Search Results (5)

The alkali–dolomite reaction (ADR) describes the interaction between alkalis in concrete and dolomite which results in dedolomitization, leading to cracking and deterioration of the concrete. A large number of research has explored the chemical products associated with the ADR, mechanisms of expansion, and methods of identification, but our understanding of the occurrence and progression of the ADR chemical reaction is substantially limited. Key factors controlling the ADR chemical reaction are generally not understood. This paper investigates the migration process of alkali ions in dolomitic limestone and reaction process with dolomite crystals and alkali. Dolomitic limestone samples were selected for experimentation. The amount of Sodium (Na⁺) was measured as a means of assessing alkali ion migration. We measured the degree of dedolomitization using X-ray diffraction (XRD). Microstructure was evaluated using field emission scanning electron microscopy (FESEM). This research provides new insights into dedolomitization. The pore network provides the physical pathway for alkali ion migration. Concentration gradients drive the migration of alkali ions, and their interactions control the efficiency of alkali ion migration. Full article

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. Full article

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. Full article

In this paper, a tetramethylammonium hydroxide (TMAH) solution and homemade cement without alkali were used to eliminate the influence of the alkali-silica reaction (ASR) on the expansion of dolomitic rocks, and a NaOH solution was used as a comparison agent. The expansion of concrete microbars and dolomite powder compacts prepared from dolomitic rocks was tested. The expansion cracks and reaction products were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results showed that TMAH reacts with dolomite crystals in dolomitic rocks to form brucite and calcite. Through X-ray diffraction and SEM-EDS analysis, it can be determined that the chemical reaction between TMAH and dolomite crystal was dedolomitization. The expansion stress test and concrete microbar expansion test suggest that the alkali carbonate reaction (ACR) can produce expansion. Although both the ASR and the ACR were observed in the NaOH reaction system, but ASRgel was not found in the cracks, indicating that the ASR may be involved in the expansion process of concrete microbars and that the ACR is the root cause of the expansion. However, under the curing conditions of the TMAH solution, many ACR products were found around the crack, indicating that the expansion of the concrete in this system was caused entirely by the ACR. Full article

The comprehensive geological, hydrogeological and hydrogeochemical model of the Liulin karstic spring area in the eastern limb of the Ordos syncline was established by a combination of chemical thermodynamics, chemical kinetics and hydrogeology. The study area was divided into four zones based on the saturation indices of calcite, dolomite and gypsum, which were computed by the groundwater-chemical simulation software PHREEQC (a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations), with consideration of the geological and hydrogeological conditions and hydro-geochemical reactions. The weight and volume modulus of carbonate rocks and sulphate rocks in each zone were calculated by the method of correlation analysis to evaluate the dissolution law of karst groundwater. The results showed that in the zone I (saturation index of calcite βc ≤ 1) the dissolution of calcite was the major geochemical reaction, the weight modulus of calcite was higher than that of dolomite and gypsum, and the pore space generated by the dissolution of calcite was one order of magnitude larger than that of dolomite and gypsum. In zone II (saturation index of calcite βc > 1 to saturation index of dolomite βd ≤ 1) the corrosion moduli were all smaller than that in zone I, the solubility of dolomite and gypsum increased, and calcite reached saturation. The space occupied by the calcite sediment was less than that dissolved by dolomite and gypsum. In zone III (saturation index of dolomite βd > 1 to saturation index of gypsum βg ≤ 1), calcite and dolomite had reached saturation, accompanied by dedolomitization, and the amount of gypsum dissolution increased obviously. The conclusions indicate that the hydrogeochemical environment plays an important role in mineral dissolution. Full article