UDK 691:620.1:691.5	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 46(3)223(2012)
MICROSTRUCTURAL AND PHYSICAL-MECHANICAL ANALYSES OF THE PERFORMANCE OF NANOSTRUCTURED AND OTHER COMPATIBLE CONSOLIDATION PRODUCTS FOR
HISTORICAL RENDERS
MIKROSTRUKTURA IN FIZIKALNO-MEHANSKE LASTNOSTI NANOSTRUKTURNIH IN DRUGIH KOMPATIBILNIH PROIZVODOV ZA UTRJEVANJE ZGODOVINSKIH OMETOV
Giovanni Borsoi, Martha Tavares, Maria do Rosario Veiga, Antonio Santos Silva
Laboratorio Nacional de Engenharia Civil, Av do Brasil 101, Lisboa, Portugal gborsoi@lnec.pt
Prejem rokopisa - received: 2011-09-27; sprejem za objavo - accepted for publication: 2012-02-24
The surface consolidation of historical renders, directed to restore cohesion and stability, is based on the use of materials with aggregating properties. This operation is usually achieved with the use of inorganic or mineral consolidants, which are preferred to organic ones, due to the better compatibility and durability.
Based on the results of previous studies, two mineral-compatible products were selected: a commercial dispersion of calcium hydroxide nanoparticles in propanol and a calcium-silicate product, consisting of a limewater dispersion of ethyl silicate. The consolidation products were applied to mortar specimens in order to assess their efficacy by determining their microstructural and physical-mechanical properties, before and after the consolidation treatment. Microstructural (optical and SEM microscopy) and chemical analyses of the consolidation products and of the consolidated samples were performed. The physical-mechanical analyses, i.e., the superficial hardness, is reported too.
Keywords: consolidation products, compatibility, nanoproducts, SEM/EDS
Utrjevanje površine zgodovinskih ometov z vidika ohranjanja kohezije in stabilnosti temelji na uporabi materialov z vezivno sposobnostjo. To se navadno doseže z uporabo neorganskih ali mineralnih utrjevalcev, ki so v prednosti pred organskimi zaradi boljše skladnosti in zdržljivosti.
Na podlagi predhodnih študij sta bili izbrani dve vezivi: komercialna disperzija delcev kalcijevega hidroksida v propanolu in proizvod na osnovi kalcijevega silikata, ki vsebuje apnovico, dispergirano v etil silikatu. Namen uporabe vzorcev veziv na vzorcih malte je bil ugotoviti njihovo učinkovitost z določitvijo mikrostrukture in fizikalno-mehanskih lastnosti, pred obdelavo z vezivom in po njej. Izvršene so bile raziskave mikrostrukture (svetlobna in SEM-mikroskopija), kemijska analiza vzorcev veziv in vzorcev po utrjevanju, fizikalno-mehanski preizkusi, poročamo pa tudi o trdoti površine.
Ključne besede: utrjeni proizvodi, kompatibilnost, nanoproizvodi, SEM/EDS
1 INTRODUCTION
hydroxide, ethyl silicate, calcium oxalate and calcium tartrate.
A common degradation phenomenon in historic mor- The aim of this work is the experimental character-
tars is the loss of cohesion of the binder-aggregate sys-	ization of two different, compatible, consolidant prod-
tem, which is usually followed by the superficial mate-	ucts, i.e., a traditional compatible product, such as a
rial loss and a loss of mechanical strength, usually as a	limewater, mixed with ethyl silicate, and a commercial
consequence of chemical and biological phenomena that	alcoholic dispersion of nanoparticles of calcium hydrox-
can modify the nature of the binder1.	ide' which presents an innovative consolidant product.
The restitution of cohesion between the mortar's
particles, turned friable by the loss of binder, is achieved	2 MATERIALS
through the application of organic or mineral conso-	^.l Specimens - Mortar samples preparation lidants. The first experimentations on silicates, fluorides,
barite and limewater were done in the 19'h century2; In order to simulate a mortar with a loss of cohesion,
subsequently in the 20'h century there was the intro-	different mortar specimens were prepared; the binder/ag-
duction of polymers, such as acrylics and epoxy resins,	gregate ratio of 1 : 4 (in volume) was chosen in order to
which are easier to apply and present better adhesive-	get the desired effect of a low-cohesion mortar, without
ness, but do not obey the fundamental rules of physical-	significant loss of material. The aggregate used was
chemical compatibility with the substrate. Inorganic	graduated siliceous sand obtained from a mixture of
consolidants are becoming preferred due to their better	three different calibrated sands with mean particle sizes
compatibility and durability; the best known inorganic	<2 mm. After the optimization of the mortar composi-
consolidants are calcium hydroxide (limewater), barium	tion, different samples were prepared, such as mortar
prisms 40 mm x 40 mm x 160 mm and ceramic bricks 28 cm X 19 cm with a single mortar layer of 1.5 cm thickness.
2.2 P^^oP^^ties and application o^ the consolidant products on lime-mortar specimens
The effectiveness of the limewater, considered the most traditional consolidant product, is known and previous studies provided good results3 4, beyond the economic advantage and full compatibility. However, limewater usually contains not more than 2 g/L of calcium hydroxide, which only guarantees a low consolidation effect2, unless it is applied in a large number of cycles.
We have decided to explore the efficiency of limewater, matured in a closed container for some years, mixed with a commercial ethyl silicate (Estel 1000®, CTS). The application of this product causes the formation of amorphous silica gels, which act as a consolidant, ensuring an increase of the mechanical resistance5. A low concentration of ethyl silicate was used (5 %), in order to moderately increase the mechanical strength.
Nanolime dispersions of calcium hydroxide are white-to-opal solutions containing stable calcium hydroxide nanoparticles dispersed in an alcoholic medium, usually isopropanol. The nanoparticles have a hexagonal-shaped form and a size range between 50 nm and 600 nm6; the reduced dimension of Ca(OH)2 particles guarantees a deeper penetration inside the smaller pores. We have used a commercial nanolime (Nanorestore®, CTS).
An analysis of the selected consolidation products was made considering the important characteristics linked to an optimal application, including the pH, setting times and dry residues (Table 1).
Table 1: Characteristics of the consolidation products Tabela 1: Značilnosti veziv
Consolidation product	pH	Dry residue (g/L)	Setting time (min)
Limewater + Ethyl Silicate (5 %)	9.2	3.51	20
Nanolime	7.2	1.78	120
The applications were made in a conditioned room, at 23 °C and 50 % RH, using a manual-spraying technique (ten consecutive applications) at a distance of 20-30 cm3'7.
3 METHODS
3.1 Characterization of the consolidant treatments
The evaluation of the efficacy of the consolidant
treatments was carried out through the use of different tests, executed before and after treatments (90 d from the consolidant product application).
The improvement of the mechanical resistance was checked by the durometer hardness (Shore A, PCE
Group)8,9. The surfaces of mortar specimens were observed with an Olympus SZH stereoscopic microscope and the images were recorded digitally.
The microstructural observations and the elemental analyses were performed on specimens previously sputtered with a gold film by SEM JEOL JSM-6400, coupled with an Oxford Instruments energy-dispersive spectrometer (EDS).
4 RESULTS AND DISCUSSION
4.1	Durometer hardness (Shore A)
The superficial hardness of the specimens was verified 90 d after the application of the product through a durometer (Shore A). As shown in Figure 1, an improvement in the superficial hardness of the treated specimens is evident.
The nanolime consolidant presents a moderate increase in the superficial hardness (18 %) compared to the untreated specimens, while the treatment of limewater mixed with ethyl silicate registered a greater increase (28 %); the values reflected the trend of previous studies that were made on ancient lime-based mortars710.
4.2	Microscopic observations by stereozoom microscopy
Stereozoom observations were made in order to evaluate the morphological and microstructural variations due to the consolidation treatments. In comparison with the untreated specimens (Figure 2a), which present wide pores and micro-cracks, the specimens treated with a limewater dispersion of ethyl silicate (Figure 2b) show a more compact surface and an increase in the porosity. Otherwise, the product presents a discontinuous distribution, forming planar agglomerates in the surface.
On the other hand, specimens treated with nanolime (Figure 2c) present a more uniform distribution of the consolidation product and homogeneous infilling of the matrix voids; moreover, fewer microcracks were visible.
Figure 1: Superficial hardness and relative increases of the treated mortars (durometer Shore A)
Slika 1: Trdota površine in njeno relativno povečanje v obdelanih maltah (trdometer Shore A)
4.3 Microstructural observations by SEM-EDS
The specimen treated using limewater with ethyl silicate (Figure 3a, b) shows the formation of platelike aggregates of calcium-silica gels. Indeed, the quick reaction in the alkaline aqueous solution of limewater rapidly forms calcium-silica gel; this gel transforms itself into a xerogel due to the evaporation of the solvent, and the presence of CaCO3 seems to modify the xerogel vesicular microstructure.
Figure 2: Stereozoom microphotographs (magnification 40-times): a) untreated specimen; b) specimen treated with limewater solution of 5% ethyl silicate and relative planar aggregates (arrows); c) specimen treated with nanolime
Slika 2: Posnetki s stereomikroskopom (pove~ava 40-kratna): a) neobdelan vzorec; b) vzorec, obdelan s 5-odstotno raztopino apnovice in etil silikata in relativno ploskimi povr{inami veziva (pu{~ica); c) vzorec, obdelan z nanodelci apna
Macroscopically, both products show few differences in comparison to the untreated mortar, and seem to induce only a slight whitening on the surface.
Figure 4: SEM microphotographs of the mortar, treated with nanolime: a) homogeneous distribution of the nanolime in the mortar paste; b) clusters of nanolime particles (arrows) mixed with the original binder
Slika 4: SEM-posnetek malte, obdelane z nanodelci apna: a) homogena razporeditev nanodelcev apna v malti; b) skupek nanodelcev apna (pu{~ica), zme{an s prvotnim vezivom
Figure 3: SEM/EDS images of the specimen, treated with limewater-ethyl silicate solution: a) presence of platelike shaped of calcium-silica gel on the mortar surface and b) corresponding EDS spectrum
Slika 3: SEM/EDS-sliki vzorca, obdelanega z raztopino etil silikata v apnovici: a) prisotnost plo{~atih oblik kalcij-silicijevega gela na povr{ini vzorca malte in b) ustrezen EDS-spekter
According to recent studies11, calcium carbonate actually aids the development of shorter linear chains of tetrahedral silica and linear silicate structure, which can explain the rapid formation of a granular gel with a platelike shape.
The SEM/EDS observations of the mortars treated with the nanolime product show micro-sized clusters of calcitic formations; the distribution and morphology of these nanostructured particles show a homogeneous consolidation film. The consolidation film of nanolime is characterized by the presence of plate-like nanoparticles that aggregate into micro-sized clusters, which are compact and polydispersed (Figure 4).
According to previous studies12, the carbonation of nanolime particles originate in oriented crystal grains, which promote the agglomeration of the particles.
Moreover, beyond the chemical, physical and mechanical compatibility, Rodriguez-Navarro et al.13 have shown that plate-like lime nanoparticles have a great capacity to absorb water (which acts as a lubricating film), guaranteeing a good plasticity and avoiding the mechanical stress inside the treated mortar.
5 CONCLUSIONS
The analysis evidenced some differences between the two products. The obtained results of the mechanical resistance, evaluated through the durometer hardness and the flexural and compressive strength, show that the highest mechanical increase was obtained with the limewater dispersion of ethyl silicate, while the alcoholic dispersion of nanolime particles guarantees a moderate improvement in the mechanical resistance. Microscopical and microstructural observations using stereozoom microscopy and scanning electron microscopy show that the limewater dispersion of ethyl silicate has a consolidation effectiveness on the treated surface, due to the formation of plate-like aggregates of calcium silica gels; however, these planar aggregates can physically interfere in the penetration depth of the consolidant. A limewater dispersion of ethyl silicate is also a good consolidation product, ensuring the restitution of superficial cohesion to the treated mortar. In any case it is recommended only for mortars with a superficial loss of cohesion, because of the reduced depth penetration.
Otherwise, nanolime particles permit a homogeneous distribution on the treated substrate; the platelike nanoparticles present a specific crystallographic orientation that could contribute to an agglomeration process. The nanolime dispersion appears as promising conso-lidant product for lime mortars with a loss of cohesion, ensuring an optimum penetration and distribution in the matrix binder; however, this dispersion does not seem to guarantee a large improvement in the mechanical resistance, so the use of nanolime is recommended for mor-
tars with reduced loss of cohesion, or to combine the use of this product with other consolidation product.
Acknowledgements
This study was developed within a project (Limen-contech - Conservation and durability of historical renders: compatible techniques and materials) financed by
FCT - Fundagao para a Ciencia e a Tecnologia (Portugal).
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