Development of geopolymer bonded wood composites

Thesis (PhD)--Stellenbosch University, 2021.

ENGLISH ABSTRACT: In the wake of finding alternative sustainable and environmentally friendly products to conventional construction materials, geopolymers offer large potential as a low carbon footprint material. Their excellent properties and the ability to be synthesized from industrial waste make them promising alternative binder sin wood-based composites where durability, environmental sustainability, structural integrity,and low cost of final products are of utmost importance. This study investigated the application of unary and binary precursor based geopolymer sin the development of composite products for use in outdoor conditions. The unary geopolymer is based on 100% ground granulated blast slag,while the binary precursor is composed of 75% class F fly ash and 25% metakaolin. The precursors were activated with a combination of sodium hydroxide and sodium silicate solutions formulated at a weight ratio of 1:2.5. The lignocellulosic materials used include sugarcane bagasse(Saccharum officinarum) and forest biomass waste from the clearing of locally occurring invasive alien species including Long-leaved wattle (A. longifolia), Black wattle (A. mearnsii) and Port Jackson (A. saligna). The production process involved using a mixed factorial experimental design. The variables considered included precursor-activator ratio (PA), curing pattern (CP), amount of lignocellulosic material (LM) and alkali concentration (MCon). For the unary system, the variables were CP, LM and MCon. PA and CP were considered at 2 levels, while LM and MCon were considered at 3 levels. The effects of the main factors and their interactions on the observed composite properties were evaluated using analysis of variance (ANOVA). The boards have comparable physical properties to cement-bonded particleboard according to the EN 632-2: 2007 standard. However, for the unary system only A. saligna boards produced with 6M NaOH and cured at 40°C for 24 h met the mechanical strength requirements, while in the binary system, only A. longifolia boards produced with 12M NaOH, PA ratio of 2:1 and cured at 100°C for 6h met the mechanical strength requirements. The boards were also thermally stable as the residues retained at the end of thermal analysis was above 70%. setting, the degree of geopolymeric reaction was impeded.The lignocellulosic materials were subjected to alkalization, acetylation, and hot water extraction to remove the lower molecular components,which could impede geopolymerization kinetics and enhance their surface characteristics. This was aimed at improving the durability of LM in the matrix and the overallproperties of the boards. The influence of each treatment on the lignocellulosic materials was evaluated using HPLC, SEM and FTIR, while the resulting boards were tested to specification and characterized using SEM and FTIR. The treatments improved the surface characteristics of the fibres and the fibre yield was not impacted significantly. FTIRindicated formation of more geopolymer products after fibre treatment, which was confirmed by SEM micrographs. The treated samples exhibited a compact and densely populated gel-like amorphous microstructure with fewer unreacted precursor particles.In the unary system, the mean modulus of rupture (MOR) increased by 3.25% for hot water extracted,23.61% for acetylated and23.94 % for alkalized AM boards. In the binary system, the mean MOR increased by 18.31% for hot water extracted, 6.03% for acetylated and 18.22% for alkalized AM boards. The study concluded that South African woody invasive plants (IPs)and sugarcane bagasse are suitableto produce both unary-and binary precursor-based geopolymer wood composites of comparable properties to cement-bonded particle boards.

AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming

Doctoral