PSI - Issue 10

I. Karatasios et al. / Procedia Structural Integrity 10 (2018) 211–218 I. Karatasios e tal. / Structural Integrity Procedia 00 (2018) 000 – 000

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values (28- 40 %), leading to low bond strength (Table 3). Finally, the use of ternary mixtures with cement-lime metakaolin exhibited compatible porosity and strength values, both in three-point and four-point flexural bending tests.

Stone

Mortar

a c Fig. 4. Electron microscopy photomicrographs of (a) stone (left) - mortar (right) interface (ST01-RM1) with micro-crack created after failure at 4-point flexural bending; (b) Stone matrix ST01 (joined with RM1 mortar) indicating quartz fragments and calcareous binder and; (c) RM1 mortar binder indicating quartz fragments and calcareous binder with alumino-silicate phases, very similar to the stone matrix (next picture). b

Stone -open porosity (%)

Mortars - open porosity (%)

10 15 20 25 30 35

27.93

24.52

17.7

12.9

10.3

8.5

0 5

cement

NHL

Lime-Cement-Metakaolin

Fig. 5. Porosity variations between different mortar mixtures, according to their binder type

4. Conclusions

Overall, the interpretation of microstructural and mechanical properties results highlighted the essential role of four-point flexural bending for assessing the bond strength of stone-mortar interface, as well as the predominant role of composition, physical and chemical characteristics of the binder on the development of the adhesion properties of repair mortars. Considering the overall mechanical and physical properties of mortar mixtures, their adhesion strength and failure during 4-point flexural bending, the three mixtures that could better satisfy the adhesion requirements of the different types of marly stones are RM1, RM3 and RM6. All three mixtures that were promoted as adhesive material for the stone fragments exhibited very good adhesion to the stone surface and compatible microstructure with the stone matrix, at the micro-scale (Fig.3). Lime based mixture exhibited better workability but higher porosity and lower strength. According to laboratory results, the adhesion strength between stone surface and join mortars decreased in inverse proportion to porosity. Higher porosity mortars exhibited lower strength. Higher water demand resulted also in elevated porosity, which eventually contributes to the decreased strength. The addition of fine quartz sand didn‟t affect the adhesion ability of mortars, in contrast it acted beneficiary, since it prevented the formation of shrinkage cracks during setting.