Dedolomitization Reaction Research Articles

The role of hydrotalcite in the chloride binding of hydrated C3A pastes containing dolomite

To better understand the limitations of the hydrotalcite formation and the capacity of hydrotalcite to bind chloride in dolomite-containing Portland cement-based materials, the reaction of dolomite in C3A pastes and the corresponding chloride binding behavior were investigated in this study. The phase assemblages, the chloride binding capacity, and the pH value were determined. The results indicate that hydrotalcite can only form in a limited range of (active magnesium)/(available aluminum) ratios. The dissolution of dolomite, which provides the magnesium activity, rather than the dedolomitization reaction, appears to be what drives the dolomite reaction with C3A and is primarily dependent on dolomite content. The hydrotalcite formation is not affected by Ca(OH)2. Hydrotalcite seems to possess a greater capacity to bind chloride than monocarboaluminate in hydrated C3A pastes, especially at high chloride concentrations. Chemical chloride binding contributes approximately 10% to the total chloride binding capacity of hydrotalcite. The remaining dolomite reduces chloride binding by increasing carbonate ion activity, and Ca(OH)2 reacts with dolomite to decrease carbonate ion activity and thereby promote chloride binding.

Microstructure of Dolostones of Different Geological Ages and Dedolomitization Reaction.

Dolostone is widely distributed and commonly used as concrete aggregates. A large number of studies have shown that there are significant differences in the expansibility of different dolostones, and the key factors determining the expansibility of alkali carbonate rocks have not been clarified. In this paper, rocks were selected from five different geological ages: Jixianian, Cambrian, Ordovician, Devonian, and Triassic ages. The ordering degree and the content of MgCO3 of dolomites in rocks of different geological ages were determined by X-ray diffraction (XRD). The degree of dedolomitization reaction in rocks cured in 80 °C, 1 mol/L NaOH solution was determined by quantitative X-ray diffraction (QXRD). The morphology of dolomites in rocks was determined by a polarizing microscope. The products of the dedolomitization reaction were determined by field emission electron microscopy (FESEM-EDS). According to the test results, the following conclusions are drawn. There is a good positive correlation between ordering degree and the molar fraction of MgCO3 of dolomites. When the MgCO3 mole fraction of dolomites varies from 47.17% to 49.60%, the higher the MgCO3 mole fraction, the greater the ordering degree of dolomite. By analyzing the degree of the dedolomitization reaction of different dolostone powders cured at 80 °C in 1 mol/L NaOH solution, it is found that the older the geological age of dolostone, the slower the dedolomitization reaction rate and the lower the degree of dedolomitization reaction. The lower the ordering degree of dolomite crystal in the same geological age, the faster the rate of dedolomitization reaction and the higher the degree of dedolomitization reaction.

Reaction mechanism of dolomite powder in Portland-dolomite cement

The purpose of this study is to provide a better understanding of the reaction mechanism of dolomite powder in Portland-dolomite cement (PDC). The hydration products and microstructure of PDC pastes were investigated by using XRD, TGA, MIP, and SEM, with Portland-limestone cement (PLC) as a comparison. Results show that dolomite exhibited a considerably higher chemical reactivity than limestone as the curing temperature increased. Different from the conventional dedolomitization reaction with brucite formation, the dolomite reaction in PDC at 40 and 60 °C produced hydrotalcite and calcite at the expense of portlandite. Moreover, the formed hydrotalcite was found to result in the decomposition of the Al-bearing products, such as monosulfate, monocarbonate and ettringite, due to its lower solubility. Overall, the reaction of dolomite increased the solid volume, densified the microstructure, and thus resulted in a refined pore structure and a higher long-term compressive strength of PDC compared to PLC.

Expansive behavior of an alkali-carbonate reactive dolostone from Argentina: Proposal of an osmotic theory-based model to explain the expansion caused by the alkali attack

The expansion mechanism of dolomitic rocks in alkaline environments is complex and a current topic of debate, since two different chemical reactions can coexist: one linked with silica attack (alkali-silica reaction, ASR) and the other with dolomite attack (alkali-carbonate reaction, ACR). In previous works, a potentially reactive, fine-grained argillaceous dolomitic rock was identified in Argentina, with similar expansive characteristics to the typical American reactive carbonates. When this rock is used as aggregate in mortars or concretes, the dolomite experiences a clearly expansive dedolomitization reaction, which cannot be controlled by the classical techniques that allow the inhibition of ASR: supplementary cementitious materials (SCMs) (fly ash, natural pozzolan, silica fume, etc.) or Li-based admixtures. Since none of the classical theories proposed by the literature adequately describe the expansive behavior of this rock, an expansion mechanism based on the osmotic phenomenon is proposed in the present work. This mechanism describes the behavior of the Argentine dolostone more appropriately and allows explaining the effect of the main factors involved in the concrete expansion (pH of the pore solution, nature of the alkali cation associated with the OH−, role of water in the expansion, temperature, aggregate particle size, effect of the SCMs, etc.). The basis of the osmotic model is presented, and its advantages are discussed compared with the classical ACR mechanisms proposed by the literature.

Effects of different alkaline solutions on the expansivity of rock prisms of an alkali-reactive dolostone from Valcheta (Río Negro, Argentina)

A fine-grained argillaceous reactive dolostone was identified in Argentina. In alkaline conditions, a clear dedolomitization reaction occurred, accompanied by a deleterious expansive process. The purpose of this paper is to investigate the correlation between the rock prism expansion and the reaction progress. Rock expansion (in high and low porosity samples) was evaluated by a modified ASTM C586 test method, using NaOH, KOH, LiOH, and Ca(OH)2 solutions. The reactivity was verified in concrete mixtures applying a method similar to ASTM C1293, using the dolostone as coarse aggregate. To evaluate the reaction progress during the tests, mineralogical changes in the rock were quantified using XRD (Rietveld). In addition, the mercury porosimetry test was performed to investigate the changes in the rock pore structure. Finally, chemical analyses of the contact solutions (CO32−, OH−, Na+, K+, Li+) were done. Considering the results obtained, some controversial aspects of the theories proposed in the literature are analyzed.

Carbonate rocks as fillers in blended cements: Physical and mechanical properties

This study investigated the physical and chemical effects of dolostone (DF) as fillers (10 to 30% by weight) in blended portland cements (BPC) and compared with limestone (LF) ground with similar fineness. The addition of both fillers increases the packing density of BPC up to 40% replacement and the standard mortar, while thewater film thickness was greater than those corresponding to PC up to a w/cm ~ 0.45. For increasing filler replacement, themini-slump decreases, the setting time is in advance and the hydration heat curves show ashorter dormant period, acceleration of C3S-hydration and greater sulfate depletion.These phenomena were more significant for LF. The hydrated compounds and it apparition sequence were similar at 20 °C for DF and LF, but no sign of harmful dedolomitization reaction was foundat 40 °C during 365 days.The compressive strength of the mortar after 28 days depends on the percentage of filler replaced, regardless of its mineralogical composition. Results suggest that DF, as occurs with LF, is a suitable additionas main component to BPC withsatisfactory and stable properties.

Mineralogical Characterization of Dolomitic Aggregate Concrete: The Camarasa Dam (Catalonia, Spain)

The Camarasa Dam was built in 1920 using dolomitic aggregate and Portland cement with two different compositions: type A (dolomite and Portland cement) and type B (dolomite and sand-cement). The sand cement was a finely powdered mixture of dolomite particles and clinker of Portland cement. The mineralogy of concrete was studied by optical microscopy, scanning electron microscopy, and x-ray powder diffraction. Reaction of dedolomitization occurred in the two types of concrete of the Camarasa Dam, as demonstrated by the occurrence of calcite, brucite, and/or absence of portlandite. In the type A concrete, calcite, brucite, and a serpentine-group mineral precipitated as a rim around the dolomite grains and in the paste. The rims, a product of the dedolomitization reaction, protected the surface of dolomite from the dissolution process. In type B concrete, in addition to dolomite and calcite, quartz and K-feldspar were present. Brucite occurred in lower amounts than in the type A concrete as fibrous crystals randomly distributed in the sand-cement paste. Although brucite content was higher in the type A concrete, type B showed more signs of loss of durability. This can be attributed to the further development of the alkali-silica reaction in this concrete type.

Dedolomitization and reservoir quality: insights from reactive transport modelling

AbstractDedolomitization or calcitization, the replacement of dolomite by calcite, modifies the porosity and permeability of carbonate rocks. This study evaluates the effects of dedolomitization on the porosity of carbonates by means of geochemical and reactive transport simulations. The obtained results indicate that dedolomitization is a slow process that may be triggered by cold meteoric water but also by warm groundwater at temperatures of ≤50°C. Dedolomitization is more efficient with fluid fluxes on the order of tens of metres per year. Most of the simulations performed in this study resulted in a loss of porosity due to calcitization. The rate of the dedolomitization reaction increases significantly when calcium‐bearing minerals with faster dissolution kinetics and smaller molar volume than calcite, such as anhydrite, are present in the system. In such situations, the porosity of the rock increases significantly (approximately 10%) during a time span of hundreds of thousands of years. Therefore, depending on its original composition, fluid flow conditions and fluid composition, the porosity of a tight dolostone can be enhanced and reach porosity values of reservoir rocks after being calcitized.

Reaktywność Alkaliczna Kruszyw Węglanowych

In the construction industry carbonate aggregates are commonly used in processes such as concrete production. Aggregates which contain (in their mineral composition) dolomite and an admixture of clay minerals and amorphous silica, can react with alkalis. These reactions can lead to a destructive expansion in concrete. This article explains the mechanisms and the essence of this phenomenon. What is more, some effective and fast methods of the estimation and evaluation of Polish aggregates consisting of carbonate rocks suggests effective methods to determine the usefulness of Polish carbonate aggregates in concrete production are discussed in the paper. Underneath the quality criteria to determine the reactivity of the aggregates will be given. It has been agreed that alkaline reaction and expansion are two separate phenomena related to each other genetically. The aggregates in which reactions caused by clay-siliceous admixtures occur are subjected to expansion. Mineral composition of expansive aggregates as well as their texture indicate that epigenetic dolomites with a distinctive texture are the most reactive.The phase transformations do not end with a complete disintegration of dolomite. They have a cyclical character. They consist of interchangeable reactions of dedolomitization and dolomitization of secondary calcite formed as a result of dolomite's disintegration. The secondary calcite can be effected by Mg+2 ions from pores' solutions and it can form a secondary dolomite. The Mg2+ ions originate from brucite [Mg(OH)2], created in dolomitization process. As a consequence of its reaction with silica, brucite can dissolve and enrich secondary calcite with magnesium. Therefore the reactions which take place in reactive carbonate aggregates and concrete that is made of it are in fact ongoing processes which consist of dolomite's changes into calcite and vice versa. They are reactions between dedolomitization products (brucite, silica) and products from outside (water, alkalis). The described dedolomitization reactions are a phase of the process that enables expansion due to formation of pressure in inter-granular cracks, with pressure being a result of dry clay-minerals' expansion under the influence of water solutions.Loosening of the aggregate's structure as an effect of dedolomitization reaction makes it easier for water solutions to migrate far into the aggregate's grains followed by their contact with clay minerals.

Dedolomitization in different alkaline media: Application to Portland cement paste

The dedolomitization reaction kinetics is studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with alkalinity between 0.1 and 1 M KOH, at 25 and 75 °C. The experimental results are numerically simulated to calculate the apparent dissolution constant rates for dolomite, k dol. At low temperature, two dissolution stages were observed. In an early stage, part of dolomite powder dissolves quickly until an apparent steady is reached. After several days, the reaction continues at a lower rate. The calculated dissolution rate for dolomite in the first stage is one order of magnitude higher than that of the second stage. At 75 °C, the k dol is two orders of magnitude higher than at 25 °C. The addition of alkali increases the k dol at high temperature, but reduces it at room temperature.

Surface alteration of dolomite in dedolomitization reaction in alkaline media

The phases precipitated on the dolomite surface and the influence of alkalis on the properties of this surface were characterized. Experiments consisted of the immersion of single crystals of dolomite in saturated portlandite solutions with different alkalinity, temperature, and silica content. Most calcite forms in the solution as {104} rhombohedra. Brucite crystallizes as platelets, piles of platelets, sponge-like and {001} truncated ditrigonal pyramids attached to the dolomite surface. The morphology of brucite crystals depends on the pH of the solution. Apparent dissolution rate constants for dolomite at 75 °C are one order of magnitude higher than at room temperature. Initially, calcite grows on the dolomite surface with the same structural orientation {104}. In a second stage, when calcite has reached equilibrium with the solution, the new-formed {104} calcite surface starts playing an important role on the precipitation of brucite, while calcite continues crystallizing in the bulk solution. The calcite surface, negatively charged, can adsorb specifically metal cations. Mg 2+, liberated by the dissolution of dolomite, would move attached to the surface, until they are trapped on these negatively charged surfaces, while OH − supplied by the alkaline solution favors the nucleation and growth of brucite. The attachment of Mg 2+ to the surface controls the place where brucite grows.

Kinetics of dolomite–portlandite reaction: Application to portland cement concrete

The dedolomitization reaction kinetics are studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with different alkalinity, temperature and silica content. The experimental results are reproduced through computer simulation, which allows the estimation of the apparent dissolution constant rates for dolomite. These are discussed together with other parameters influencing the kinetics, in particular the modification of the specific surface of dolomite. The parameters obtained make it possible to predict the behavior of the system beyond experimental periods. Both experimental and simulated results are discussed in connection with the expansion and cracking occurrences observed in portland mortars and concretes made with dolomitic limestone.

Mechanism of dedolomitization and expansion of dolomitic rocks

Based on thermodynamic analysis of the dedolomitization reaction and calculation of the volume change associated with this reaction by the principle of the tightest pile of particles, the influences of pH value on alkali-dolomite reaction and expansion were investigated. Dedolomitization reaction and its expansibility were evaluated by examination of dolostone powder-cement compacts and dolomitic rocks containing few clays and quartz. Dolomite reacts with hydroxyl to form CO 3 2− ion, brucite and calcite, which are fine, generally smaller than 1 μm, and enclose many voids. It is confirmed by seveal means that dedolomitization reaction can directly bring about expansion. The rates of both the reaction and expansion depend markedly on the pH value of solutions. The higher the pH value, the larger the rates. When pH value is low enough, the reaction does not take place, thus no expansion appears.

Experimental Hydrothermal Dedolomitization: ABSTRACT

Hydrothermal replacement of dolomite by calcite has been examined through experimental reactions carried out in teflon-lined stainless steel bombs, at temperatures ranging from 50° to 200°C (122° to 392°F), at End_Page 456------------------------------ equilibrium vapor pressure, and for durations of days to months. Solution compositions were varied to examine the effects of ionic strength, Mg2+/Ca2+ ratio, and SO42- concentration on the direction and/or rate of reaction. The starting solid for these experiments was a stoichiometric and well-ordered synthetic dolomite; several experiments were also seeded with solid reagent-grade calcium carbonate (calcite). In pure CaCl2 solution (0.1 M) replacement was complete in less than one week at temperatures of 100°C (212°F) and greater. Replacement was minor after several weeks at 50°C (122°F). Longer term experiments at 50° are still underway. A series of experiments established that dedolomitization only occurs at Mg2+/Ca2+ ratios of less than 1/10 (total ionic strength = 0.1) at all temperatures from 100° to 200°C (212° to 392°F). Because this reaction proceeds far faster than the reverse reaction (dolomitization), we believe this result establishes the most precise calcite/dolomite phase boundary to date. The presence of SO42- was found to strongly inhibit the dedolomitization reaction, even a concentrations far less than in seawater. Seeding the reaction with a small amount of calcite had little effect on the rate of reaction, suggesting that nucleation is not rate-limiting in dedolomitization. Finally, increasing ionic strength from 0.01 up to 0.5 increased the rate of reaction. End_of_Article - Last_Page 457------------

Mechanism of the alkali-carbonate rock reaction

Summary This paper reviews the main hypotheses proposed for the alkali-carbonate rock reaction, and presents evidence supporting the mechanism advanced by this laboratory. The latter postulates that the alkali-expansivity of certain carbonate rocks containing dolomite is due to a wetting of previously unwetted clay minerals; the wetting is made possible by the de-dolomitization reaction which produces access channels for moisture. The new evidence consists of (a) no volume change of pure dolomite crystals immersed in 2M NaOH for 4 months; (b) the decreasing expansion of solid plus liquid of rock powders in 2M NaOH with increasing particle size of pure dolomite, using a dilatometer technique; (c) the observation from scanning electron micrographs of alkali-reactive and non-reactive rocks that much of the clay constituent appears to be distributed close to the dolomite rhombs, thus accounting more readily for the idea that dedolomitization opens up channels for moisture to gain access to the clay; and (d) a detailed study of some 50 carbonate rocks from the highly calcitic to the highly dolomitic with special emphasis on petrographic examinations and their correlations with results of laboratory performance.

Alkali reactivity of dolomitic limestone aggregate

Summary Additional evidence was obtained in support of the hypothesis that excessive expansion in the alkali-carbonate rock reaction is caused by moisture uptake by the previously ‘unwetted clay’ constituent. The dedolomitization reaction is shown to be necessary only to provide access of moisture to the ‘locked-in’ clay. A microscopic method was used to determine the specific surface of the dolomitc, the quantitative nature of which made it possible to define a practical upper limit below which expansion does not occur. It was also shown that relatively small percentages of illitic clay can account for the excessive expansion shown by reactive carbonate rocks. The new techniques used included measurements of dimensional change of thin rock wafers and of compacts of clay and powdered rock.