Effects of cellulose, hemicellulose, and lignin on the morphology and mechanical properties of metakaolin-based geopolymer

https://doi.org/10.1016/j.conbuildmat.2018.04.028Get rights and content

Highlights

Abstract

Natural fiber-reinforced geopolymer has attracted wide attention in construction and building materials due to its low cost, low density, and excellent mechanical properties. Cellulose, hemicellulose, and lignin are the three basic components of natural fibres, and were investigated to reveal their influence on the metakaolin-based geopolymer. Comparative evaluations were investigated via morphology analysis and mechanical strength analysis. The results showed distinct microstructures and mechanical properties of the geopolymer-based materials with cellulose, hemicellulose, and lignin, respectively. A low content (5 wt%) of lignin, cellulose, and hemicellulose enhanced the flexural and compressive strength of pure geopolymer. Higher lignin and hemicellulose led to the porous morphology, lower density, and brittle fractures of geopolymer-based composites, which reduced the flexural and compressive strength in these geopolymer-based composites. It was noted that the degree of geopolymerization was clearly lowered by the alkaline degradation of hemicellulose. With the increase in cellulose content, in contrast, the denser structure and fewer pores of the geopolymer matrix were detected, as well as ductile failures of geopolymer-based composites. Good bonding was also shown between the geopolymer matrix and cellulose fibres without remarkable degradation. The results of this study will facilitate a better understanding of the effect of lignocellulosic biomass in natural fibre-reinforced geopolymers and should serve as the basis for further research and applications.

Introduction

Geopolymer, by reduction of CO2 emissions, has emerged as an environmentally friendly alternative to Portland cement thanks to its high compressive strength, low shrinkage and creep abilities, excellent inflammability, and exemplary durability [1], [2], [3], [4], [5]. Thus, geopolymer can be used in a wide range of applications, such as civil infrastructure (roads and railways), sustainable construction and building materials, and specific industrial applications (radioactive immobilization and contaminant encapsulation) [6], [7], [8], [9], [10]. However, like most ceramics, pure geopolymer suffers from brittleness problems, which can be overcome with fibre reinforcement in the geopolymer matrix [11], [12].

Growing attention to environment and climate change has piqued people's interest in natural material production and environmental protection materials. It is generally recognized that natural fibres derived from wood, bamboo, cotton, flax, etc. [12], [13], [14], [15] have been widely applied to reinforce the geopolymer matrix or to produce lignocellulosic biomass-based materials with geopolymer binders due to these fibres’ advantages with regard to natural abundance, recyclability, low cost, low density, excellent mechanical properties, and nontoxicity [16], [17].

Many previous studies dealt with natural fibres as a unit. In fact, natural fibre, in terms of chemistry, is a biopolymer composite composed mainly of a network of cellulose (40–60 wt%), hemicelluloses (20–40 wt%), and lignin (10–25 wt%) [18], [19]. The content of these three chemical components in several natural fibres [20], [21], [22], [23] is listed in Table 1. As shown, cellulose, hemicellulose, and lignin are the major components of natural fibres (72.3–99.5 wt%), albeit in quite different lignocellulosic biomass from one species to another. If the different effects of cellulose, hemicellulose, and lignin on geopolymer can be found, it will be possible to better understand the bonding mechanism between natural fibres and geopolymer, and to choose which natural fibre is best suited to the development of natural fibre-reinforced geopolymer composites.

The objective of this work was to investigate the influence of the common chemical composition of natural fibres, especially cellulose, hemicellulose, and lignin, on geopolymer properties. Comparative evaluations were investigated via morphology analysis and mechanical strength (flexural and compressive strength) analysis. The results of this study will facilitate improved understanding of the lignocellulosic biomass in natural fibre-reinforced geopolymers to serve as the basis for further research and applications.

Section snippets

Materials

High purity grade cellulose fibres (medium), hemicellulose (xylan), and lignin were supplied by Sigma (St Louis, MO, USA). A commercial metakaolin (MK) (Metamax®, BASF, Germany) was used as the aluminosilicate source. The chemical compositions of MK were measured by X-ray fluorescence (XRF) (XRF-1800, Shimadzu, Japan), the results of which are presented in Table 2. The particle size distribution (PSD) of MK (dispersed in water) was assessed with a Mastersizer 2000 particle size laser analyzer

FTIR analysis

The FTIR spectra of the geopolymer-based composites are displayed in Fig. 1. The wide vibration bands around 3500 cm–1 and 1652 cm–1 exhibited in all spectra were attributed to O–H stretching and bending, respectively [24], [25], [26]. The intensity of the peaks at 451 cm–1 was associated with Si-O-Si bending vibration [27]. Another intense band was centered around 1020 cm–1, corresponding to the Si-O-Al and Si-O-Si vibration bands of the geopolymer. In addition, the chemical composition of

Conclusions

In this study, geopolymer-based composites were produced to investigate the influence of the cellulose, hemicellulose, and lignin of natural fibres on the morphology and mechanical strength of metakaolin-based geopolymer. The conclusions can be drawn as following:

  • (1)

    A low content (5 wt%) of lignin, cellulose, and hemicellulose enhanced the flexural and compressive strength of pure geopolymer.

  • (2)

    Increasing hemicellulose content clearly lowered the degree of geopolymerization due to the alkaline

Conflict of Interest

There is no conflict of interest.

Acknowledgements

The authors are grateful for the financial support from the Fundamental Research Funds for the Central Universities [grant numbers 2017ZY28, 2016ZCQ01].

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