Лако-красочные материалы — производство

Table 3 reports the physical and mechanical properties of the designed mortars. The physical properties of the designed mortars differ insignificantly, indicating that the nano-titania addition neither modified the microstructure, nor affected the hygric behaviour of the materials. The lowest Fc values were recorded for the NHL samples with a B/A=1 (NHLT1), while the Fc values decreased with curing time in the ML samples. Even though, the Fc values recorded at four weeks curing for the MLT samples are lower than the corresponding values for samples without nano-titania (ML), nevertheless, the Fc values of the MLT samples reached higher values than the ML samples after three months and one year curing, thus indicating the beneficial effect of the nano-titania in the compressive strength. The decrease of Fc values over time in the ML compositions has been already reported by other authors (Aggelakopoulou 2011, Velosa 2009) and was most probably attributed to the microcracking formation due to shrinkage during the curing.

By comparing the results of physical and mechanical properties, the mortars NHLT2, ML and MLT were selected as adhesive means for the stones under consideration (Table 4). The mortar NHLT1 showed similar values to the others, except for its high capillary water coefficient; this is probably the reason this mortar exhibited difficulty in joining the stone specimens. As such, it was therefore not included in the finally selected mortars.

It seems that nano-titania with its hydrophylicity created an environment, which not only enhanced the hydraulic component formation, but also controlled the shrinkage, thus avoiding microcracking (Karatasios 2010). Further support to this statement is derived from the dense network of hydraulic components observed in the SEM micrographs (Figure 4b) for the MLT samples.

Table 4 reports the results of the 4-point bending test and the direct tensile test for the adhered stone-mortar specimens (Figure 1). In all tests, failure was observed at the interface between stone and mortar. The results revealed that the higher bonding strength (higher flexural and tensile strengths) was measured when applying the NHLT2 mortar, as compared to the MLT and ML mortar.

Based on the physico-chemical and mechanical characterization of all the studied adhesive mortars the MLT mortars with metakaolin, lime and nano- titania and the natural hydraulic lime mortars with nano-titania (NHLT2) have been selected as the most appropriate for the adhesion of fragment porous stones.

Conclusion

This research work addressed an important problem in the restoration sector concerning the reassembling of stone fragments from ancient monuments using non-cementitious mortars. The proposed adhesive mortars contain hydraulic lime or metakaolin and lime as binders, carbonate sand with grains between 250 and 63 pm and binder to aggregate ratio 1 or 2. The nano — titania as additive was employed in a binder replacement of 4.5-6% w/w.

The mechanical characterization indicated that the mortars with nano — titania showed increased compressive and flexural strengths and moduli of elasticity over time, when compared to the specimens without nano-titania. The results also indicate enhanced carbonation and hydration of mortar mixtures with nano-titania. The hydrophylicity of nano-titania improves the humidity retention of mortars, thus facilitating the carbonation and hydration processes. This property can be exploited into the fabrication of mortars tailored to adhering porous limestones, where humidity controls the mortar setting and adhesion efficiency. Home-designed equipment applied to measure the bonding strength of stone-mortar systems revealed that the nano-titania addition in both metakaolin-lime and hydraulic lime mortars improves the adhesive property of the mortar when applied to porous stones. The ability of nano-titania mortars to discolor stains of methylene blue under visible light irradiation can be used in favor of the avoidance of biological crusts. Considering all the above and the bonding strength, binders of metakaolin — lime and/or hydraulic lime mortars with nano-titania have been selected to carry out the adhesion of fragments porous stones in restoration interventions.

Acknowlegments

This work was carried out in collaboration with the Akropolis Restoration Service (YSMA). The authors would like to thank Emeritus Prof. Vasileia Kasselouri-Rigopoulou, (Committee for the Restoration of the Acropolis Monuments-ESMA) for collaboration and scientific support, the Committee for the Restoration of the Acropolis Monuments, Dr. E. Aggelakopoulou Head of the Technical Office for the Acropolis Monuments’ Surface Conservation and Dr. E. Sioubara, scientific responsible for the reassembling of fragment stones of Archaic period. This research has been co-financed by the European Union (European Social Fund — ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) — Research Funding Program: Heracleitus II, Investing in knowledge society through the European Social Fund.