Development of novel building composites based on 1 hemp and multi-functional silica matrix 2

11 This study focuses on the development of novel bio-composites using a silica matrix that 12 provides dual functionality: as a hydrophobic surface treatment and as a binder for hemp-shiv. 13 The hydrophilic nature of hemp shiv, a plant based aggregate, results in composites having 14 poor interfacial adhesion, weak mechanical properties and long drying times. In this work, sol- 15 gel process has been utilised to manufacture durable low density hemp based composites. 16 Morphological characterisation by scanning electron microscopy (SEM) showed that hemp shiv 17 was embedded well in the matrix. Detailed chemical analysis using x-ray photoelectron 18 spectroscopy (XPS) and gas chromatography-mass spectrometry (GC-MS) indicate the 19 presence of water soluble and ethanol soluble extractives leached from the hemp shiv which 20 are incorporated into the silica matrix inducing the binding effect. The composites were water 21 resistant and showed good mechanical performance having the potential to develop novel 22 thermal insulation building materials.


Introduction 28
Bio-based materials have become increasingly popular for producing economical engineering 29 materials in the building and construction industry. Composites manufactured using the woody 30 core of the hemp plant (Cannabis Sativa L.) known as shiv have been adopted by the building 31 industry. Lightweight composites from hemp shiv possess excellent hygroscopic [1,2], thermal 32 [3,4] and biodegradable [5] properties.

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Hemp shiv has low density due to its high porosity and it tends to absorb large amounts of water 35 [6]. The hydrophilic nature of bio-based materials makes them incompatible with hydrophobic 36 thermoset/thermoplastic polymers [7]. On the other hand, since the shiv competes with the 37 binder for the available water, purely hydraulic binders like lime or cement cannot hydrate 38 completely, leading to a powdery inner core in the hemp-lime walls which is poorly bound [8].

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The issue of adhesion with hemp-lime has stimulated considerable investment in hemp-specific 40 lime based binders. The most recent generation of binders utilises high specific surface area 41 lime in order to obtain a more reactive binder, however, they are still susceptible to adhesion 42 issues. Pre-fabrication of panels or blocks ensures factory controlled conditions which reduce 43 the extremes of adhesion issues (e.g. extensive flouring), but there still remains the inherent 44 issue that the soluble sugars on the surface of the shiv interfere with the hydration of the 45 binders, resulting in lower strength composites [9]. The durability of the material is compromised 46 due to high moisture uptake as colonial fungal growth is encouraged resulting in cell wall 47 degradation [10].

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Extractives include numerous low molecular mass compounds such as fatty acids, waxes, 52 sterols, triglycerides, steryl esters, glycosides, fatty alcohols, terpenes, phenolics, simple 53 sugars, alkaloids, pectins, gums and essential oils. It is well known that extractives can be 54 isolated using polar and non-polar solvents. Volatile extractives are represented by highly 55 volatile compounds which can be separated by water distillation. They are mainly composed of 56 monoterpenes and other volatile terpenes including terpenoids as well as many different low of a hydrophobic silica treatment as a binder for hemp shiv to produce novel robust light weight 87 composites with enhanced water resistance.

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The surface elemental and chemical composition of the specimens were analysed using X-ray 121 photoelectron spectroscopy (XPS). Prior to XPS analysis, samples were oven-dried at 80 °C for 122 96 hours. XPS spectra of the samples were recorded with an X-ray photoelectron spectrometer 123 (Kratos Axis Ultra, UK). All spectra were collected using a monochromatic Al Kα X-ray source

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The specimens were removed from the moulds and transferred to a conditioning room at 19 °C 163 and 50% relative humidity. Another set of samples were prepared by mixing hemp shiv (75 164 vol%) and ethanol-water solution (25 vol%) and rest of the conditions were kept constant as

(A) (B) (C) (D) (E) (F) (G) (H)
In the present study, hemp shiv based composites have been manufactured by using silica sol 307 as a binder. The binding matrix has been characterised and its morphology, chemical 308 composition and physical properties have been studied in comparison with silica glass. The

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The silica is able to covalently bond to hemp shiv through the hydroxyl groups of cellulose [26].

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During the drying process, the gel starts condensing, releasing ethanol and water and develops 319 a silica network. The extracts from the shiv that are entrapped in the silica network alter the 320 characteristics of the silica. From the SEM analysis, it was seen that the silica morphology is 321 modified. The structure of the new modified silica with incorporated extracts is less brittle when 322 compared to the pure silica glass.

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The chemical composition of the silica specimens is mainly composed of carbon, oxygen and 325 silicon. Chemical characterisation using EDX reveals that the modified silica (binding matrix) 326 has a higher carbon content than the pure silica. Detailed XPS analysis indicates that due to sol 327 interaction with hemp shiv, the silica chemistry has been significantly altered.

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To analyse the extracts that were leaching out from hemp shiv during the silica based 340 treatment, the process was simplified by using a solution of ethanol and water for the extraction 341 process. Ethanol is able to dissolve waxes and isolate lipophilic extractives. These ethanol-342 soluble extractives were analysed using GCMS and it was found that the extract was mainly

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When compared to conventional hemp-lime composites, it is evident that the production costs of 383 the hemp-silica composites would be higher due to the hydrophobic treatment on hemp shiv.

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However, this cost could be off-set by savings elsewhere, both in production ingredients composite is expected to be better due to their significantly lower density than hemp-lime.

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Overall early indications are that the global warming potential of this composite would be 392 approximately 5% lower than that of a conventional composite. The life span is expected to 393 increase by 50% due the improved resistance to water that is responsible for degradation of the

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The authors declare that they have no conflict of interest.