Development and Testing of Geopolymer Mortar
3.2 Experimental Investigation
3.3.1. Setting time
Chapter 3 Development and Testing of Geopolymer Mortar 3.3 Experimental Observations
The effect of the addition of FA to the mixes containing only NaOH as alkali activator can be observed from Fig 3.13 and 3.14. Similar trends in the results were observed in mixes with a/b ratios 0.60 and 0.65. The addition of FA retarded the setting time of the mixes.
Gradual increase in setting time occurred with the increase in FA content for both a/b ratios.
This phenomenon is attributed to the fact that unlike UGGBS, FA cannot form geopolymerisation products instantly at early stages when subjected to curing at ambient temperature [106 – 107], thereby delaying the setting of the mixes. Moreover, since FA particles are spherical shaped as seen in Fig. 3.11. Hence, its addition allowed better mobility of particles in the mixes which eventually contributed to delay of setting time. However, the increase was more pronounced in mixes with FA content of 30 % and above, compared to those with FA content of 20 %.
Figure 3.13: Effect of FA content on setting times of GPM for a/b of 0.6.
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Setting time (in minutes)
Mix designation
Final Setting Intial Setting
GM6 GM9 GM10 GM11 GM12
Chapter 3 Development and Testing of Geopolymer Mortar
Figure 3.14: Effect of FA content on setting times of GPM for a/b of 0.65.
Addition of SP (both SN and PE based) to the mixes at constant FA content contributed to the retardation of early setting by preventing the geopolymerisation products from formation of bonds at early period, Fig. 3.15. The reason for this is similar to that of addition of FA in the mixes which provide better mobility to particles by producing less viscous mixes and thus retarding the setting times. In fact, the accelerating capacity of SP which is otherwise shown in decreasing the setting times of PC based systems is not shown in these geopolymeric mixes. Till 1.5 % SP dosage, the retarding effect was better exhibited by PE based SP compared to the SN based ones. In contrast to this, at SP dosage of 3 % high retarding effect was observed due to SN based SP in the mixes. It is presumed that addition of 3 % SN based SP caused new chemical reaction with NaOH, rendering it ineffective in carrying out the geopolymerisation process for instant formation of geopolymeric products. However, no such definite chemical reaction have been identified so far. But the sudden change of appearance of the dark greyish mortar to light brownish colour (see Fig. 3.16) and exhibition of segregation of particles give an impression of possible chemical reaction within the mix at SP content of 3 %. At lower dosage, however no such colour change could be observed. The phenomenon may indicate absence of chemical reaction at lower dosage. Laskar and Bhattacharjee [110] while carrying out study on geopolymer concrete reported that with
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Setting time (in minutes)
Mix designation
Final Setting Intial Setting
GM8 GM13 GM14 GM15 GM16
increased dosage of SP, segregation occurs forming a dense lower phase and a creamy upper phase within the mix. Criado et al. [113] observed similar changes due to addition of SP in the geopolymeric mixes.
Figure 3.15: Effect of SP content and type on setting times of GPM.
Figure 3.16 (a): Appearance of fresh geopolymer mortar, GM 17 which consisted of 0.5 % SN based SP.
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Setting time (in minutes)
Mix designation
Final Setting Intial Setting
GM10 GM17 GM18 GM19 GM20 GM21 GM22
Chapter 3 Development and Testing of Geopolymer Mortar
Figure 3.16 (b): Appearance of fresh geopolymer mortar, GM 19 which consisted of 1.5 % SN based SP.
Figure 3.16 (c): Appearance of fresh geopolymer mortar, GM 21 which consisted of 3.0 % SN based SP.
Figure 3.16 (d): Appearance of fresh geopolymer mortar, GM 22 which consisted of 3.0 % PE based SP.
Fig. 3.17 indicates that the change in alkali concentration changed the effect of SP on the setting behaviour of mixes. Increase in alkali concentration at constant FA content of 30 % and SP ratio of 1.5 % (both SN and PE based) decreased the setting time indicating the fact that increase in alkali concentration increased the rate of formation of bonds between the geopolymerisation products in the mixes. The trend that is observed by other researchers in geopolymeric systems without SP [108, 111], is also followed by these mixes containing SP.
Drop in setting times for mixes with 10 M alkali activator was very less compared to that for mixes with 12 M and 14 M alkali activator when mixes with 8 M alkali activator were considered as reference. This was exhibited by mixes with both SN and PE based SP.
Chapter 3 Development and Testing of Geopolymer Mortar
Figure 3.17: Effect of alkali activator concentration on setting times of GPM.
The time of SP addition also influenced the setting time of the mixes, Fig. 3.18. Setting retardation was highest in case of Type I SP addition. However, the difference in the setting times from the mixes with other mixing types is nominal. In all, PE based SP with Type I addition outperformed others in creating retarding effect in the mixes.
Figure 3.18: Effect of SP addition type on setting times of GPM.
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Setting Time (in minutes)
Mix designation
Final Setting Intial Setting
GM23 GM24 GM19 GM20 GM25 GM26 GM27 GM28
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Setting Time (in minutes)
Mix designation
Final Setting Intial Setting
GM19 GM20 GM29 GM30 GM31 GM32
The setting time of GPM was found to be much lower than that of PCM. Initial setting time of PCM mix, CM1 was 95 minutes and final setting time was 265 minutes. In some of the GPM mixes, the initial setting time was found to be lesser than that recommended in the IS 8112 2013 [114]. However, addition of admixture and alteration of alkali activator concentration led to enhanced initial setting time. The final setting time was significantly less than the maximum values in all the mixes which makes GPM a strong contender for recognizing it as a concrete repairing agent.