PSI - Issue 41

Maria-Evangelia Stogia et al. / Procedia Structural Integrity 41 (2022) 744–751 Maria Evangelia Stogia et al. / Structural Integrity Procedia 00 (2019) 000–000

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achieved by the blended cement with higher metakaolin replacement. This is because more aluminate is available in the cement with higher content of metakaolin as mentioned by Qian et al. (2019)]. This explains the higher value of the flexural strength of the L47.5-M47.5-C5 comparing with the L50-M50-C0 that does not include cement. The mechanical strength of MK-cement mortars was improved and the promotion is more obvious on higher MK contents, according to Zuo et al. (2020). After curing time of 28 days, the mechanical strength of all samples is further promoted. Especially for those based on MK content of 20 and 35 wt %, the increase for flexural strength reaches up to 45 %. The pozzolanic effect of MK can promote the formation of high-density C-S-H gel with low-density C-S-H gel decrease during curing. The last investigated binder L35-M35-C30 exhibits lower compressive and flexural strength than the L40-M40-C20 binder, even though it contains more cement content and the MK/ lime ratio in both pastes is the same. This mechanism is explained by Dinakar et al. (2013) as a result of the dilution effect of replacing a part of cement by the equivalent quantity of MK. In MK concrete, the filler effect, pozzolanic reaction of MK with calcium hydroxide and compounding effect (synergetic effect of mineral admixture) react opposite to the dilution effects.

4,0

3,5

3,0

2,5

2,0

1,5

1,0

Flexural strength [MPa]

0,5

L100-M0-C0 L80-M20-C0 L65-M35-C0 L50-M50-C0 L47.5-M47.5-C5 L40-M40-C20 L35-M35-C30 0,0 Binary and ternary pastes [NHL5 (L) - Metakaolin (M) - CEM (C)]

Fig. 6.Flexural strength of the examined paste at 28 days of age.

3.3. Discussion of the results Compressive strength results show similarities with those of flexural strength, with a clear strength increase in mortars as the MK content increases and are according to the trend noticed in the literature, e.g. Velosa (2016). The increase in the mechanical properties of all binders that contained NHL5 was attributed to the hydration of several hydraulic compounds that form hydrated calcium silicates (C-S-H phases) and similar trend was noticed by Garijo et al. (2020). In conclusion, MK can promote the mechanical strength and toughness, especially on the later stage of curing. The higher content of MK has more prominent promotion effect. However, some researches, e.g. Badogiannis et al. (2004), showed that excess MK easily delayed the hydration of cement increasing consistency and water consumption. 4. Conclusions The mechanical properties for binary and ternary binders for restoration applications were investigated and the conclusions can be summarized as follows: 1. The experimental results indicate that binary and ternary binders were successfully developed that can be used in the restoration interventions of traditional buildings and Cultural Heritage buildings. 2. The lime/pozzolan mortars have improved mechanical properties compared to the lime itself; the use of greater percentages of pozzolan in a mortar does not necessarily imply improved mechanical performance.