Evaluation and beneficiation of harvested and high sulfur fly ash for use in concrete and mitigation and service life modelling of alkali-silica reaction in concrete
Open Access
- Author:
- Kaladharan, Gopakumar
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 09, 2021
- Committee Members:
- Farshad Rajabipour, Chair & Dissertation Advisor
Shelley Stoffels, Major Field Member
James Rosenberger, Outside Unit & Field Member
Christopher Gorski, Major Field Member
Aleksandra Radlinska, Major Field Member
John Regan, Program Head/Chair - Keywords:
- Alkali-silica reaction
mitigation
admixtures
harvested fly ash
sampling plan
flue gas desulfurization
high sulfur fly ash
beneficiation
service life modelling - Abstract:
- Fly ash is the most commonly used supplementary cementitious material (SCM) in concrete production today. In recent years, however, the quantity of freshly produced fly ash in the United States has declined by nearly 60% and this has caused a fly ash shortage to produce high-quality and durable concrete. To address this challenge, in the first part of this dissertation, alternative sources of fly ash including harvested and high sulfur/alkali fly ash were evaluated and suitable beneficiation strategies were identified to address challenges in their performance. The potential issues with using landfilled fly ash as a concrete pozzolan was summarized based on a literature review. Accordingly, a statistical sampling approach was developed to determine the viability of a given landfill as a pozzolan source and this approach was applied to a Class F fly ash landfill in Pennsylvania. Samples taken from the landfill were tested based on ASTM C618 requirements and statistical analyses were performed to determine the necessary beneficiation strategies to make the material viable for use as concrete pozzolan. Blended mortar and concrete mixtures incorporating the beneficiated fly ash were tested for slump, plastic and hardened air content, compressive strength, and mitigation of alkali-silica reaction (ASR). High sulfur and/or high alkali fly ashes are produced in coal-fuel power plants with semi-dry or dry flue gas desulfurization (FGD) systems that produce fly ash comingled with FGD products. Such fly ashes do not meet the ASTM C618 SO3 content limit (5.0% max.) and may be unable to mitigate the ASR in concrete due to the high alkali content of the fly ash. The chemistry and mineralogy of the sulfur present in these ashes can vary significantly (e.g., CaSO4, CaSO3, Na2SO4) based on the FGD technology used and this in turn affects the performance of these fly ashes in cementitious systems. Thus, the single SO3 content limit prescribed by ASTM C618 is not sufficient to capture the complexity and performance of these fly ashes and this has resulted in elimination of potentially viable SCMs. In this research, the effect of these fly ashes on various concrete performance parameters such as workability (flow and flow retention), pore fluid pH, setting time, strength development, and potential for deleterious expansion were evaluated by considering both real and simulated fly ashes (i.e., by blending of specification-compliant fly ash with different sulfur compounds). While many measured properties of the resulting concretes were acceptable, poor performance was observed in the case of setting time and pore fluid pH with some fly ashes. In such cases, novel beneficiation strategies were devised and implemented to successfully address the problem. The second part of this dissertation addresses the mitigation of ASR in concrete and service life prediction of susceptible structures. A new generation of ASR-inhibiting chemical admixtures were developed for concrete. These admixtures, which are in the form of soluble inorganic and organic salts, are cheaper and more abundant than lithium admixtures, yet provide more consistency in terms of quality, supply, and performance in comparison with SCMs. A methodical approach was developed to identify such admixtures that primarily mitigate ASR by reducing the pH of concrete pore solution. The mechanism of pH reduction was identified and the set of criteria that a potential admixture must meet were developed. The suitable admixtures were screened using ASTM C1293 as a proof-of-concept. Additionally, the performance of these admixtures in concrete and mortar mixtures was extensively evaluated. The properties studied include flow, flow retention, concrete slump, plastic air content, setting time, compressive strength of mortar and concrete, pore fluid pH at 0, 7, 28, 90, and 180 days, formation factor (along with porosity and pore connectivity), and drying shrinkage. The admixtures were found to maintain the reduced pH in the long term and had minimal impact on workability, plastic air content, and strength. These admixtures may increase the pore connectivity of the system without increasing the porosity. Drying shrinkage was also found to be slightly higher when compared to ordinary portland cement (OPC), but within the limits specified by the ASTM standards. A final list of eight promising admixtures were identified. Finally, estimating the service life of structures affected by ASR is essential for effective investment of resources for maintenance, repair, or replacement of crucial infrastructure. Current ASR prediction models either do not consider all the important factors that impact the magnitude and rate of ASR progression or are too complicated to implement in practice. In this study, mini-mortar bar specimens were used to evaluate the development of ASR expansion using five evenly spaced levels of aggregate reactivity (ASTM C1293 1-year expansion value), pore solution pH, temperature, and moisture access. The experiments were designed using the response surface methodology and a quadratic regression model was used to relate the ASR expansion rate to the testing conditions. Further, a service life model was developed for predicting the time to failure (time when expansion reaches 0.04%) based on the concrete mix design and exposure conditions. The predictions of the developed model were in good agreement with the outdoor exposure blocks data available in literature. Overall, this dissertation addresses key challenges in improving the sustainability and durability of concrete. The evaluation and beneficiation protocol developed in this dissertation for the use of landfilled fly ash will hopefully encourage their use and ultimately increase the supply of fly ash available for producing high quality and green concrete. Additionally, the work done on high sulfur and/or high alkali fly ash will encourage more research into these materials and possibly lead to a re-evaluation of the current standard specifications that currently limit their use. The cheap and abundant admixtures developed for mitigation of ASR will provide the industry with a reliable strategy for addressing this durability challenge. Additionally, the service life model developed for ASR susceptible structures addresses the impact of the most important parameters and can be used for effective asset management.