PSI - Issue 67

5

Hwan Lee et al. / Procedia Structural Integrity 67 (2025) 107–114 Author name / Structural Integrity Procedia 00 (2024) 000 – 000

111

to reduce the expansion to below the allowable 0.1% expansion limit. In contrast, in our study, it was found that there was a significant reduction (~68%) in ASR expansion at half the dosage (0.05%) when compared to the Ramezani et al. study.

0.7

0.7

C NMK_5

C CNT_0.01 CNT_0.03 CNT_0.05

0.6

0.6

NMK_7.5 NMK_10

0.5

0.5

0.4

0.4

0.3

0.3

% Expansion

% Expansion

0.2

0.2

0.1

0.1

0

0

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

Days

Days

(b)

(a)

Fig. 2 ASR expansion of (a) nano metakaolin and (b) CNT with different dosages

2.2. Compressive strength T he compressive strength of the mortar cubes is illustrated in Fig. 3. The replacement of cement with 7.5% and 10% NMK resulted in an increase in compressive strength at all tested ages (see Fig. 3a). Conversely, a 5 wt% NMK substitution resulted in decreasing compressive strength, with reductions of 5.8% and 10.8% observed at 28 and 90 days, respectively. Notably, 7.5 and 10 wt% NMK substitutions enhanced the 90-day compressive strength by 16.4% and 22.9%, respectively. Previous studies have indicated that the incorporation of NMK can significantly improve mechanical properties, likely due to the dual influences from the filling effect and pozzolanic reaction. However, the literature also suggests that there is an optimal quantity of NMK that maximizes compressive strength. For instance, Morsy et al. observed that when mortar mixes were made with 20% of Class F fly ash and varying proportions of NMK (0, 2.5, 5, 7.5, and 10 wt%), the compressive strength of samples cured for 60 days increased only up to 7.5 wt% NMK addition, beyond which it decreased (Morsy et al. 2014). The decrease beyond this optimum is thought to result from NMK agglomeration around cement particles, which could obstruct the hydration reaction. Similarly, Fadzil et al. found that in ultra-high-performance concrete (UHPC) mixes prepared with 10% metakaolin and 0, 1, 3, 5, 7, and 9 wt% NMK, only the mixes containing 0 and 1 wt% NMK showed an increase in 28-day compressive strength (Fadzil et al. 2014). In contrast, mixes containing 3 to 9 wt% NMK exhibited a decrease in strength. This suggests that while NMK can improve compressive strength, there is an optimal dosage, beyond or below which replacement of cement with NMK becomes detrimental. 0.01 wt%, 0.03 wt%, and 0.05 wt% of CNT addition increased the 90-day compressive strength by 16.3%, 13.9%, and 24.3%, respectively (see Fig. 3b). Based on a review paper done by Silvestro et al, 74% of the 91 articles reported an increase in compressive strength up to 30% at 28 days (Silvestro et al. 2020). This beneficial effect of CNT on compressive strength is attributed to several factors: (a) The nucleation effect of CNT for more C-S-H formation (Silvestro et al. 2020), (b) the pore refinement effect leading to a dense composite (Xu et al. 2015), and (c) modification of hydration products (e.g. high-density C-S-H) (Barbhuiya and Chow 2019).