Cell wall microstructure, pore size distribution and absolute density of hemp shiv.

Y Jiang,M Lawrence,M P Ansell,A Hussain

doi:10.1098/rsos.171945

Abstract

This paper, for the first time, fully characterizes the intrinsic physical parameters of hemp shiv including cell wall microstructure, pore size distribution and absolute density. Scanning electron microscopy revealed microstructural features similar to hardwoods. Confocal microscopy revealed three major layers in the cell wall: middle lamella, primary cell wall and secondary cell wall. Computed tomography improved the visualization of pore shape and pore connectivity in three dimensions. Mercury intrusion porosimetry (MIP) showed that the average accessible porosity was 76.67 ± 2.03% and pore size classes could be distinguished into micropores (3–10 nm) and macropores (0.1–1 µm and 20–80 µm). The absolute density was evaluated by helium pycnometry, MIP and Archimedes' methods. The results show that these methods can lead to misinterpretation of absolute density. The MIP method showed a realistic absolute density (1.45 g cm−3) consistent with the density of the known constituents, including lignin, cellulose and hemi-cellulose. However, helium pycnometry and Archimedes’ methods gave falsely low values owing to 10% of the volume being inaccessible pores, which require sample pretreatment in order to be filled by liquid or gas. This indicates that the determination of the cell wall density is strongly dependent on sample geometry and preparation.

Highlights

Bio-based insulation materials, such as hemp, flax and wheat straw, offer a number of benefits in comparison with more established mineral and oil-based alternatives, such as mineral wool and PUR-based products [1,2]
For measurements performed in a dry state, the hemp shiv was dried in an oven at 60°C until a constant mass was reached, it was cooled to ambient temperature in a sealed container
It can be seen that the stem cross section has an indented shape, similar to a four-leaf clover, which enhances the rigidity of the stem

Summary

Introduction

Bio-based insulation materials, such as hemp, flax and wheat straw, offer a number of benefits in comparison with more established mineral and oil-based alternatives, such as mineral wool and PUR (polyurethane rigid foam)-based products [1,2]. Apart from timber, hemp-lime is perhaps the most researched bio-based building material because it is a cheap and low density material (bulk density: 0.08–0.16 g cm−3) with associated. Hemp- 2 lime (often referred to a ‘hempcrete’) is a composite material made up of the woody core of the hemp stalk (shiv) used as an aggregate in a lime-based binder. The binders are typically based on hydraulic lime but specially formulated to avoid the inhibition of hydration caused by the sugars present in the hemp stalk

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