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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the dissolution- precipitation of C 3 S that offers the ideal conditions for the dissolution of the metakaolin to form C S-H. This reaction consumes 30 % to 40 % of metakaolin present in the mortar. Beyond 28 days, the pozzolanic activity of MK slows due to the decrease in the rate of portlandite within mortars. Therefore, the L50-M50-C0 paste exhibits slower compressive strength than the previous binary pastes as there is amount of MK that did not react with the lime. Qian et al. (2019) proposed that in normal environments, where the CO 2 ingress is not limited, the optimal MK content needs to be verified, since the carbonation reaction is favored in such condition and counterbalances the portlandite consumption in the pozzolanic reaction. In overall, lime hardens by carbonation, a slow process that contributes to strength in the long-term, while cement hardens by the hydration of calcium silicates, a process that contributes to a rapid strength gain, mostly until 28 days of age. Natural hydraulic lime hardens by a combination of both processes. The blended mortar that presented the highest potential to be used in restoration works, i.e. physical characteristics and moderately higher mechanical strength at early age, was the lime-hydraulic lime mortar containing 50 % natural hydraulic lime by mass of the total binder, as suggested by Silva et al. (2015).

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Compressive strength [MPa]

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

Fig. 5.Compressive strength of the examined pastes at 28 days of age.

3.2 Flexural strength results Fig. 6 displays the results of the four-point bending tests, where the flexural strength of the 100 wt % lime paste was equal to 0.9 MPa. The average flexural strength of all binary binders after 28 days of curing was found to be approximately 1.5 MPa. Regarding the ternary binders, their average flexural strength exceeds 2 MPa. The difference between them is due to the presence of cement, which possesses superior mechanical properties in comparison to the other materials. However, the average flexural strength is similar between the pastes with percentage of cement 20 wt% and 30 wt% and not higher in this with the maximum cement content as expected. This is observed, because the hydration reactions in cement may have not proceeded yet. Especially, adding cement (0.3 wt %) in the L35-M35 C30 paste increases by 55 % the flexural strength of the L65-M35-C0 paste. Similarly, adding cement (0.2 wt %) in the L40-M40-C20 paste increases by 40 % the flexural strength of the L50-M50-C0 paste. The flexural strength increases in binders L80-M20-C0 and L65-M35-C0 as the lime content in the binder decreases as Ramesh et al. (2019) noticed in lime/cement mortars. As the metakaolin/lime ratio increased, the flexural strength increased which is in agreement with Aggelakopoulou et al. (2011). Higher mechanical strength improvement can be