Synthetic Marbles Research Articles

Synthetic White Marble-Like Material Produced from Natural Raw Materials

An alternative natural marble (such as Thassoss marble, Greek) was synthesized and produced using natural raw materials, namely silica sand, magnesite, limestone, phosphate and fluorspar. A ceramization casting of glass powder was employed to produce a similar tile to the common natural marble with outstanding properties. Different batch compositions subjected to mixing, melting, quenching, pulverizing, casting and sintering were employed. X-ray diffraction analysis and scanning electron microscopy were used for characterization. The physical and mechanical properties of the produced synthetic samples were investigated. These properties include density, water absorption, porosity, hardness, compression and flexural strength. Some of these properties of synthetic marble are better than the common tile similar to marble or granite.

The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep

We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800°C and at strain rates between 10−6 and 10−3 s−1. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation.

The effect of dissolved magnesium on diffusion creep in calcite

We experimentally tested a series of synthetic calcite marbles with varying amounts of dissolved magnesium in a standard triaxial deformation machine at 300 MPa confining pressure, temperatures between 700 and 850°C, stresses between 2 and 100 MPa, and strain rates between 10 −7 and 10 −3 s −1. The samples were fabricated by hot isostatic pressing of a mixture of calcite and dolomite at 850°C and 300 MPa. The fabrication protocol resulted in a homogeneous, fine-grained high-magnesian calcite aggregate with minimal porosity and with magnesium contents between 0.07 and 0.17 mol% MgCO 3. At stresses below 40 MPa the samples deformed with linear viscosity that depended inversely on grain size to the 3.26±0.51 power, suggesting that the mechanisms of deformation were some combination of grain boundary diffusion and grain boundary sliding. Because small grain sizes tended to occur in the high-magnesium calcite, the strength also appeared to vary inversely with magnesium content. However, the strength at constant grain size does not depend on the amount of dissolved magnesium, and thus, the impurity effect seems to be indirect. At stresses higher than 40 MPa, the aggregates become non-linearly viscous, a regime we interpret to be dislocation creep. The transition between the two regimes depends on grain size, as expected. The activation energy for diffusion creep is 200±30 kJ/mol and is quite similar to previous measurements in natural and synthetic marbles deformed at similar conditions with no added magnesium.

Grain growth and inclusion formation in partially molten carbonate rocks

To learn more about the kinetics and mechanisms of coarsening and melt inclusion formation, we investigated the effects of melt content, viscosity, and topology on the microstructural evolution of partially molten and melt-free calcite aggregates. Synthetic marbles with eutectic melts were produced by annealing mixtures of calcite and either calcium hydroxide or lithium carbonate for up to 80 h at a confining pressure of 300 MPa and temperatures of 973–1,023 K. The melts produced in the two systems are expected to differ significantly in viscosity. Generally, coarsening rates decrease with increasing melt fraction, probably because the diffusion length across melt pockets increases. Analysis of grain shapes in the samples with about 40% melt indicated that coarsening was accommodated by agglomeration in the samples of the calcium/lithium carbonate system. In the calcium carbonate/hydroxide system, classical Ostwald ripening occurred. For melt contents about 10% and below, melt-filled pores are either dropped from or dragged along with migrating grain boundaries, depending on the pore size and the grain boundary curvature. These data can be used to constrain the conditions where fluid or melt inclusions form under natural conditions. Combining our results and previous studies illustrates a systematic relation between the grain boundary mobility in calcite aggregates and the diffusion kinetics associated with second phases residing on the grain boundaries. In particular, boundaries with no porosity are most mobile, those boundaries dragging melt-filled pores are slower, those with gas-filled pores are slower yet, and those containing solid phases are slowest or may even be motionless.

Microcrack growth and healing in deformed calcite aggregates

Modelling of fluid flow in the Earth's crust depends on our understanding of the physical processes responsible for porosity-production and porosity-reduction. This study concerns the role of thermally-activated processes on the evolution of porosity and permeability in rock samples containing microcracks. Experimentally deformed Carrara marble and a synthetic marble were heat-treated at effective pressures of 50 and 150 MPa and temperatures up to 923 K with argon as pore fluid. In situ measurements of the connected porosity and the permeability during heat-treatment indicate a complicated evolution of porosity and permeability. During heating up to 600/700 K, the porosity changes very slowly with increase in temperature and time, decreasing by 0.01–0.02, while the permeability increases by an order of magnitude. At higher temperatures, the porosity reduction rate accelerates, and both the connected porosity and the permeability decrease with increasing temperature and time. The permeability is proportional to the cube of the connected porosity. Microstructural changes during heat-treatment of Carrara marble include (a) the replacement of the finely-crushed fragments within the intergranular damage zones by rounded or polyhedral grains 5–20 μm in diameter; and (b) the partial to complete healing of twin boundary cracks and tips/sub-branches of transgranular cracks. In the finer grain-size synthetic marble, heat-treatment blunts the tips of grain boundary cracks; partial healing of grain boundary cracks is associated with the formation of ridge-channel structures on many grain interfaces. The evolution of porosity, permeability and microstructure points to the roles of thermal cracking, plastic compaction, and diffusional crack healing in controlling fluid transport properties of cracked rocks at elevated temperature. Significantly, the porosity–permeability relationships during isostatic crack healing are very different from those associated with crack growth during deformation.

Effect of quartz inclusions on plastic flow in marble

To investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks, we produced synthetic marbles with a grainsize of 5–25 µm, containing quartz of similar size, with volume fractions of 0, 5.1, and 20.4 vol.%, and between 3 to 9% porosity. These rocks were deformed in triaxial compression at a strain‐rate of 3 × 10−5 s−1 confining pressure of 200 MPa, and 600°C. Marbles containing 20.4 vol.% quartz were almost 5 times stronger than those containing no quartz. The work‐hardening rate at given strain increased with quartz content. Marbles containing 20.4 vol.% quartz work‐hardened up to 10% strain whereas in single‐phase samples work hardening was negligible beyond 3% strain. The dislocation density in the calcite increased with increasing quartz content. The data are consistent with models for the strength of ceramic matrix composites, or with the hypothesis that strength is increased by decreasing porosity.

Brittle and semibrittle deformation of synthetic marbles composed of two phases : Dresen, G; Evans, B J Geophys ResV98, NB7, July 1993, P11921–11933

Brittle and semibrittle deformation of synthetic marbles composed of two phases : Dresen, G; Evans, B J Geophys ResV98, NB7, July 1993, P11921–11933

Brittle and semibrittle deformation of synthetic marbles composed of two phases

To investigate the influence of rigid, second‐phase particles on the strength of marble in the semibrittle deformation regime, we performed conventional triaxial mechanical tests on synthetic samples formed by hot isostatic pressing two‐phase aggregates of fine‐grained calcite and 5–20 wt % of either SiC, Al2O3, or SiO2. The samples were tested at strain rates of 10−5 s−1 and at room temperature over a range of pressures from 5 to 300 MPa. Microstructures were studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Adding rigid inclusions with incoherent matrix/particle interfaces increased the range of pressure over which the transition between localized brittle failure and crystal plastic flow occurred. The strength of the pure marbles was inversely proportional to porosity between 0 and 8%. In the brittle field, two‐phase aggregates are generally 50 MPa weaker than pure marbles of similar porosity. Second phases and associated cracklike porosity tend to distribute brittle deformation in our samples. Aggregates with 20% inclusions show stable cataclastic flow at 5 MPa confining pressure. At the same pressure, pure samples fail catastrophically along a distinct fault. At higher pressure in the semibrittle field, the strengths of pure and two‐phase aggregates converge. The deformation resistance of the material containing particles is consistently higher than that of the pure samples. TEM observations also suggest an additional contribution to the deformation resistance and hardening rate by interactions between the dispersed particles and crystal defects.

Acid-base interactions: adhesion and the elimination of cavities in the production of synthetic marble

Cavities in synthetic marble can be eliminated not only through the use of microspheres but also with silica aggregates of a particular size range at given ratios of aggregates (sand/CaCO 3) and reactants (monomer/moulding powder). This result arises from acid-base interactions in the polymer/silica/calcium carbonate system. The elimination of cavities is accompanied by an enhanced resistance to compression loading. Factory products based on these findings have no cavities.

Grain growth in synthetic marbles with added mica and water

Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 μm (D 0) to 65 μm in 24 hours. Growth rates fit the equation, D n -D 0 =Kt, where K is a constant and n≃2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, D max, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once D max is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.