The primary objective of this research is to investigate the nano- and micro-scale mechanisms of late age ettringite formation and how these mechanisms relate to macro-scale expansion in concrete materials, enabling possible mitigation strategies for damage due to the late age formation of ettringite. This research will examine the chemical and physical structure of the reactants and products, as well as the pore structure evolution, involved in the process of late age ettringite formation and subsequent expansion and cracking. A more thorough understanding of the crystalline pressures induced on the microstructure will result by refining existing models with analytical techniques such as nanoindentation and residual stress analyses.
Concrete is the most widely used engineering material in the world, yet is still poorly understand in many regards. The prevention of delayed ettringite formation, which is known to cause concrete cracking, would significantly improve the infrastructure life span of the United States and abroad. Educational activities will include traveling to local middle and high schools to provide hands-on experiments that give the students a real-life view of how engineering is an important part of the modern world. Local primary school groups will be invited to campus to participate in hands-on learning experiences at Tennessee Tech?s STEM Center. In addition to the educational activities, a residual stress analysis workshop is proposed at the end of the project to disseminate the strong background in this field at Tennessee Tech to other cement and concrete researchers; thus, expanding beyond traditional K-12 outreach.
A comprehensive research program was conducted to fully understand the mechanisms of delayed ettringite formation in cementitious materials. Delayed ettringite formation occurs in portland cement-based materials that have experienced an internal temperature greater than 70 C and can lead to significant expansion and cracking of a concrete structure. The objectives of the research were to investigate the nano- and microstructural mechanisms of formation of late age ettringite as these differences relate to macro-scale behavior. To understand these mechanisms, it was necessary to study the development of ettringite formation beginning at the nanoscale. Thus, this research investigated the mechanisms of ettringite formation by examining the chemical and physical structure of the reactants and products, as well as the pore structure evolution, involved in the process of late age ettringite formation and subsequent expansion and cracking. The experimental research utilized (1) physical expansion measurements over a period of 5+ years, (2) innovative accelerated leaching tests, (3) high resolution x-ray diffraction, (4) pore structure analysis via small angle x-ray scattering (SAXS), (5) water vapor sorption isotherms, and (6) preliminary residual stress analysis. A comprehensive nano and micro-structural model, merging the experimental data with existing and refined thermodynamic and computational models, was constructed. DEF-related expansion is extremely complex to understand due to the irregular formation of ettringite. n this research, it was seen that the samples that expanded early versus the samples that did not, also showed larger areas under the pore size distribution graphs within the range of 15 to 60 nm. Typically, pores would have to fill completely with DEF crystals before damage might occur; however, single crystals under supersaturated conditions, within mesopores (2-50 nm in diameter), are thought to be the reason for the DEF mass expansions (19). In the samples that expanded at later ages, the pore size distributions were similar to those that did not show any expansions up until at least 75 days. Between 75 and 1700 days, these peaks shifted to the right, but not as far as the early expansive samples. This peak shift was similar to the shift seen in the non-expansive samples. The late age peak shifts in all of the samples may be accredited to one or all of the following conditions: Filling of pores less than 10-15 nm with DEF, Filling of pores greater than 10-15 nm with DEF, Closing of pores smaller than 10-15 nm due to hydration of C-S-H, Closing of pores greater than 10-15 nm due to hydration of C-S-H. If pores greater than 10-15 nm close at a faster rate than the pores less than 10-15 nm, it would cause the illusion that the majority of pores would be expanding. Pores, however, continue to shrink due to the continued hydration of C-S-H. Based on 1500 day measurements, numerous samples have displayed expansions that would be detrimental in structures. As stated in the research objectives, several cement composition and mix design factors were found to contribute to expansion. In regards to the accelerated test method development, the summary of the results are as follows; Sulfate is a key factor in the determination of DEF potential in cementitious materials heat cured at 100oC. Sulfates have a highly positive correlation with percent expansion and seem to play a significant, positive role in determining percent expansion observed in mortar bars. Alkalis (potassium and sodium) also seem to play an acute role in the onset of expansion and the overall percent expansion observed in mortar bars for samples heat cured at 100oC. This is consistent with earlier findings by several studies [1-3,9,18]. A possible explanation for this finding is the one given by Famy et al [3] that alkali hydroxides affect the rate of desorption of sulfates from the outer C-S-H. Samples heat cured at 100oC with a sulfate ionic weight ratio peak above 15mg/g of dry weight have a higher propensity to expand by over 0.10% (which is regarded as deleterious without restraint). All the cement types that were investigated in this study that had the aforementioned ratio value expanded significantly by over 0.10%. The SO4/Al2O3 ratio may be useful as a secondary check for the DEF-related expansion potential for samples heat cured at 100oC. A higher SO4/Al2O3 ratio will generally indicate a higher propensity for high DEF-related expansions observed in mortar bars. The research conducted has enabled the students working on the project to develop interdisciplinary skills necessary for professional and personal development. This research has supported, wholly or in part, four M.S., one Ph.D., and 13 undergraduate students. Furthermore, most of the M.S. students were former undergraduate research assistants and continued their graduate studies at Tennessee Tech under the PI’s direction. Of the 15 total students supported by this project, six have been from underrepresented groups (female and minority).
