Date of Award

Winter 12-18-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Warda Ashraf

Second Committee Member

Eric N. Landis

Third Committee Member

Roberto Lopez-Anido

Abstract

Cellulose Nano Fibrils (CNF) having a high aspect ratio, elastic modulus, tensile strength and reactive surface area for functionalization was considered as a promising nanomaterial for improving chemo-mechanical properties of the cementitious matrixes. CNFs are typically less than 0.2 mm in length and 50nm in width that can be extracted from plants and trees. Overall, in this thesis the study is broadly divided in to three stages: (i) In the first stage, a hybrid silica coated CNF (Si-CNF) was developed and investigated for its effects on cement based systems in addition to Pure CNF (PCNF). (ii) In the second stage, effect of varied fine content percentages of PCNF and lignin containing CNF (LCNF) in Portland cement paste systems were evaluated. (iii) And for the final stage, long term stability of cellulose nanofibrils (CNF) in high alkaline cement pore solutions was studied. For the first two-phases of the study described here, a comprehensive investigation on the effects of CNFs (Si-CNF, PCNF and LCNF) on dispersion stability of fibrils, cement paste workability, hydration, microscopic phase formation, compressive strength and fracture properties were studied. For the final stage of study on long-term stability of CNFs, several laboratory experiments that focused on crystallinity degree of CNF, intensity ratios of hydrogen bonds in CNF, alkali ion concentrations in artificial pore solution (APS), pH concentration of APS and morphology of CNFs were investigated. The central hypothesis assumed for developing Si-CNF is, coating CNF with silica nanoparticles will improve the dispersion and long-term stability of the CNF, thus resulting in further enhancement of the microstructure and mechanical properties of cement-based composites. Silica coating helped in the dispersion of CNFs by increasing overall zeta potential measurements. The effect of Si-CNF on cement hydration was found to be dependent on the water to cement (w/c) ratio. Specifically, CNFs accelarated the early age cement hydration at 0.35 ratio and it retards the hydration at 0.45 w/c ratio. Such water dependent effect was attributed to the negatively charged hydroxyl and carboxyl surface sites of CNF which can bind alkali ions or cement particles. The increase of compressive strength due to the addition of Si-CNF was prominent at 0.35 w/c ratio compared to that of 0.45 w/c ratio. Addition of Si-CNF was found to increase the flexural strength and compressive strength of cement paste up to 52% and 22%, respectively. The main objective for 2nd phase of the study is to determine the optimum fine content percentages for PCNF and LCNF in portland cement pastes, such that mechanical grinding cost can be reduced at lower fine content percentage usage. The CNFs used in this study were obtained through the process of mechanical grinding of pulps, in which the fibers are broken down to fraction of small particles called ’fines’. The fine content percentages considered in this study are 60%, 75%, 80%, 90% and 95% for both PCNF and LCNF. LCNFs being colloidally semi-stable showed improvement in workability of cement pastes at 0.35 w/c ratio compared to PCNF. Maximum improvement in the compressive strength of the cement paste cube samples was achieved at 75% fine content percentages for both LCNF and PCNF. Addition of 75% fines content percentage of PCNF and LCNF was found to increase the flexural strength of the cement pastes up to 112% and 96%, respectively. Overall, based on the results obtained from this study, optimum fine content percentages for CNFs is considered to be around 60% to 75%. The main hypothesis assumed for investigating the durability of CNFs in alkaline pore solution of cement (APS) was, in the presence of high alkaline medium CNFs having high amorphous phase initially will be transformed into more crytalline phase due to the process of alkaline hydrolysis. Degradation of CNF films in APS were obtained using XRD, which represents the degree of crystallinity (crystallinity index, C.I) in CNF. Increased C.I by 86% for CNF indicates higher crystallinity in CNFs due to alkaline degradation of cellulose. Increase in intensity ratios of inter to intra molecular O-H bonds of cellulose is about 0.989 and 0.982 for PCNF and LCNF films respectively. Increase in intensity ratio can be related to transformation of amorphous phase in cellulose to crystalline phase through the process of alkaline degradation.

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