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Title: Durability of reinforced GGBS/FA geopolymer concretes
Authors: Nguyen, Thanh T.
Keywords: Geopolymer concrete
Fly ash (FA)
Slag (GGBS)
Issue Date: 2017
Publisher: © Thanh T. Nguyen
Abstract: Geopolymers are a subset of alkali-activated materials, synthesised from by-products of low-calcium aluminosilicates such as fly ash (FA) and metakaolin (MK). These clinker-free binders have been developed as a potential partial replacement of CEM I (ordinary Portland cement) in order to help reduce the environmental footprint of concrete. Geopolymer concretes (GCs) can also provide mechanical properties comparable to those of CEM I concretes. The inclusion of ground granulated blast-furnace slag (GGBS) offers benefits in improving the hardening process of the concrete at ambient temperature and reducing the permeability of the binder. However, there have been a limited number of studies on the durability of FA-based GC for structural applications, in particular GGBS/FA blended GCs with up to 50 % GGBS. The current study hence aimed to investigate the durability properties for the structural construction of GCs optimised for the compressive strength. It is also essential to improve understanding regarding the fresh state of GC in order to develop effective preparation and casting processes. The factors influencing the compressive strength and consistency of FA-based GCs were investigated, including varying the constituents, the initial curing temperature and the additional GGBS. The relationships between the compressive strength and water content of both blended GCs with GGBS/FA ratios of 0/100, 20/80, 35/65 and 50/50, and the initial curing temperatures of 10, 20, 30 and 75 oC, as well as CEM I concretes as references were evaluated in order to identify the mix designs targeted at 35 MPa and 60 MPa. The durability properties of these concretes were then investigated at 91 days using several selected testing methods standardised for analysis of CEM I concretes including water permeability, rapid chloride migration, concrete resistivity, carbonation and steel reinforcement corrosion. An effective combination coating process for the steel mould using Z30 Fluid and Waxoyl was developed. The consistency of GCs increases with the water content, but decreases with the increase of GGBS proportion. The flow table is recommended to indicate the consistency of GCs (compared to the slump) because the flow diameter distributed equally and linearly over an appropriate range from a low to medium workability, as classified for CEM I. Prolonged curing at 10 oC for 28 days is sufficient for blended GCs with at least 20 % GGBS to achieve a compressive strength of 33 MPa. The presence of GGBS increases the compressive strength of blended GCs cured at either 20 or 75 oC to above 80 MPa. Whereas, the optimal mix design of the FA-based GC with an alkaline to binder ratio of 0.3 can be obtained when using the total aggregate to GC ratio of 0.77; fine aggregate to total aggregate ratio of 0.35; sodium hydroxide molarity of 12 M; sodium silicate to sodium hydroxide ratio of 2.5; superplasticizer to binder ratio of 1.5 %; and initial steam curing temperature of 75 oC for 24 h. However, an alkaline to binder ratio of 0.6 exhibited the maximum 7-day compressive strength of 82 MPa for the FA-based GC. The compressive strengths of the GCs decrease with the increased water content and become more sensitive at the higher water to solid ratios, on which the effect of increasing temperatures from 10 to 30 oC becomes insignificant. Moreover, the compressive strength Durability of reinforced GGBS/FA geopolymer concretes vi development of the GCs shows an insignificant increase at the age of 91 days, except for 35 % GGBS specimens and cured at 20 oC or higher with a slight decrease. In general, increasing the compressive strength of GCs from 35 MPa to 60 MPa reduced the durability properties by factors of 1.5 to 2, except for the porosity (VPV) with a slight decrease. According to the VPV results, the porosity of the GCs is comparable to that of the equivalent CEM I concretes regardless of the curing temperature and GGBS content. Meanwhile, the GCs cured at more ambient temperatures seem to have either a coarser or more interconnected pore structure leading to a higher sorptivity coefficient than the CEM I counterparts by a factor of 2 to 5. However, at the elevated curing temperatures of 75 oC, the sorptivity of GCs became comparable with the CEM I concretes. Besides, the inclusion of GGBS up to 50 % shows inconclusive influences on the porosity and pore interconnectivity but it hinders the chloride migration coefficient. The time to failure inferred from the current and time relationship monitored in the accelerated corrosion test takes up to 5 times longer for the GCs than the CEM I equivalents. Conversely, the GGBS/FA blended GCs appear to be more susceptible to carbonation, even in the natural atmospheric condition. Moreover, 1% CO2 concentration seems to be aggressive to FA-based GC cured at 75 oC with a carbonated depth of more than 5 cm after 54 days, whereas in the natural condition, the carbonation depths of these samples were very low and similar to that of the high strength CEM I concretes after 147 days. It can be concluded that care must be taken when using the GCs with 50 % GGBS and cured at more ambient curing temperatures for the structural concrete, in particular in aggressive environments containing extensive chlorides and/or carbon dioxide, in which an elevated curing temperature of 75 oC is recommended.
Description: This Thesis is restricted until 02/07/2020. A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
Sponsor: Loughborough University, School of Civil and Building Engineering.
URI: https://dspace.lboro.ac.uk/2134/25776
Appears in Collections:Closed Access PhD Theses (Civil and Building Engineering)

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