Shohana Iffat

Date of Award

Summer 2022

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Fabio Matta

Abstract

Oxidized graphitic nanoparticles such as multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs), can produce enhancements in physicomechanical properties of cement composites that are relevant to structural and durability performance, provided that said nanoparticles are well dispersed in the composite. Lower-bound MWCNT concentrations are reported in the literature in the range 0.01-0.05% in weight of cement (wt%). Such concentrations may not be costeffective for practical applications and may also be excessive since they would result in a nanoparticle surface area at or above 105 m2 for 1 m3 of cement paste, or 104.5 m2 for 1 m 3 of concrete, which may be large enough to facilitate nanoparticle agglomeration in cement matrices. These agglomerates may contribute to the inhomogeneity of the cement matrix and introduce defect sites that negatively contribute to mechanical properties associated with strength, stiffness, and toughness.

In this research, low concentrations of MWCNTs in the range 0.001-0.05 wt% were explored as amendments in cement paste, thus one order of magnitude smaller than lower-bound concentrations reported in the literature. It was found that such decrease in MWCNT concentration continues to provide significant enhancement to compressive strength and stiffness compared to plain cement paste. In fact, neither significant enhancements in the 28-day compressive strength and stiffness, nor noticeable changes in the nano- and micro-structure, were observed by increasing the MWCNT concentration by 50 times, from 0.001 wt% to 0.05 wt%. Instead, as originally hypothesized, MWCNT agglomerates were consistently observed in the cement matrix when using a 0.05 wt% MWCNT concentration.

The results above highlighted the merits of exploring more effective means of leveraging the specific surface area (SSA) of graphitic nanoparticles by considering alternative morphologies, in addition to truly low concentrations. To this end, partially unzipped carbon nanotubes (PUCNTs) are introduced in this research as nanoamendments for cement composites. Oxidized PUCNTs are of interest because they combine the aspect ratio and mechanical strength of MWCNTs, and the high graphene edge content and dispersibility of GNPs. In addition, PUCNTs offer a greater specific surface area compared to MWCNTs, which makes them suitable for exploring reduced concentrations. In this part of the research, truly low concentrations of PUCNTs in the range 0.001-0.05 wt% were utilized in cement paste. It was found that reducing the PUCNT concentration by one order of magnitude, from 0.05 wt% to 0.005 wt%, led to significantly enhanced nano- and micro-structure as well as compressive strength and elastic stiffness. These results were supported by SEM micrographs, which consistently showed PUCNT agglomerates for a concentration of 0.05 wt%; instead, for a concentration of 0.005 wt%, well dispersed PUCNTs were consistently observed, together with preferential and accelerated formation of C-S-H. This evidence agrees with dynamic light scattering test results on PUCNT suspensions where hydrodynamic size and zeta potential values indicate less dispersibility of PUCNTs at concentrations equivalent to 0.05 wt% in the cement paste manufactured with these suspensions.

The research into more effective graphitic particle morphologies continued by investigating graphene oxide nanoribbons (GONRs), which are obtained by fully unzipping MWCNTs. GONRs combine the properties of carbon nanotubes and graphene, i.e., high surface area, graphene edge content, and aspect ratio. In fact, GONRs are easy to disperse in stable aqueous suspensions without compromising the sp2 basal plane to defects. Due to their edge content resulting from oxidation, GONRs have higher contents –COOH edge groups compared to MWCNTs and GNPs, and to a lesser extent PUCNTs. A dispersibility study in aqueous solution was conducted using GONRs with different oxidation level (i.e., oxygen functional group content), and in different concentrations. Uniform GONR dispersions were quantitatively verified through DLS testing, based on hydrodynamic size and zeta potential measurements, for concentrations in the range 0.0125-1.25 g/L (equivalent to 0.0005 to 0.05 wt% of cement in concrete with water-to-cement ratio of 0.4), and for oxygen content in the range ~30-40 wt% for up to 7 days after initial sonication. These dispersed and stable aqueous suspensions of GONRs are suitable to explore novel graphitic-nanoamendment solutions for cement composites, including actual concrete, using more rational and practical concentrations.

Rights

© 2022, Shohana Iffat

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