Cise Unluer
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: Engineering::Materials::Material testing and characterization; Engineering::Materials::Composite materials; Engineering::Mechanical engineering::Prototyping
• Online Access: https://hdl.handle.net/10356/93583; http://hdl.handle.net/10220/49934
• Institution: Nanyang Technological University

Digital concrete construction has recently become the subject of very rapidly growing research activities all over the world. It opens a new horizon and unlimited possibilities for the concrete industry, especially in terms of geometrical flexibility, reduction of manpower and costs, increased productivity, speed of construction, and sustainability. The potential of digital production of concrete is represented by exponentially increasing innovative projects and research, which are often addressed using the generalized term “3D-concrete-printing”. Recent advances in concrete printing have mainly focused on developing Portland cement (PC) based binders that are both extrudable and buildable. The production of PC is associated with many environmental challenges including the air pollution and devastation in land and resources. One of the promising measures is the development of sustainable cements, which can be achieved via two main pathways: (i) partial PC replacement with industrial by‐products and wastes and (ii) new cement formulations based on different chemistry with lower environmental impacts. This thesis is dedicated to material synthesis of two promising sustainable building materials such as (i) high volume fly ash (HVFA) cement and (ii) geopolymer based cement for extrusion-based 3D printing application. Rheology of fresh concrete is critically important in 3D printing process, that determines the material ability to be extrudable and buildable. With an aim of utilizing minimum 60-70% fly ash, HVFA cement was formulated while understanding the key parameters influencing yield stress, viscosity and thixotropy properties. Thixotropy was quantified using shear thinning and viscosity recovery protocols. In addition, structural-build up was measured in the dormant period, to control the buildability of printed structures. Considering the rheological requirements of a large-scale concrete printing, nanoclay was added as viscosity modifying agent (VMA) to improve the control mix performance.The nanoclay was found to improve the thixotropy but had no effect on structural-build up rate which limited the part buildability. To accelerate the build-up rate, micro silica fume was incorporated in the mix design and better buildability was achieved due to significant improvement in the strength development property. The rheology and reaction product of the final mix design were characterized to resolve the underlying mechanisms using laser scattering, isothermal calorimetry, X-ray Powder Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) techniques. In the second part of the thesis, fly ash based geopolymer was synthesized with improved rheological properties for 3D printing process. High volume of fly ash was supplemented with blast furnace slag and micro silica fume to provide adequate microstructural packing required for printability. Shape retention, which is related to viscosity recovery was found to be improved with silica fume addition up to 10-15% without much affecting the extrudability. Solution of potassium silicate was used as activator in different concentration (molar ratio) and solution-to-binder ratio to understand the geopolymer rheology based on yield stress and viscosity values. The activator solution was found to reduce the yield stress and enhance the cohesiveness, similar to the use of superplasticizer in conventional PC system. Therefore, nanoclay was again used to improve the printability of geopolymer while minimizing the viscous effect of activator solution. In contrast, powder (potassium) silicate activated system show rheological responses similar to those of HVFA cement system. XRD and FESEM were used to quantify the resultant geopolymer reaction product and relate it to the 28 days mechanical strength. In the last section, directional properties of both 3D printed HVFA and geopolymer samples were studied in terms of compression, flexural and tensile bond strength. Due to layering effect, 3D printed samples exhibited orthotropic properties with reduced bond strength with increasing inter-layer time gap. Effects of deposition speed and print height were also investigated, which concludes that the material build-up rate plays an important role in determining the final effect along with the printing parameters. Therefore, the build-up rate was accessed via early age mechanical strength testing, where the fresh concrete was uniaxially deformed to obtain green strength and stiffness. Based on the green strength development and deformation behaviour, print path and speed were optimized to build two large-scale 3D printed structures for demonstrating the proposed criteria and mix designs are suitable in practice. The contradictory challenge of 3D printing concrete i.e both pumpable as soft fluids and buildable as solid elements were solved in this thesis by understanding the rheology of two promising sustainable materials before, during and after the printing. Based on experimental observations, it is concluded that PC based plain mortars are deemed suitable for concrete printing while geopolymers need significant material tuning based on the printing process.