On the mechanisms for improved strengths of carbon nanofiber-enriched mortars
Introduction
Various approaches have been used by researchers and industry to increase the strength and durability of concrete structures and/or to reduce the volumes and amount of natural resources required in the construction sector. Approaches such as: amending compositions of ordinary Portland cement (OPC) to change alite:belite ratios [1,2]; increasing the fineness of the cementitious materials; including the use of non-standard aggregates [3]; or using supplementary cementitious materials [4,5] all aim to increase durability. Other methods include the addition of different types of fibers [[6], [7], [8]] or nano-fillers (e.g., carbon nanotubes and carbon nanofibers) to enhance the resilience of concrete by improving flexural/tensile strength and strain capacity of concrete.
Carbon nanofibers (CNFs) are structural analogues of carbon nanotubes (CNTs), but generally of greater fiber lengths and aspect ratio. Together, CNTs and CNFs are among the strongest manmade materials known with tensile failure stresses ~11–63 GPa and elastic moduli approaching 1000 GPa [9]. Despite investigations by many researchers on the properties of CNT/CNF enriched cement pastes and mortars (e.g. [[10], [11], [12]]), little is known concerning the microstructural composition of CNF enriched mortars.
The purpose of this study was to investigate the use of an aqueous based carbon nanofiber admixture in mortar mixes to understand the mechanisms behind the measured changes in strength in CNF reinforced mortars, focussing specifically on the analysis of the hydration products and the development of the microstructure. At the present time, no consensus exists regarding how, or even if, nano-particles nucleate, accelerate and/or amend the hydration reactions of cement, hence the main research aim of this paper was to investigate these possible effects of CNFs on cement hydration reactions within a mortar matrix.
Section snippets
Materials
Australian Type GP (General Purpose) Ordinary Portland Cement (OPC) complying with AS 3972-2010 [13], with the chemical composition presented in Table 1, was used as the binder in this study. Reagent grade (≥95%) Ca(OH)2 and CaCO3 powders purchased from Sigma-Aldrich and pure monoclinic tricalcium silicate (passing #325 sieve) from CTL Group (Illinois, USA) were used as references (e.g., for comparison of Fourier-transform infrared spectroscopy, thermogravimetry, X-ray powder diffraction data).
Mechanical properties and impacts of carbon nanofibers addition
The measured average compressive strengths of the reference mortar and the mortar samples with 1 wt% and 10 wt% CNF-admixture contents at 28 days were 22.5 MPa, 31.8 MPa and 25.2 MPa, representing a ≈ 41% and ≈ 12% strength increase in compressive strength, respectively, for the L-CNF and H-CNF mortars over that of the reference sample. Strength improvements of CNF mortars are considered to be related to either (i) increased CSH nucleation by surfaces of the CNFs, (ii) bridging by the
Summary and conclusions
The main purpose of this study was to investigate the effects of addition of carbon nanofibers on the mechanical strength of mortars and to describe the underlying phenomena leading to the differences in performance. Based on the obtained results, it is evident that CNFs can improve strength if they are well-dispersed in the cementitious matrix and only a small amount of 1% admixture (by weight of cement) can enhance the strength significantly (~41%).
A combination of analytical techniques
CRediT authorship contribution statement
Andras Fehervari:Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization.Alastair James Neil MacLeod:Formal analysis, Investigation, Writing - review & editing.Estela Oliari Garcez:Formal analysis, Writing - review & editing.Laurie Aldridge:Formal analysis.Will P. Gates:Formal analysis, Writing - review & editing, Visualization, Supervision, Funding acquisition.Yunxia Yang:Investigation, Writing - review & editing.Frank Collins:Writing - review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors are thankful for the funding support of Eden Innovations Ltd. and the Australian Research Council (Linkage Project funding scheme LP150100832). The first author also would like to thank Mr. Andrew Johnson for manufacturing the sample crasher used in this study.
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