5.1 Introduction
Chapter 5 presents a summary of the present study, the major conclusions from the conducted research.
The main objective of this study was to further the past research carried out in geopolymer concrete mix design and develop a scaling up process of work carried out by the Physics Department at Curtin University, in which they produced geopolymer pastes that set quicker under ambient conditions. By adding aggregate to these mixes and therefore producing a geopolymer concrete, several mix designs were tested by introducing different additives to the concrete. The two processes differed by more than just adding aggregate, as it was discovered that the longer handling time of the concrete restricted the effectiveness in adding calcium hydroxide to geopolymer concrete. It was also apparent how much water is stored within the aggregate, as no extra water needed to be added to the mix at all, due to how wet the aggregate was in Mixes Three to Seven.
In order to maintain a constant approach between each of the batches, mixing procedures, materials used and mix designs for the most part were kept constant.
5.2 Production of Geopolymer Concrete
5.2.1 Pre-production Issues
The most important work carried out before the mixing of the concrete was the preparation of the alkaline liquid. This liquid was a combination of a sodium hydroxide solution and sodium silicate. The sodium hydroxide solution was formed by dissolving pellets into distilled water under pre‐calculated proportions. Upon carrying out this
dissolution it was seen that the reaction carried out was exothermic, and heat was generated as the solid dissolved.
Sodium silicate was used as obtained through a local supplier. These two elements were combined at the beginning of the day of mixing and kept sealed until use. The sodium hydroxide solution though was able to be produced a few days prior to mixing so long as any precipitate formed in that standing time was re‐dissolved again before use. No super plasticisers were used in the laboratory work in this research.
Aggregates were not prepared prior to use to provide a realistic comparison to that of a larger scale in industry. The water content of the aggregates were taken, though, and noted what effect this content had on the final results.
5.3 Results and Observations
5.3.1 The Use of Silica Fume to Aid Ambient Curing
The addition of silica fume to geopolymer concrete produced a faster setting mix;
however it had a negative effect on the compressive and tensile strength. In this mix, silica fume was added at a quantity of 8.3% of the geopolymer paste as a replacement for fly ash. The 28 day compressive strength for Mix Two peaked at 12.8 MPa comparative to Mix One’s 30.0 MPa (Figure 4.8).
The addition of this silica fume to the concrete mix also caused a swelling of the cylinder, resulting in a porous expansion above the top of the mould upon setting as seen in Figures 4.6 and 4.7.
5.3.2 The Effect of Free Water Content on Geopolymer Concrete
It was seen that under ambient conditions in the middle months of the year in Western Australia, the curing of standard geopolymer concrete with no additives takes almost a
The effect of raising the free water content in geopolymer concrete was similar to that of ordinary concrete, reducing its strength. This was confirmed in Mix Four, where by doubling the free water content of Mix One, the 28 day strength resulted in one third of it with 10.8 MPa (Figure 4.10).
5.3.3 The Use of Calcium Hydroxide to Aid Ambient Curing
The addition of calcium hydroxide within a geopolymer concrete mix causes the concrete mix to set quicker. In the research carried out, the use of calcium hydroxide did not improve the compressive strength of the concrete despite it setting quicker.
It can be seen in Table 4.16 that increasing the amount of calcium hydroxide into a geopolymer concrete mix proportionally increases the compressive strength of the concrete mix. It was seen that an addition of 3% of the geopolymer of calcium hydroxide produced a concrete mix that set within 24 hours and exhibited a compressive strength extremely similar the standard reference mix (11.0 MPa). It was also seen that 0.5% and 1% of calcium hydroxide added in fact decreased the compressive strength of the mix (Figure 4.16).
Upon replacement of 5% of fly ash with calcium hydroxide in a geopolymer mix, the concrete flash set at approximately 10 minutes into placing the concrete into moulds.
It was also seen that the chemicals in the mix were furiously reacting after thorough mixing in of the calcium hydroxide. Though the strength of Mix Three (5% calcium hydroxide utilized) substantially higher than the reference strength, it was seen as a failed mix because of the rapid setting and therefore would not be applicable in large scale operations (Table 4.10).
5.3.4 Other Observations During Research
During this research, all ambient cured geopolymer concrete specimens developed a layer of efflorescence on the outside as seen in Figures 4.4 and 4.5. This efflorescence
is thought to be unreacted sodium hydroxide in a white crystalline form (Temuujin, van Riessen and Williams 2009). This however, did not occur in the experimental research carried out in the steam curing of geopolymer concrete specimens.