Title: Nano-Engineered Cement for 3D Printing Concrete
Description: 3D printing concrete (3DPC) became one of the most investigated topics in cement and concrete research in the last decade. Research within this topic includes examining the role of admixtures on the fresh-state properties, specifically cement rheology, along with the mechanical performance of the printed materials and structures. 3DPC offers a promising platform for sustainable binders, optimized design, and economic and rapid construction that can help reduce the high CO2 impact of Portland cement. This thesis is aimed towards engineering nano-modified cement binders for 3DPC by characterizing the effects of dispersion on fresh, hardening and hardened properties, examining the potential of combining inorganic nanoparticles with organic admixtures to tailor select rheological properties, and developing printing performance measurements, and applying the findings in promising applications. Nanoclays (NC) are one of the most attractive rheological modifiers for 3DPC as they increase structuration and buildability with minimal increase to viscosity or pumping requirements.

This work starts by studying the impact of different dispersion techniques (sonication, magnetic stirring, dry powder mixing) on the rheological properties of NC-modified cement paste. In addition, a novel dry dispersion technique that coats cement grains with nanomaterials was developed and compared to conventional dispersion methods. The results revealed that dry dispersion enhanced NC efficiency in increasing the static yield stress of cement paste, effectively reducing NC content requirements by 33% compared to solution dispersion to achieve the same print height. The observed changes in rheological properties at different NC contents and dispersion techniques implied that the origin of structuration in NC-modified cement pastes is mainly driven by the interactive forces between NC needles (NC-NC) more so than with cement (NC-cement). This incentivized exploring partial treatment of cement with dry dispersion at a relatively high NC content of 10 wt.% to maximize the aforementioned NC-NC interactions.

The results confirmed that the mixture of 10 wt.% NC-coated cement with uncoated ones performed similar or better than mixtures where dry dispersion was applied to all cement grains while maintaining the same NC dosage. From the collective findings of this investigation, it was deduced that partial treatment of cement with NC using dry dispersion can maximize NC efficiency in increasing the static yield stress for 3DPC. Moreover, because NC-NC interactions were more influential than NC-cement interactions on the structuration of cement paste, NC are likely to be successful in increasing the static yield stress and buildability of alternative binders for 3DPC. The promising performance of NC-coated cement motivated further exploration of dry dispersion on other nanomaterial types. The impact of dry dispersed versus solution sonicated NC, silica and calcium nanoparticles (SNP and CCNP), and graphene nanoplatelets (GNP), on hydration kinetics and mechanical performance was investigated. Results of isothermal calorimetry and quantitative x-ray diffraction for nano-modified cement pastes showed the critical role of the method of dispersion on the progression of cement hydration, which in turn altered strength development.

For example, SNP-coated cement paste exhibited a delay in ettringite formation by a few hours compared to solution sonication, which likely caused the delayed compressive strength development observed in SNP-coated cement mortars. Nevertheless, for mortars modified with NC, SNP, and CCNP, processing via dry dispersion and solution sonication showed comparable 28-day compressive strengths, implying the successful application of dry dispersion for all three nanomaterials. In addition, results showed an increase in electrical conductivity of GNP-coated cement pastes with dry dispersion whereas GNP were not dispersible with sonication without surfactants or functionalization. The collective results show the efficacy of dry dispersion as an alternative dispersion technique to sonication but one that offers ready-to-mix or just-add-water nano-engineered cement products. Therefore, nano-coated cements via dry dispersion could be very beneficial for remote 3DPC applications or commercialization of nano-engineered binders.

One of the drawbacks of NC reported in literature and confirmed in this work is the significant increase in stiffness that causes filament breakage and tearing during extrusion. To remedy this, a new hybrid rheological modifier combining NC with methyl cellulose (MC) was introduced to tailor cement paste rheology and meet 3DPC requirements. The hybrid mixture of NC and MC proved to increase NC efficiency by up to 900 Pa/1 wt.% of MC in cement paste without jeopardizing its extrudability, essentially decreasing the NC content requirement, and associated costs, to achieve greater print heights. In addition, the hybrid admixture maintained similar or better mechanical performance compared to unmodified cement mortars whereas addition of NC or MC alone showed reduced 28-day mechanical strengths. To capture the effects of the new admixture on ink or filament properties, three recently proposed printer-based ink tests were applied – elastic buckling of thin walls, slug test and cable sag test.

The results confirmed that despite the softening effect of the hybrid admixture on elastic modulus of cement paste compared to NC alone, the critical buckling height, which measures structural stability, was not similarly impacted. In addition, ink cohesion measured by both slug and cable sag tests improved with the hybrid admixture compared to NC or MC alone. The collective results suggest that the hybrid admixture can tailor cement rheology to meet 3DPC requirements by enhancing ink or filament properties while maintaining mechanical performance. The last investigation applied the previous findings to enable 3D printing and facilitate CO₂ mineralization for a new alternative binder. Magnesium oxide (MgO), similar to Portland cement, hardens through hydration but only develops mechanical strength through carbonation. However, atmospheric carbonation is a self-depreciating diffusion process where the carbonation of the exterior retards and limits further internal carbonation. Building upon the new understanding of the origin of structuration of NC in cement pastes and the high performance of the hybrid combination of NC and MC, the new admixture was used to enable 3D printing of MgO binders.

The results confirmed that NC enhanced shape stability by increasing static yield stress while MC maintained ink cohesion, thereby effectively making MgO pastes printable. Compression tests of 3D printed and conventionally mold cast MgO paste cylinders showed that 3D printing can significantly increase strength by up to an order of magnitude. Examining the effects of different infill patterns (<100% and 100% infill density) and water-to binder ratios, results indicated that the increase in strength is attributed to 3D printing effects like the lack of protective formwork, which increased water evaporation and consequently increased carbon diffusion and intake. This study was the first to be published on tailoring the rheology of MgO binder and studying the effects of infill patterns on the compressive strength of 3D printed MgO pastes. The summary of results demonstrates that 3D printing can introduce significant benefits for carbon cured material systems, such as reactive MgO based systems, to potentially reach CO2 neutrality or negativity.

Chapter 1 is the introduction, which describes how the main goal of this work is to explore the use of nanomaterials for 3DPC. Chapter 2 presents a literature review on 3DPC properties, cement rheology, and nanomaterials. Chapter 3 discusses the effects of NC dispersion, including the novel dry dispersion technique, on the structuration behavior of cement pastes. Chapter 4 dives deeper into the application of dry dispersion on other types of nanoparticles, i.e. SNP, CCNP and GNP. Chapter 5 revisits NC with the addition of MC to tailor cement rheology for 3DPC. Chapter 6 utilizes the results of Chapters 3 and 5 by examining 3D printed MgO paste and carbon intake. Chapter 7 summarizes this work and lists all the chapters’ conclusions. Additional discussions of the printer and extrusion head that were designed and built in Columbia CEEM Carleton Laboratory are also included in the appendix of this work, detailing gantry versus delta printers for 3DPC and the development of a low-cost concrete extrusion head with an open-to-atmosphere hopper that eliminates the need for a pumping system.

Lastly, multiple in-situ printing properties and ink performance tests developed by the author, which utilize the printing system to characterize the fresh properties of inks on site, were expanded and detailed.