The New Jersey Institute of Technology's
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Title:	The impact of material and environmental properties on the ettringite - metaettringite thermochemical reaction
Author:	Strand, Aaron
View Online:	njit-etd2022-084 (xviii, 175 pages ~ 7.5 MB pdf)
Department:	Department of Civil and Environmental Engineering
Degree:	Doctor of Philosophy
Program:	Civil Engineering
Document Type:	Dissertation
Advisory Committee:	Adams, Matthew P. (Committee chair) Wecharatana, Methi (Committee member) Bandelt, Matthew J. (Committee member) Assaad, Rayan Hassane (Committee member) Nadimpalli, Siva P.V. (Committee member)
Date:	2022-05
Keywords:	Cement Energy storage Ettringite Metaettringite Stability Thermochemical energy
Availability:	Unrestricted
Abstract:	The use of energy is an essential need for people, with demand constantly increasing due to an increasing population and an increase in living and comfort standards. Since 1965, world energy consumption has quadrupled. During peak utilization, the strain on the grid infrastructure can be immense. As energy demand fluctuates during different times of the day and year, the energy production needs to scale accordingly This creates inefficiencies within the system such as over building generation capacity or losses from start up or ramping up production. This can be alleviated through load leveling where excess energy generated during low demand periods can be stored and used during higher demand periods. Also, storage systems are needed with the increasing use of renewables in energy generation. Long-term energy storage through reversible thermochemical energy processes is being increasingly examined due to their capability of high energy densities. One possible form of this storage is with the ettringite-metaettringite conversion process. The thermochemical reaction using ettringite is focused on the hysteresis in the dehydration and rehydration between the two states, where decomposition of the crystal will result in energy being unrecoverable. The ettringite crystal is rarely found in nature, but it is commonly found as a product from the hydration of hydraulic cements. Specialty types of cement binders such as calcium sulfoaluminate (C$A) cement; the binary systems of calcium aluminate cement (CAC) with calcium sulfate (C$); and the ternary systems of ordinary portland cement (OPC) with CAC and C$ can be used in which ettringite can be found as a primary hydration product. The primary concerns related to these systems are the stability of the ettringite-metaettringite reaction and the efficiency of energy storage. The effect of the cement type and dehydration temperature is studied for the macrostability of the specimens by measuring the number of cycles that could be completed before failure of the specimens. The compressive strength of the pastes is used to investigate the stability of the systems during the dehydration and rehydration cycle before failure. The microstructural properties of the cement pastes are investigated through the use of X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). This work shows that the type of cement and dehydration temperature does impact the stability and microstructure of the systems. The stability of long-term storage of specimens is also investigated for the C$A cement. Storage of the C$A paste system is possible when the relative humidity (RH) is kept below 45%. The energy released on rehydration of the paste systems is studied for the three cements at five different dehydration temperatures. Presented in this dissertation are the heat flow and cumulative heat for these systems during the first 24 hours of rehydration. Results indicate that the type of cement and dehydration temperature do impact the amount of energy released during rehydration. Coupled with the results on stability, the OPC-CAC-C$ cement, which performed the best for stability, performed the worst of the three for energy recovery. The C$A system is also studied at three dehydration temperatures for the heat released when subjected to multiple cycles. The results indicate that there is a decreasing trend of energy release as cycle count is increased for two of the three temperatures.