Atmospheric deterioration of ancient and modern hydraulic mortars
Introduction
Historic masonry is made up of different materials (bricks, stones, mortars, etc.), all of which are strongly affected by degradation processes. Much has been published in the literature on the environmental damage encountered on building materials, such as stones and bricks (Baer et al., 1991; Van Balen et al., 1997), but little information is available on the impact of atmospheric multipollutants on mortars, either ancient or modern.
A few studies have been performed on lime mortars, i.e. prepared using lime and sand, where damage occurs following the same typologies and mechanisms found on sandstones (Sabbioni and Zappia, 1992; Sabbioni et al., 1998). However, practically no results are available for hydraulic mortars, in spite of their fundamental role in modern construction and their widespread use in the restoration of historic masonry, where they are employed as structural, rendering, repointing and consolidant materials or as support for original frescoes. Today, clear signs of damage due to atmospheric deposition can be observed also on these materials, but the little information there is only comes from tests performed in the laboratory (Zappia et al., 1994; Martinez Ramirez et al., 1997), where it has basically been shown that cement mortars exposed to pollutant SO2 present the highest degree of sulphation, compared to other hydraulic mortars (i.e. pozzolan mortars), notwithstanding their lower calcium carbonate content.
Concerning hydraulic binders, since ancient times, materials of different origin have been added to lime (an air-hardening binder) in order to obtain hydraulic mortars. The term “hydraulic” refers to two related properties: the property of hardening when water is added to the dry binder and the capacity of the hardened mortar to be water-resistant.
The Romans discovered that if sand of volcanic origin (of the type present near Pozzuoli) was used to substitute ordinary sand in lime mortar, the mortar became hydraulic (Vitruvio, 1960; Van Deman, 1912; Blake, 1947; Lugli, 1957; Ashby, 1991). This effect is mainly due to the chemical composition (high SiO2, Al2O3 reactive contents), high amorphous content and high specific surface area of the pozzolan, which reacts with lime and water to form calcium silicates and aluminates hydrates. The same effect could be obtained by substituting ordinary sand with ground-fired clay (chamotte) or finely ground bricks or tiles (cocciopesto).
Hydraulic lime is a special type of lime, which, regardless of the presence of pozzolan, has itself the ability to harden under water (Goria, 1976; Picon, 1997).
Hydraulic lime constituted the “link” with the modern hydraulic binder developed in the mid-19th century: Portland cement. The term cement is applied to many different materials, the one most commonly referred to in engineering being ordinary Portland cement (OPC). The use of cement spread rapidly at the end of the 19th century to assume a position of absolute predominance in the 20th century.
Hydraulic mortars are found not only in archaeological and historic monuments (both as original components and restoration materials), but also in those of the 19th and 20th century, as well as structures of the current period, which must be considered as an integral part of the Cultural Heritage. For this reason, it is particularly surprising that the effects of atmospheric pollution on these building materials have not yet been studied.
The aim of this paper is to summarise the results achieved in a study on the effects of atmospheric deposition on archaeological, historic and modern monuments where hydraulic mortars have been used. It is an attempt to take a significant step forward in the scientific knowledge on the environmental impact on the degradation of hydraulic mortars and to provide a useful documentation of degradation phenomena.
Section snippets
Experimental
In order to collect different types of ancient and modern hydraulic mortars, samples were collected from numerous sites in Italy, Belgium and Spain, in urban, suburban, rural or maritime areas. In particular, attention was focused on hydraulic structures (such as aqueducts, reservoirs and thermal baths) of Roman archaeological sites in southern Italy (Latium and Campania regions), where hydraulic mortars containing pozzolan materials were widely employed.
Cement and hydraulic lime-based mortars
Results
The mortars collected from various ancient and modern buildings in Europe were first analysed in order to study the nature of the binder (presence or absence of hydraulic characteristics), which is a necessary step in understanding the mortar degradation and in choosing appropriate restoration strategies. Subsequently, the damage encountered on the sampled mortars induced by atmospheric pollution was investigated with the combined use of the above mentioned analytic techniques.
Optical
Discussion
By applying the methodological set-up for the identification of the type of mortar binder (Van Balen et al., 1999), pozzolan, cement and hydraulic-lime-based mortars were recognised when employed in the construction of new buildings or in the repair and maintenance of ancient and historic masonry.
Ancient Roman mortars showed that all the samples studied were of pozzolan mortar. Microscope studies revealed how natural pozzolan serves a dual role in the mortars: on the one hand, the amorphous
Conclusions
The present work represents the first systematic synthesis of environmental analyses performed on hydraulic mortars.
Using the specific methodology expressly set up, it was possible to distinguish the type of binder present in the sampled mortars. All the Roman mortars were found to be pozzolan based, thanks to the presence of pozzolanic admixtures. In turn, modern and restoration mortars collected from Belgian and Spanish masonries were composed of hydraulic lime and cement as binder.
The
Acknowledgments
The present study was supported by the EC, within the Environment and Climate programme, Project EDAMM – Environmental Deterioration of Ancient and Modern Hydraulic Mortars (Ct. no. ENV4-CT95-0096).
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