3D printing in construction offers significant advantages in cost, material, and time efficiency, but material-related challenges need to be overcome for accelerated adoption. One of these challenges is understanding the rheological properties of cement paste, the primary fluid component of concrete, and how they are related to its microstructure. Moreover, the change in these properties over time must be monitored as the paste is a dynamic system in the fresh state, the period most relevant for the 3D printing process. The dissertation primarily explores and emphasizes the viability of small amplitude oscillatory shear techniques for understanding and differentiating between the microstructural evolution of cement pastes without and with printability-enhancing additives.
Understanding the rheology of cement pastes, especially the time-evolution of viscoelastic properties, is crucial for 3D printing as they affect the flow of the material and structural stability throughout the printing process. Viscoelastic properties can be measured using oscillatory rheological experiments, which have been found suitable for cementitious materials and provide key properties like storage modulus and loss modulus, among others. There has been a growing interest in using such rheological techniques as there still exist many unanswered questions regarding rheological-microstructure and microstructure-printability relationships. The mixture of cement and water by itself is not printable; additives are generally required. Additives like nanoclays, calcium carbonate whiskers, and viscosity-modifying agents can enhance the printability of concrete by improving structural buildup and flow behavior. However, their microstructure-printability relationship remains unexplored, and this investigation has tried to shed some light on it. The dissertation is structured into chapters that discuss rheological measurements, the impact of additives, and various testing methods to support hypotheses about microstructure-rheology relationships and 3D printability.
Chapter 1 involved the use of small amplitude oscillatory sweep techniques to study ordinary Portland and Portland limestone cements. Yield stress and viscosity are commonly measured rheological properties for printability, but these tests may provide little information about the microstructure as they are destructive in nature. Oscillatory sweep tests can be non-destructive and provide information about the microstructure before structural breakdown, which is important for the material that is already extruded. This material is at rest but is also undergoing hydration, which necessitates monitoring the evolution of material properties over time. The relatively few studies that exist that have studied this time-evolution have focused primarily on the evolution of storage modulus, while the change in the critical strain parameter, which is itself important for measuring the storage modulus, has remained unexplored.
In this chapter, an ordinary Portland cement (OPC) and a Portland limestone cement (PLC), mixed at different w/c ratios, were subjected to amplitude sweeps to observe the time-evolution of critical strain during the induction period. A Python algorithm was developed for extracting several different rheological properties along with critical strain. As hydration progressed, critical strains were found to increase exponentially and had an inverse correlation with the w/c ratio. The increase was quantified by an equation with a good fit using w/c ratio and time as the dependent variables. It was also shown that critical strain and storage modulus have different growth profiles, which could mean that the underlying microstructural factors for those properties are different. It was also shown that the choice of criterium for locating critical strain significantly affected the calculated critical strain and highlighted the importance of standardization of such criteria.
Chapter 2 extended the application of the oscillatory techniques and hypotheses toward cement pastes with additives that could improve 3D printability. Chemical admixtures and mineral additives are generally added to cement-based materials to achieve adequate printability. This investigation employed additives with different physical and chemical properties to observe their impacts on printability and hydration kinetics. Amplitude sweeps were used to measure changes in various rheological properties during the dissolution and induction periods in plain and additive-modified pastes. This chapter shows that amplitude sweeps can be effective methods for differentiating between cement pastes with different additives. The chapter also showed the importance of monitoring properties other than critical strain and storage modulus, specifically the yielding strain, for facilitating an understanding of microstructure-rheological property relationships when combined with other characterization techniques. Establishing these relationships can eventually help explain why printability-enhancing additives that are already used are effective and can provide a tool to explore more additives in the future.
Chapter 3 explored the use of in-situ characterization tests to help support the claims made in previous chapters. pH testing on various cement pastes highlighted the correlation between pH and storage modulus. Electrical impedance measurements were conducted to monitor cement hydration and microstructural development. The resistance of the pastes increased over time, with an initial slow rate followed by a rapid exponential increase, correlating with critical strain. The pH and resistance results showed they could be promising in-situ measurement techniques for monitoring prints on-site. The chapter also includes a discussion on the properties of methylcellulose, specifically its foaming capability and polymeric behavior, which potentially affected the rheological results and printability.
Chapter 4 discussed the methodologies and results of frequency and time sweeps in rheological tests, focusing on the storage modulus. It examined how different factors, such as amplitude, frequency, and additives, affect continuous measurement. Amplitude and frequency sweeps are interconnected, requiring both to be performed in tandem to determine the best combination of amplitude and frequency for time sweep tests. Frequency sweeps on different cement pastes showed that storage modulus curves change over time, with smoother curves at different frequencies depending on the age of the paste. Additives affect the frequency sweep results, leading to different ranges of ideal frequencies and storage modulus values. Time sweeps were conducted by varying the oscillation amplitude and frequency, and it was found that varying them during the time sweep can improve the quality of storage modulus evolution curves. The results also suggested that the minimum strain rate required varies over time, and can be achieved by changing either the strain amplitude or the frequency. The chapter also included a preliminary investigation on structural rebuilding, which showed that all rheological properties that were monitored recovered fully, at a more rapid rate during rebuilding.
The results and hypotheses presented in Chapters 1, 2, and 4 can serve as foundations for improving measurement protocols for oscillatory tests and, combined with Chapter 3, can guide further explorations of viable techniques to study microstructure evolution during the induction period of cement pastes.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/4zgh-2d62 |
| Date | January 2024 |
| Creators | Badjatya, Palash |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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