Enhancing the durability of natural plant fibres in cementitious composites with hydrothermal treatment and application of silica-based coatings

Abstract

The sustainable conservation and utilization of nature and its resources in the built environment can mitigate against global warming. This can be achieved by recycling and enhancing the durability of natural plant fibres in cementitious composites to reduce the carbon footprint in the environment. However, there is need to stabilize hydrophilic plant fibres that easily decompose in alkaline matrices by subjecting the fibres to treatments as well as replacing cement with supplementary cementitious materials (SCMs) to reduce the alkalinity of cementitious matrices. In this study, the influence of hydrothermal treatment cycles (HTTCs) on the behaviour of three vegetable plant (flax, hemp and palm) fibres was investigated. Then these macro plant fibres subjected to HTTCs and their untreated fibres as well as macro PVA fibres were incorporated in ternary Portland cement (PC) matrices of varying compositions (particularly with either fly ash or palm oil fuel ash) and characterized. Furthermore, soft (micro fibrillated pine) and hard (nano fibrillated eucalyptus) woods were reinforced in ternary PC matrices with fly ash to evaluate the effect of fibril size and content on flexural and fracture (computed using two displacements, crack mouth opening displacement (CMOD) and crack tip opening displacement (CTOD)) behaviour. After which, protective silica (Si)-based coatings applied on plant fibres were synthesized via the sol-gel process with two (nitric and acetic) acid catalysts used singly or in combination, two functionalizing hydrophobic (a short and a long carbon chained) silanes and varied molarities of water and acid catalysts; and subsequently evaluated. Lastly, the effect of hydrothermal treatment (HT) and applied Si-based coatings on hybrid sized fibre-cementitious composites (HSFCC) combining fibres of macro, micro and nano lengths in the short term, after accelerated ageing cycles (AACs) and natural weathering (NW) were assessed. The outputs indicated that HT enhanced plant fibres with mostly high cellulose and low ketones and organic matter contents. Flax and hemp fibres with high cellulose contents had increased tensile strength after HT which led to improved bridging ability and performance in fibre cementitious composites (FCC) than palm fibres with high organic matter content. These plant fibres with high cellulose contents can be used to replace synthetic PVA fibres in FCC, particularly after HT. The resistance to pull out offered by the matrices on fibres depends on the kind of SCMs used as cement replacement, with the most resistance when palm oil fuel ash is used compared to fly ash. CMOD rather than CTOD distinctively defined the contribution of nano eucalyptus fibrils in FCC with additional increase in content above 0.3% resulting to negligible changes in flexural and fracture properties. Hence, limiting the content of nano fibrils to 0.3%, while micro pine fibrils and macro plant/synthetic fibres were added at higher percentages in HSFCC leading to optimised strength and structural fibre-matrix bridging at various (nano to macro) levels. Regarding the protective Si-based sol-gel coatings, increasing the water molarity rather than reducing acid catalyst molarity enhanced the coating properties like water resistance and hydroxyl intensity. These optimised aqueous silica-based sols catalysed by a single acid or a combination of two, formed hydrophobic Si-based coatings on plant fibres with high water resistance when cured for about 5 min at temperatures of 80 or 105°C. Water droplets retained on coated fibres confirmed the hydrophobicity of the Si-based coatings which contains nanopores that allows the passage of water vapour in and out of the fibres. Cohesion and strain of freshly prepared HSFCC were enhanced with the addition of either Si-based coated plant fibre hybrids or uncoated PVA hybrids attributed to the interfacial friction between the fibre surface, applied Si-based coatings and hydration products formed. Increased cementitious hydration reactions occurred when pozzolans of larger surface areas were combined with pozzolans with smaller surface areas, particularly when there are available pores in those with larger surface areas that stores and releases moisture gradually during hydration. The addition of plant and synthetic fibre hybrids with or without Si-based coatings in pure PC matrices delayed hydration, altered the time of peak heat flow but increased the heat released after a day. In blended matrices, the addition of Si-based coated plant fibre hybrids decreased heat released except in ternary PC matrices with fly ash and palm oil fuel ash and the use of c4 coated hydrothermally treated (HTT) hemp fibre hybrids in quaternary PC matrices with silica fume, fly ash and palm oil fuel ash. An increased hydration of pure PC matrices with age led to reduced elastic moduli and hardness after indentation whereas quaternary PC matrices exhibited the highest hardness due to the larger sizes of unhydrated clinker in the matrices. The introduction of plant and synthetic fibre hybrids in HSFCC reduced the hardness and moduli further while enhancing carbonation with age, especially after AACs and NW. These results indicate that the use of hydrothermal treatment and silica-based coating on plant fibres made them comparable to synthetic fibres such as PVA when used in hybrids that combine lengths of various sizes as reinforcement in cementitious composites.

Date of Award	27 Mar 2024
Original language	English
Awarding Institution	University of Bath
Sponsors	Commonwealth Scholarships Commission
Supervisor	Kevin Paine (Supervisor) & Juliana Calabria-Holley (Supervisor)