PSI - Issue 64

Hamzeh Shdeifat et al. / Procedia Structural Integrity 64 (2024) 1360–1368 Shdeifat et al. / Structural Integrity Procedia 00 (2019) 000–000

1365

6

Table 3. Results of compressive strength and bulk densities

Relative compressive strength (%)

Compressive strength (MPa)

Bulk Density (kg/m 3 ) 1787.37 1743.90 1721.65 1721.65

Mix ID

R

1H

2H

1H

2H

KC20R1.2 KC14R1.2 KC14R.6

41.07 32.32 51.68 51.68

34.56

44.54 51.08 23.72 19.22

84.16% 86.63% 42.11% 37.89%

108.46% 158.04% 45.9% 37.20%

28

21.76 19.58

KC14R.6(TS)

As displayed in Fig. 5, both mixes KC20R1.2 and KC14R.12 exhibited significantly higher post-fired compressive strength when compared to mix KC14.R.6. A clear correlation between the compressive strength at ambient temperature and the residual strength, suggesting that a more compact microstructure leads to greater reduction in residual strength. It is reported that a compact microstructures is highly susceptible to explosive spalling, as vapour pressure is unable to escape the compact microstructure without breaking its way through (Peng et al., 2018).

0 10 20 30 40 50 60

10% 15%

10%

0% 5%

0.3%

-15% -10% -5%

-2%

-7.34% -7.90%

-8% -7.59% -8.39%

Change in volume (%)

Compressive Strength (MPa)

KC20-R1.2 KC14R1.2 KC14R.6 KC14R.6 (TS)

KC20R1.2 KC14R1.2 KC14R.6 KC14R.6 (TS)

1H 2H

R 1H 2H

Fig. 5. Compressive strength of FA geopolymer

Fig. 6. Volume change of FA geopolymers

It is noted that the post-fired strength was much higher than reference strength for both mixes KC20R1.2 and KC14R.12. This remarkable increase in strength is attributed to the densification process that the geopolymer undergo at elevated temperatures. During this process, the density of the geopolymer gel increases, resulting in a more rigid microstructure (Lahoti et al., 2018, Bakharev, 2006). This is in a good agreement with the volume change values in Fig. 6. The trend indicates that the driving factor behind the increase in residual compressive strength is shrinkage. However, mix KC14R.6 recorded high shrinkage yet low residual compressive strength. This is because the mix was subjected to explosive spalling at early firing period which resulted in permanent damage, preventing the compressive strength to increase when shrinking. Expansive behaviour was observed predominantly in mix KC20R1.2 during the first hour of firing. In contrast to shrinkage, expansion tend to reduce the density of the microstructure, leading to reduction in compressive strength. The expansion at first hour of firing is attributed to the high activator content and oxides ratio which resulted in the presence of unreacted or partially reacted silicates in the matrix. These silicates swell at elevated temperatures, causing expansion in the formation (Provis et al., 2009, Abdulkareem et al., 2014). Introducing a geopolymer adhesive with an expansive behaviour at elevated temperatures in NSM CFRP application can be promising as the expansion will most likely result in further friction forces between the adhesive and concrete substrate, increasing the bond strength. On the other hand, the shrinkage at 2 nd hour after the expansion