Mechanical properties of fly ash/slag based geopolymer concrete with the addition of macro fibres

[摘要] ENGLISH ABSTRACT: Geopolymer concrete is an alternative construction material that has comparable mechanicalproperties to that of ordinary Portland cement concrete, consisting of an aluminosilicate andan alkali solution. Fly ash based geopolymer concrete hardens through a process calledgeopolymerisation. This hardening process requires heat activation of temperatures aboveambient. Thus, fly ash based geopolymer concrete will be an inadequate construction materialfor in-situ casting, as heat curing will be uneconomical.The study investigated fly ash/slag based geopolymer concrete. When slag is added to thematrix, curing at ambient temperatures is possible due to calcium silicate hydrates that formin conjunction with the geopolymeric gel.The main goal of the study is to obtain a better understanding of the mechanical properties ofgeopolymer concrete, cured at ambient temperatures. A significant number of mix variationswere carried out to investigate the influence that the various parameters, present in the matrix,have on the compressive strength of fly ash/slag based geopolymer concrete. Promisingresults were found, as strengths as high as 72 MPa were obtained. The sodium hydroxidesolution, the slag content and the amount of additional water in the matrix had the biggestinfluence on the compressive strength of the fly ash/slag based geopolymer concrete.The modulus of the elasticity of fly ash/slag based geopolymer concrete did not yieldpromising results as the majority of the specimens, regardless of the compressive strength,yielded a stiffness of less than 20 GPa. This is problematic from a structural point of view asthis will result in large deflections of elements. The sodium hydroxide solution had the mostsignificant influence on the elastic modulus of the geopolymer concrete.Steel and polypropylene fibres were added to a high- and low strength geopolymer concretematrix to investigate the ductility improvement. The limit of proportionality mainly dependedon the compressive strength of the geopolymer concrete, while the amount of fibres increasedthe energy absorption of the concrete. A similar strength OPC concrete mix was compared tothe low strength geopolymer concrete and it was found that the OPC concrete specimenyielded slightly better flexural behaviour. Fibre pull-out tests were also conducted toinvestigate the fibre-matrix interface.From the knowledge gained during this study, it can be concluded that the use of fly ash/slagbased geopolymer concrete, as an alternative binder material, is still some time away as thereare many complications that need to be dealt with, especially the low modulus of elasticity.However, fly ash/slag based geopolymer concrete does have potential if these complicationscan be addressed.

[发布机构] Stellenbosch University