Olga Cazalla

Carbonate and silicate phase reactions during ceramic firing

Mineralogical, textural and chemical analyses of clay-rich materials following firing, evidence that initial mineralogical differences between two raw materials (one with carbonates and the other without) influence the tex- tural and mineralogical evolution of the ceramics as T increases from 700 to 1100° C. Mineralogical and textural changes are interpreted considering local marked disequilibria in a system that resembles a small-scale high- T meta- morphic process ( e.g., contact aureoles in pyrometamorphism). In such conditions, rapid heating induces significant overstepping in mineral reaction, preventing stable phase formation and favoring metastable ones. High- T transfor- mations in non-carbonate materials include microcline structure collapse and/or partial transformation into sanidine; and mullite plus sanidine formation at the expenses of muscovite and/or illite at T ‡ 800° C. Mullite forms by mus- covite-out topotactic replacement, following the orientation of mica crystals: i.e., former (001) muscovite are ^ to (001)mullite. This reaction is favored by minimization of free energy during phase transition. Partial melting followed by fingered structure development at the carbonate-silicate reaction interface enhanced high- T Ca (and Mg) silicates formation in carbonate-rich materials. Gehlenite, wollastonite, diopside, and anorthite form at carbonate-silicate interfaces by combined mass transport (viscous flow) and reaction-diffusion processes. These results may add to a better understanding of the complex high- T transformations of silicate phases in both natural ( e.g., pyrometamor- phism) and artificial ( e.g., ceramic processing) systems. This information is important to elucidate technological achievements and raw material sources of ancient civilizations and, it can also be used to select appropriate clay com- position and firing temperatures for new bricks used in cultural heritage conservation interventions.

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.

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.

Behavior of Brick Samples in Aggressive Environments

The weathering of different brick samples ina range of aggressive environments has been studied.Brick samples were prepared using two clay types (fromGranada, Spain), different additives, and a range offiring temperatures (850–1100 °C). The brickscompositional and textural characteristics wereevaluated using XRD, SEM, hydric tests and mercuryintrusion porosimetry (MIP). The samples weresubjected to accelerate aging, including wet-dry,freeze-thaw and salt crystallization cycles. The decayof the bricks in polluted atmospheres was simulated ina static chamber containing sulfur dioxide (SO2)at 25 °C and 50% relative humidity. Samplesfired at 1000 °C proved to be the most durable,with better hydric behavior (fast drying and slowwater absorption) and fewer micropores. However, theywere not suitable for salt-rich environments (badperformance in the salt decay test). Samples fired at850 °C turned out to be more resistant to saltdecay, but they showed a poor hydric behavior (slowdrying and rapid water absorption) and littleresistance to freeze-thaw and wet-dry cycles. Samplesfired at 1100 °C had good hydric behavior, theyperformed well in the salt decay test, but they didnot perform as well as the samples fired at1000 °C in most accelerated aging tests. Gypsumformed on all the brick samples submitted to SO2atmosphere regardless exposure time (e.g. gypsumappears following just 24 h of exposure),composition, or firing temperature. Samples withdeposited particulate matter collected from vehicleexhausts (diesel, as well as leaded and non-leadedgasoline motor cars) resulted in the fastest gypsumdevelopment and greater abundance. On the other hand,the blank samples, and the samples withpollution-derived dust collected from historicalbuildings showed little gypsum development. Theimplications of these results in historicalbrick-building preservation in a range of aggressiveenvironments, and in polluted atmospheres inparticular, are discussed.

Three-way ANOVA interaction analysis and ultrasonic testing to evaluate air lime mortars used in cultural heritage conservation projects

Lime mortar has been used throughout history despite current substitution by hydraulic mortars (cements). The chemical composition of the lime used in its manufacture, however, is determined by local geology. In addition, the type of slaking, which depends on the amount of water used, gives rise to different types of lime. The result is that the behavior of lime mortar can vary depending on the composition and type of lime used. A three-way ANOVA analysis was carried out to determine the composition, type, and temporal evolution and the interactions of these three variables for evaluation of the characteristics of air lime mortars for their use in cultural heritage conservation projects.

Ultrasound and mechanical tests combined with ANOVA to evaluate brick quality

Abstract Mixtures of clays are often used in the manufacture of bricks, with distinct additives for diverse ends. The firing process, however, determines the final properties of the material. In this work, ultrasound and mechanical tests have been combined with a statistical tool, the analysis of variance (ANOVA), to analyse the mineralogical and physical characteristics of solid bricks manufactured from mixtures of local clays to which small amounts of additives have been included. They were then fired at different temperatures, ranging from 850 up to 1100°C. A new parameter, defined as the “resistance anisotropy” has been formulated to simplify the statistical interpretations. In addition, a two-way ANOVA interaction analysis has been used to evaluate the effect that selected factors (addition/absence of additives and firing temperature) have on the technical quality of bricks.