Phase transitions within the lime cycleimplications in heritage conservation

Resumen

This thesis deals with the study of three different processes within the lime cycle. These are: 1) thermal decomposition of dolomite (CaMg(CO3)2), 2) hydration of lime (CaO) and 3) carbonation of calcium hydroxide (Ca(OH)2). While the thermal decomposition of calcite, which represents the first step of the lime cycle, has been studied thoroughly, and its mechanisms is now well established, this is not the case of the other relevant carbonate involved in the lime cycle, that is, dolomite. In fact, and despite over one hundred year studies, the thermal decomposition of dolomite is still a poorly understood process. In order to understand the ultimate mechanism(s) of such a reaction, dolomite single crystal were calcined in air at 500 ºC up to 1000 °C and in situ, in the TEM (high vacuum), following irradiation with the electron beam. In situ TEM shows that the decomposition involves the initial formation of a face centered cubic mixed oxide structure (Ca0.5Mg0.5O) with the following reactant/product orientation relationships: [001]dolomite//<111>oxide, <-441>dolomite//<100>oxide, {11.0}dolomite//{110}oxide, {11.8}dolomite//{110}oxide and {10.4}dolomite^{100}oxide ~12º. This phase undergoes de-mixing into oriented crystals of Mg-poor CaO and Ca-poor MgO solid solutions upon long term e-beam exposure. Ex situ TEM, XRD, 2D-XRD, and FESEM analyses show the formation of porous pseudomorphs made up of oxide nanocrystals with similar parent/product orientation relationships, but with limited Ca/Mg substitution (up to ~9-11 %) due to de-mixing (spinodal decomposition) of the metastable (Mg,Ca)O precursor. High ion diffusivity at T > 500 °C (ex situ experiments) favors the formation of pure CaO and MgO crystals during coarsening via oriented aggregation and sintering. These results show that the thermal decomposition of dolomite is topotactic (shear-transformation) and independent of pCO2 (i.e., vacuum or air). Formation of Mg-calcite nanocrystals (up to ~8 mol % Mg) during the so-called ¿half decomposition¿ is observed at 650-750 ºC. This transient phase formed topotactically following the reaction of CaO nanocrystals (solid solution with ~9 mol % Mg) with CO2 present in the air and/or released upon further dolomite decomposition. With increasing T, Mg-calcite transformed into calcite which underwent decomposition following the known topotactic relationship: {10.4}calcite//{110}CaO and <-441>calcite//<110>CaO. These observations solve the long standing controversy regarding the mechanism of the ¿two-stage¿ decomposition of dolomite which assumed the direct formation of calcite during the so-called ¿half-decomposition¿ of dolomite. It has been hypothesized that hydration of lime, which represents the second step of the lime cycle, can take two different reaction pathways. The first one involves direct precipitation of calcium hydroxide from a supersaturated solution formed upon the dissolution of CaO in the slaking water. The second refers to the reaction between solid CaO and water vapor. Water is vaporized under the influence of heat released during hydration. The latter (i.e., vapor phase hydration) has received less attention in the literature. In order to shed some light on the mechanism(s) of vapor phase hydration as well as the crystallographic control in the advancement of the reaction CaO pseudomorphs (formed upon calcination in air at 700 ºC up to 900 °C of calcite single crystals) were subjected to the effect of air de-voided of CO2 at 50% RH. Crystallite size measurements (XRD) yielded equal values regardless the initial size of CaO crystals. FESEM analysis show the pseudomorphic character of hydration. This two observations are consistent with a coupled dissolution/precipitation reaction. Pole figure (2D-XRD) and HRTEM analyses point to the topotactic nature of the reaction with the following orientation relationships between lime and portlandite: {11.0}portlandite//{110}lime and {00.1}portlandite//{111}lime. The topotactic nature of the reaction points to a solid state replacement of lime by portlandite. Most probably the selection of the ruling mechanism is controlled by RH and microstructural features, e.g., porosity, crystal size, which depend on the calcination T and retention time, and evolve as hydration progresses. The reaction between calcium hydroxide (portlandite) and carbon dioxide, the so-called carbonation, which occurs via the reaction Ca(OH)2 + CO2 ¿ CaCO3 + H2O, results in the formation of precipitated calcium carbonate (PCC). This is the third, and the last stage of the lime cycle. Here, carbonation was performed in three different routes: ¿ precipitation at the water-air interface in solutions saturated with respect to Ca(OH)2 (lime water) and exposed to air, with and without excess of solid Ca(OH)2 ¿ carbonation of portlandite single crystals placed in air at 93% RH for 4 moths ¿ carbonation of two types of hydrated lime pastes (HLP) by exposition to air at 93% RH for 4 months All experiments were performed at room temperature (20 ± 2 ºC) and pCO2 ~10-3.5 atm. Additionally to experiments, simulation of pore water chemistry in saturated calcium hydroxide solution in contact with air was performed using PHREEQC software. The carbonation in lime water (pH ~12.4) results in early precipitation of unstable ACC, which subsequently undergoes transformation to calcite, with no metastable vaterite/aragonite. TEM-SEAD reveals that ACC is characterized by short range order (proto-calcite), which predetermines the formation of calcite. Under excess of solid Ca(OH)2 in lime water, precipitation of scalenohedral calcite is favored. In the absence of additional Ca(OH)2, calcite shows rhombohedral features. Carbonation of portlandite single crystals results in pseudomorphs, that fully preserve the external hexagonal shape. 2D-XRD analysis discloses the epitactic nature of the process, showing the following crystallographic relationship: <001>calcite//<001>portlandite. This behavior is explained by considering the structure of hexagonal portlandite which is made up of alternating Ca2+ and OH- layers piled up along the c-axis. Such a structure matches the calcite structure where alternating layer of Ca2+ and CO32- pile up along the three-fold axis of this hexagonal phase. The crystallographic control in the advancement of the process along with the preservation of the external shape of the parent phase (pseudomorphism) point to a coupled dissolution/precipitation replacement of portlandite by calcite. Such a process progresses due to the contrasting molar volume of and solubility differences between parent and product phases which leads to porosity development and enables the advancement of the reaction front to the core of the replaced phase. In-situ XRD analysis of solution with excess solid Ca(OH)2 (portlandite) subjected to carbonation in air, along with field emission electron microscopy (FESEM) observations of carbonated portlandite single crystals, revealed that dissolution of portlandite is strongly anisotropic. Dissolution of portlandite prism faces progress faster than dissolution of basal planes. This is consistent with the features of portlandite crystal structure. Carbonation of lime pastes involves two processes: precipitation of calcite from solution and direct replacement of portlandite crystals by calcite via coupled dissolution/precipitation reaction. Morphology and crystal size of calcite precipitated during carbonation of HLP are strongly affected by the surface area and pore size distribution of lime pastes, which also evolves while carbonation progresses. The processes considered in this thesis have a great importance in many different fields: industrial, geochemical, pharmaceutical. In each of these areas, control over a process, leading to a product with desirable properties, is based on the understanding of its principles. The understanding of the mechanisms of the these reactions can shed some light on the problem of use of traditional lime based products in the field of cultural heritage conservation.