On the mechanisms for improved strengths of carbon nanofiber-enriched mortars

doi:10.1016/j.cemconres.2020.106178

Cement and Concrete Research

Volume 136, October 2020, 106178

https://doi.org/10.1016/j.cemconres.2020.106178 Get rights and content

Highlights

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Carbon nanofibers do not promote formation of additional calcium silicate hydrates.
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CNFs delay the hydration of alite and shift it towards the formation of portlandite.
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Carbon nanofibers foster formation of calcium carbonate phases.
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Carbon nanofiber mortars have lower total porosity with greater micropore volume.

Abstract

As nanomaterials, carbon nano-fillers have manyfold benefits in concrete applications, and while of considerable research interest, their impacts on cement phases are still not fully understood. We applied multiple techniques, including mercury intrusion porosimetry, X-ray diffraction, thermal gravimetry, infrared spectroscopy and electron microscopy to investigate in detail the effects of a pre-mixed carbon nanofiber admixture in mortars. The addition of carbon nanofibers was observed to: (i) alter pore structure to a lower total porosity with greater proportion of micropores, (ii) delay the hydration reaction of alite, (iii) shift hydration of alite towards the formation of portlandite and (iv) foster formation of calcium carbonate phases. The ensemble of results indicate that improved performance is unlikely to be due to formation of additional calcium silicate hydrates, as has been suggested in the literature, but instead is due to the combined effect of nano-fiber bridging and pore-filling by portlandite and calcium carbonates.

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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On the mechanisms for improved strengths of carbon nanofiber-enriched mortars

Highlights

Abstract

Introduction

Section snippets

Materials

Mechanical properties and impacts of carbon nanofibers addition

Summary and conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgement

Cem. Concr. Res.

Composites Part A

Constr. Build. Mater.

Cem. Concr. Res.

Cem. Concr. Compos.

Constr. Build. Mater.

Cem. Concr. Res.

Powder Technol.

Cem. Concr. Res.

Cem. Concr. Res.

Compos. Sci. Technol.

Constr. Build. Mater.

Cryst.-Chem. Appl. Clay Sci.

Cem. Concr. Res.

Compos. Sci. Technol.

Cem. Concr. Compos.

Cem. Concr. Compos.

Constr. Build. Mater.

Constr. Build. Mater.

Cem. Concr. Res.

Geochim. Cosmochim.

Cem. Concr. Res.

J. CO2 Util

High belite cement from alternative raw materials

Mater. De Construc.

Hydration of high belite cement

Analytical review of the mix design of fiber reinforced high strength self-compacting concrete

J. Build. Eng.

Optimizing the use of fly ash in concrete

Fiber-Reinforced Cements and Concretes

Fiber-reinforced concrete with application in civil engineering

Adv. Civ. Eng.

Development of high strength self-compacting fibre reinforced concrete for prefabricated concrete industry

Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load

Sci

Carbon nanotube–cement composites: a retrospect

IES Journ. Part A, Civ. & Struc. Eng.

General Purpose and Blended Cements

PA, ASTM C305 Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency, C305 - 14

PA ASTM C109/C109M Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimens), C109 - 16a

Methods of Testing Concrete - Method 9: Compressive Strength Tests-Concrete, Mortar and Grout Specimens

Model structures for C-(A)-S-H(I)

Acta Cryst

The dynamics of capillary flow

Phys. Rev.

Scanning electron microscopy and nanoindentation analysis of modified multi-walled carbon nanotubes/cement composites

J. Adv. Microsc. Res.

Mechanical properties and durability of ultra high strength concrete incorporating multi-walled carbon nanotubes

Mater. Struct.

Potential of carbon nanotube reinforced cement composites as concrete repair material

J. Nanomater.

Effect of multi-walled carbon nanotubes (MWCNTs) on the strength development of cementitious materials

J. Mater. Res. Technol.

Reinforcing effect of carbon nanotubes/surfactant dispersions in Portland cement pastes

Adv. Civ. Eng.

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