Eduardo M. Sebastián Pardo

Lime Mortars for the Conservation of Historic Buildings

Abstract Awareness of the need for compatible materials for the preservation of the architectural heritage has resulted in the revival of lime-based mortar technology and applications. However, knowledge of the preparation process and procedure influencing the final quality of lime mortars is limited, and controversy persists in the conservation community regarding the most appropriate material for conservation treatments (for example, hydrated lime versus aged lime putty). This paper reviews current knowledge on lime mortar technology, including burning, slaking, aging and carbonation of lime. Special emphasis is given to the effects of aging on the morphological evolution of hydrated lime and on the carbonation process, since these aspects have not been discussed thoroughly in the technical and conservation literature. The improvements observed in the physical properties of hydrated lime after prolonged storage under water can be attributed to particle size reduction (<1μm) and morphology changes (from prism to plate-like crystals). Studies on the carbonation of non-aged commercial hydrated lime and traditionally aged slaked lime revealed higher carbonation rates in the case of aged lime. Some recommendations are given for the processing of lime and the preparation of lime mortar for conservation treatments. The use of aged lime putty is recommended because this material, with higher plasticity and water-retention capacity, results in mortars of higher strength that carbonate faster.

Durability of bricks used in the conservation of historic buildings — influence of composition and microstructure

Differences in mineralogical and textural evolution during firing of calcareous and non-calcareous bricks are studied and correlated with their behaviour in hygric and weathering tests. Results reveal significant differences in the evolution of vitrification degree, porosity and pore size distribution. Such evolution depends mostly on raw clay composition and firing temperatures. A higher degree of vitrification and of compressive strength is displayed by calcareous rather than non-calcareous bricks at lower firing temperatures of between 700 and 900 °C. However, their resistance to salt crystallisation and freezing is not notably improved because of unfavourable pore size distribution and crack development. The latter are caused by the transformation of calcite into calcium oxide at around 800 °C, which reacts readily with moisture to form calcium hydroxide, thus leading to a volume increase (lime blowing). This problem can be avoided by closely controlling grain size and content of carbonates in the raw clays. High firing temperatures of 1100 °C in the case of calcareous clay and 1000 °C in the case of non-calcareous clay are required to produce durable bricks that remain unaltered upon weathering. The improved durability appears to be due to a more favourable pore size distribution and a reduction in porosity. Results from textural and hygric studies of the brick samples indicate that these parameters can to a significant extent be controlled by varying raw clay composition and firing temperature, thus making it possible to fabricate replacement bricks for particular conservation purposes. This paper addresses limitations regarding the interpretation of test results, as well as the lack of a systematic application of existing standards for evaluating the state of conservation of historic bricks and for establishing specifications for replacement bricks.

A review of selected inorganic consolidants and protective treatments for porous calcareous materials

Abstract Incorporating the results of a meeting held in London in December 2000, sponsored by English Heritage and The Getty Conservation Institute, this article reviews both consolidants and protective treatments for the conservation of deteriorated limestone and lime plaster. Carbonate deposition (including both inorganic solutions and biomineralization), barium hydroxide treatment, ammonium oxalate and tartaric acid treatments are covered. The article reviews selected literature, identifies open questions and promotes discussion of a range of issues, encompassing application techniques, performance, compatibility and retreatability. While many questions concerning these important systems have been addressed in published sources, there are significant opportunities for new research.

Geochemical characterization of sedimentary basins by statistical analysis: The Mio-Pliocene sequences of the Vera Basin, SE Spain

Abstract The origin and history of sedimentary rocks can be deciphered from the trace elements in their clay minerals. The inheritance from weathering profiles is detected by the analogy in trace-element amounts of the Principal components analysis (PCA) was used to find the relationships among the chemical elements, the minerals and the parent rocks, and to select the most discriminating elements. Sr, Ni, Cr, Cu, Co, V, Sc and Zn appeared so to be representative of the clay fractions and the parent rocks, and were therefore selected to obtain information about the parent rocks of the sediments. A hierarchical ascending classification (HAC) was performed with these elements, it regroups samples with similar geochemical characteristics. Graphical comparison of mean geochemical composition among HAC groups provided a better identification of inherited similarities between some sediments and the parent rocks. Most of the clay minerals in the sediments appeared so to have been formed by weathering of the metamorphic basement. Only one laminite sample from Cuevas de Almanzora section in the north of the basin was identified as formed by weathering of volcanic rocks. Volcanic weathering fluids enriched some clays from the Coscojar, Garrapancho and Garrucha sections in Sr. Some Cu and Zn enrichments acquired during early diagenesis are also evidenced by graphic comparisons. B and Mg contents were used to indicate paleoenvironments. They distinguished two different sedimentary environments, separated by a paleogeographical high situated at the site of Garrapancho Hill. Higher B contents in the southern part of the basin indicate a more marine environment than in the north composed of several subbasins with different salinities correlated with varying inputs of fresh water. The Mg contents are always higher in the north, suggesting a more confined environment. The sediments in the south were deposited in higher-energy environments as shown by the presence of olistostromes whereas in the north marls and clayey laminites suggest a calmer and more monotonous environment.

MINERALOGICAL EVOLUTION OF DI- AND TRIOCTAHEDRAL SMECTITES IN HIGHLY ALKALINE ENVIRONMENTS

The mineralogical evolution of di- and trioctahedral smectites (i.e. montmorillonite and saponite) exposed to high-pH environments has been studied to determine the influence of compositional differences on clay dissolution and the formation of new phases. The present study helps to gauge the effects of highly alkaline solutions on the swelling capacity of smectitic clays and experimental results are extrapolated to predict the behavior of smectite-rich soils in various technical applications such as nuclearwaste storage and architectural conservation. The present study revealed extensive dissolution of montmorillonite in 5 M NaOH or 5 M KOH solutions and the neoformation of various zeolites, thereby reducing the clay’s swelling capacity significantly. Saponite, in contrast, experienced less pronounced changes, including transformation into a randomly interstratified saponite-chlorite and a Si-rich amorphous phase. These changes only provoked a partial reduction in swelling capacity. The results imply that under repository conditions (e.g. alkaline environment caused by hyperalkaline fluids released during concrete leaching), the slow and limited transformation of saponite into corrensite-type minerals would be beneficial for preserving the clay’s swelling capacity and, therefore, its effectiveness as a sealing material. Conversely, the loss of swelling capacity as a result of zeolite formation in montmorillonite observed in the present experiments would limit the clay’s effectiveness as a sealing material in waste repositories. In the case of earthen architecture conservation, alkaline consolidation treatments aimed at reducing the soils’ swelling capacity and, thereby, improving water resistance, would only be effective for treating earthen structures made of soils rich in dioctahedral smectites. Soils containing trioctahedral smectites, in contrast, are not likely to improve their water resistance because the swelling capacity will only be partially reduced.