Kerstin Elert

Influence of mineralogy and firing temperature on the porosity of bricks

The changes in brick porosity upon firing (700 up to 1100 � C) and its relation to the mineralogical composition are examined. Two types of raw clay with a composition representative of that used in brick-making industry were selected to manufacture the bricks: one contains notable amounts of carbonates, with a grain size of under 1 mm, and the other is predominantly quartzitic and lacking in carbonates. We demonstrate that the presence or absence of carbonates strongly influences the porosity development and, therefore, the brick texture and physical-mechanical properties. The carbonates in the raw clay promote the formation of fissures and of pores under 1 mm in size when the bricks are fired between 800 and 1000 � C. The absence of carbonates results in a continuous reduction in porosity and a significant increase in the pore fraction with a radius (r) > 1 mm as the firing temperature rises and smaller pores coalesce. Porosity and pore size distribution results obtained from the combined use of hydric tests (HT), mercury intrusion porosimetry (MIP) and digital image analysis (DIA) of scanning electron microscopy photomicrographs are compared. A clear correlation between the water absorption and drying behaviour of the bricks and the porosity plus pore size distribution is observed. DIA discloses the evolution of size, shape and connectivity of macropores (r > 1 mm) and evidences that MIP results underestimate the macropore content. Conversely, MIP gives a good estimate of the open porosity and of the distribution of pores with r <1 mm. It is concluded that the combined use of these complementary techniques helps to fully characterise the pore system of bricks. These results as well as the study of the evolution of the speed of ultrasound waves vs. time yield useful information to evaluate the bricks physical–mechanical behaviour and durability. The relevance of these findings in the conservation of historic buildings is discussed. # 2003 Elsevier Ltd. All rights reserved.

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.

Liesegang pattern development in carbonating traditional lime mortars

Liesegang patterns, generally rings, bands, spheres or spirals, form in far–from–equilibrium systems in nature and in the laboratory by self–organized periodic precipitation of sparingly soluble phases following a nonlinear reaction–diffusion process. Although Liesegang patterns have been known for more than hundred years, there is still disagreement as to the mechanisms underlying this phenomenon. Most studies have focused on Liesegang pattern formation in gels, quantitative studies of quasiperiodic patterns in non–conventional porous media (e.g. construction materials) being rare. Here, we report the development of ‘revert’ three–dimensional Liesegang patterns (i.e. concentric ellipsoids) in traditional lime mortars undergoing carbonation. Portlandite (Ca(OH)2) in a quartz (SiO2) sand aggregate, transforms into calcite (Ca(CO)3) in contact with atmospheric CO2, resulting in banded cementation of the lime mortar. Surprisingly, well–developed Liesegang patterns only occur in mortars prepared using ‘aged’ lime putty, kept under excess water for years, following an ancient Roman recipe to improve slaked lime quality; the carbonation of these mortars being faster than in pattern–less ones. The smaller Ca(OH)2 particle size in the long–term–aged putty enhances dissolution and increases the ion–concentration product, while creating a higher volume of pores with r < 0.1 &mgr;m. These small pores can sustain very high supersaturation ratios with respect to CaCO3, resulting in higher nucleation rates, a crucial fact for pattern development previously neglected. These results may have strong implications for the understanding of Liesegang patterns, as well as for the conservation of architectural heritage.

Application limits of Q-switched Nd:YAG laser irradiation for stone cleaning based on colour measurements

Abstract The application limits of the laser-cleaning technique for different types of building stones have been investigated by measuring colour variations. The selected stones differ in their chemical and mineralogical composition, colour, texture and crystallinity degree. The experimentation was carried out with a Q-switched Nd:YAG laser. The colour variations on stones associated with different operative fluences were measured using a colorimeter. Further, surface morphological changes were examined under SEM. From the calculation of colour differences, a damage threshold fluence was established for each stone type. The response of the stones to laser radiation at a particular fluence was found to be mainly conditioned by their chemical and mineralogical composition and, to a less extent, by their textural characteristics.

Role of marble microstructure in near-infrared laser-induced damage during laser cleaning

When marble is cleaned by nanosecond neodymium yttrium–aluminum–garnet lasers (1064 nm), strongly absorbing surface contaminants are removed at fluences substantially below the damage threshold for the much less absorptive marble substrate. Recent studies have shown, however, that unacceptable roughening of the marble surface also may occur at low fluences due to removal of individual grains. In order to elucidate this effect, we have compared the low-fluence response of marbles with two different grain sizes and single-crystal calcite, in the fluence range 0.12–1.25 J cm−2. Damage was greater in fine-grained than coarse-grained marble, and did not occur in the single-crystal calcite at these fluences. The temperature rise following defect-mediated absorption triggers thermal plasma emission and generates shock waves; the concomitant surface damage depends on the size and crystallographic orientation of the crystals. Laser irradiation anneals the defects and increases “crystallite size.” The implications ...

Laser cleaning of stone materials: an overview of current research

Abstract This paper presents an overview of current knowledge and recent advances in the application of pulsed laser radiation to the cleaning of works of art, particularly ornamental stone. Special emphasis is put on the critical review of publications from the last six years. The recent history of laser cleaning in art conservation is summarised and a description of laser fundamentals, laser types and practicalities for use in conservation is given. Existing explanatory models of the underlying physics of laser cleaning are briefly described, as are the advances in laser cleaning of different stone types. Common techniques, including on-line monitoring used to detect material damage after laser irradiation, are outlined and examples are given of their application. Finally, current trends and improvements in laser-cleaning efficiency and the reduction of damaging side effects are addressed. It is hoped that this overview will bring greater attention to the issues that are important for future research on the laser cleaning of stone.

Amorphous and crystalline calcium carbonate phases during carbonation of nanolimes: implications in heritage conservation

Nanolimes are alcohol dispersions of colloidal Ca(OH)2 nanoparticles used as novel nanomaterials for the conservation of cultural heritage. Upon exposure to atmospheric CO2 at room T, and in the presence of H2O, they undergo carbonation forming CaCO3 cement which consolidates decayed porous materials such as stone or mural paintings. Despite extensive research on the synthesis and applications of nanolimes, little is known about the mechanisms and kinetics of the formation and transformation of metastable and stable calcium carbonate phases and their effects on the treatment efficacy. This is a strong handicap to their effective and widespread application. Here we show that the carbonation of nanolimes in humid air at room T involves the initial formation of amorphous calcium carbonate (ACC) and its transformation into metastable vaterite (and minor aragonite) via a dissolution–precipitation process, followed by non-classical nanoparticle-mediated crystal growth. Subsequently, vaterite (and aragonite) partially dissolves and stable calcite precipitates. All these phase transformations follow first order kinetics, where the rate controlling step is the amount of undissolved parent phase. We unambiguously demonstrate that precipitation of vaterite (up to ∼35 wt%) and aragonite (∼5 wt%) after ACC (up to ∼24 wt%) is favored by the alcohol adsorbed on Ca(OH)2 nanoparticles undergoing carbonation. Although it is known that vaterite formation limits consolidation, the fast kinetics of the solvent-mediated vaterite–calcite transformation (72% conversion in ten days) ensures that, in the short-term, the almost full consolidation potential of nanolimes can be achieved. Finally, the mechanistic and kinetic commonalities between nanolime carbonation and biomineralization/biomimetic synthesis of CaCO3 underline that the observed multistep crystallization and non-classical crystal growth might be general and applicable for the rational design of novel CaCO3 materials.

Protection and consolidation of stone heritage by self-inoculation with indigenous carbonatogenic bacterial communities

Enhanced salt weathering resulting from global warming and increasing environmental pollution is endangering the survival of stone monuments and artworks. To mitigate the effects of these deleterious processes, numerous conservation treatments have been applied that, however, show limited efficacy. Here we present a novel, environmentally friendly, bacterial self-inoculation approach for the conservation of stone, based on the isolation of an indigenous community of carbonatogenic bacteria from salt damaged stone, followed by their culture and re-application back onto the same stone. This method results in an effective consolidation and protection due to the formation of an abundant and exceptionally strong hybrid cement consisting of nanostructured bacterial CaCO3 and bacterially derived organics, and the passivating effect of bacterial exopolymeric substances (EPS) covering the substrate. The fact that the isolated and identified bacterial community is common to many stone artworks may enable worldwide application of this novel conservation methodology.Salt weathering enhanced by global warming and environmental pollution is increasingly threatening stone monuments and artworks. Here, the authors present a bacterial self-inoculation approach with indigenous carbonatogenic bacteria and find that this technique consolidates and protects salt damaged stone.

Crystallization and Colloidal Stabilization of Ca(OH)2 in the Presence of Nopal Juice (Opuntia ficus indica): Implications in Architectural Heritage Conservation

Hydrated lime (Ca(OH)2) is a vernacular art and building material produced following slaking of CaO in water. If excess water is used, a slurry, called lime putty, forms, which has been the preferred craftsman selection for formulating lime mortars since Roman times. A variety of natural additives were traditionally added to the lime putty to improve its quality. The mucilaginous juice extracted from nopal cladodes has been and still is used as additive incorporated in the slaking water for formulation of lime mortars and plasters, both in ancient Mesoamerica and in the USA Southwest. Little is known on the ultimate effects of this additive on the crystallization and microstructure of hydrated lime. Here, we show that significant changes in habit and size of portlandite crystals occur following slaking in the presence of nopal juice as well as compositionally similar citrus pectin. Both additives contain polysaccharides made up of galacturonic acid and neutral sugar residues. The carboxyl (and hydroxyl) functional groups present in these residues and in their alkaline degradation byproducts, which are deprotonated at the high pH (12.4) produced during lime slaking, strongly interact with newly formed Ca(OH)2 crystals acting in two ways: (a) as nucleation inhibitors, promoting the formation of nanosized crystals, and (b) as habit modifiers, favoring the development of planar habit following their adsorption onto positively charged (0001)Ca(OH)2 faces. Adsorption of polysaccharides on Ca(OH)2 crystals prevents the development of large particles, resulting in a very reactive, nanosized portlandite slurry. It also promotes steric stabilization, which limits aggregation, thus enhancing the colloidal nature of the lime putty. Overall, these effects are very favorable for the preparation of highly plastic lime mortars with enhanced properties.