Calcium-rich Solution Research Articles

Study on the Effect of Cations on Morphology in the Preparation of Vaterite Calcium Carbonate from Dolomite

AbstractIn this paper, the effect of trace components in dolomite on the morphology of vaterite calcium carbonate is studied in the CaCl2‐NH3‐CO2 system, with a focus on the effect of cations (Mg2+, Fe3+, Si4+, and Al3+) in the solution. Ca2+, NH4+, Mg2+, Fe3+, Si4+, and Al3+ in the digestion solution are proportionally prepared into a solution by analyzing the calcium rich digestion solution which is obtained by digesting dolomite with ammonium chloride solution. Under the optimal conditions, NH3 is introduced at a rate of 0.5 L min−1 for 1 h, CO2 is introduced at a rate of 0.5 L min−1 for 1 h and rotation speed of 500 r min−1 to prepare the vaterite calcium carbonate. The results show that the addition of Mg2+, Fe3+, Si4+, and Al3+ can promote the growth of vaterite calcium carbonate. Among them, adding Mg2+ and Si4+ can promote the dispersion of vaterite, Fe3+ and Al3+ can cause agglomeration of vaterite. Material Studio software is used to predict the crystal morphology of vaterite calcium carbonate under ideal conditions, and the calculation results are basically consistent with the experimental results.

Selective removal behavior of lead and cadmium from calcium-rich solution by MgO loaded soybean straw biochars and mechanism analysis

Modified biochars has great potential for removing heavy metals from aquatic environments, but the removal of heavy metals by biochars is usually significantly affected by the co-presence of the macro amount of metal ions, such as Ca. Enhancing the ion exchange capacity of biochar by increasing its alkali metal content is a very prospective method to improve its selectivity. In this paper, MgO loaded biochar (MBC) was synthesized by co-pyrolysis of soybean straw and MgCl2·6H2O for selective remove Pb and Cd from calcium-rich wastewater. MBC exhibited excellent selective adsorption performance for Pb and Cd in calcium-rich wastewater due to the successful loading of MgO. The adsorption capacities of MBC for Pb and Cd were 582.57 and 167.40 mg/g, and the removal efficiency of Ca below 2.5% with an initial concentration of 800 mg/L. The ion exchange capacities of Pb and Cd enhanced almost 27 and 23 times than BC. By analyzing the results of BET, XRD, SEM-EDS, XPS and FTIR, the adsorption mechanisms of MBC were mainly including ion exchange, precipitation with minerals, and interaction with oxygen-containing functional groups. The easy preparation method and high selective adsorption capacity makes MBC an ideal alternative for efficiently selective removal Pb and Cd from calcium-rich wastewater.

Microfluidic study in a meter-long reactive path reveals how the medium’s structural heterogeneity shapes MICP-induced biocementation

Microbially induced calcium carbonate (CaCO3) precipitation (MICP) is one of the major sustainable alternatives to the artificial cementation of granular media. MICP consists of injecting the soil with bacterial- and calcium-rich solutions sequentially to form calcite bonds among the soil particles that improve the strength and stiffness of soils. The performance of MICP is governed by the underlying microscale processes of bacterial growth, reactive transport of solutes, reaction rates, crystal nucleation and growth. However, the impact of pore-scale heterogeneity on these processes during MICP is not well understood. This paper sheds light on the effect of pore-scale heterogeneity on the spatiotemporal evolution of MICP, overall chemical reaction efficiency and permeability evolution by combining two meter-long microfluidic devices of identical dimensions and porosity with homogeneous and heterogeneous porous networks and real-time monitoring. The two chips received, in triplicate, MICP treatment with an imposed flow and the same initial conditions, while the inlet and outlet pressures were periodically monitored. This paper proposes a comprehensive workflow destined to detect bacteria and crystals from time-lapse microscopy data at multiple positions along a microfluidic replica of porous media treated with MICP. CaCO3 crystals were formed 1 h after the introduction of the cementation solution (CS), and crystal growth was completed 12 h later. The average crystal growth rate was overall higher in the heterogeneous porous medium, while it became slower after the first 3 h of cementation injection. It was found that the average chemical reaction efficiency presented a peak of 34% at the middle of the chip and remained above 20% before the last 90 mm of the reactive path for the heterogeneous porous network. The homogeneous porous medium presented an overall lower average reaction efficiency, which peaked at 27% 420 mm downstream of the inlet and remained lower than 12% for the rest of the microfluidic channel. These different trends of chemical efficiency in the two networks are due to a higher number of crystals of higher average diameter in the heterogeneous medium than in the homogeneous porous medium. In the interval between 480 and 900 mm, the number of crystals in the heterogeneous porous medium is more than double the number of crystals in the homogeneous porous medium. The average diameters of the crystals were 23–46 μm in the heterogeneous porous medium, compared to 17–40 μm in the homogeneous porous medium across the whole chip. The permeability of the heterogeneous porous medium was more affected than that of the homogeneous system, while the pressure sensors effectively captured a higher decrease in the permeability during the first two hours when crystals were formed and a less prominent decrease during the subsequent seeded growth of the existing crystals, as well as the nucleation and growth of new crystals.

Preparing Mg(OH)2-based materials by electro-deposition method from magnesium- and calcium-rich brine simulant

Pollution to the ocean caused by the disposal of reject brine is a serious environmental concern. Recycling the ions in reject brine for materials manufacturing can mitigate this environmental burden and create new value streams. This paper presents a way of preparing Mg(OH)2-based building materials from magnesium- and calcium-rich solutions through electro-deposition. The validity of this method in two scenarios, namely brine simulant and seawater simulant, was assessed by comparing the efficiency of deposition; and the properties of the deposits were investigated by X-ray diffractometry (XRD), thermo-gravimetric (TG), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and atomic force microscope (AFM). It was found that a Mg(OH)2-based material mixed with CaCO3 and Ca(OH)2 could be electrically deposited on the surface of the cathode. The mass growth rate, mechanical performance, and compactness of the deposits prepared from the brine simulant were better relative to those of the deposits prepared from the simulated seawater. On the micro-scale, the particles grown from the brine simulant showed a higher packing density and smaller size, which could have a positive effect on the mechanical property of the deposits. Overall, magnesium- and calcium-rich brines can be more promising for preparing Mg(OH)2-based building materials relative to seawater.

Selective and enhanced nickel adsorption from sulfate- and calcium-rich solutions using chitosan

Nickel (Ni) is an economically important metal characterized by its mechanical strength and anticorrosion properties. With the increasing industrial demand for Ni and the depleting accessible Ni primary ores, sustainable technologies are required for the recovery of this metal from alternative resources. In this study, adsorption using chitosan was investigated as a sustainable technique to recover Ni from sulfate (SO42−) and calcium (Ca) rich secondary resources. The effects of pH, contact time, and the presence of SO42− and Ca on Ni adsorption were investigated in batch experiments. Chemical speciation modeling was performed to analyze how the predominant Ni species present under different conditions may affect the efficiency of the adsorption process. The comparison of chitosan’s maximum Ni adsorption capacities in the absence (1.00 mmol/g) and presence (1.49 mmol/g) of 500 mM SO42− and 10 mM Ca indicated the positive effect of these ions on Ni adsorption. The predominance of the neutrally charged NiSO40 species in Ni/SO42− system has contributed to the enhanced Ni adsorption on chitosan as verified by X-ray photoelectron spectroscopy (XPS) analysis. However, kinetic studies confirmed that the Ni adsorption rate decreased by 4.5 times when SO42− was present. The subsequent continuous Ni adsorption from a real SO42−-rich leachate in a column setup revealed that chitosan is selective for Ni over Ca and Cr with selectivity quotients of 9.6 (KNi/Ca) and 3.0 (KNi/Cr). Overall, this study indicated that Ni complexation with SO42− enhances the Ni adsorption capacity of chitosan, but slows down the adsorption process.

Carbon dioxide conversion into calcium carbonate nanoparticles using membrane gas absorption

Mineral carbonation in an aqueous solution is one of the practical strategies in carbon capture utilization and storage (CCUS). The wet carbonation can be further used to produce calcium carbonate nanoparticles with vast applications. Since the slow CO2 absorption affects the carbonation reaction, membrane gas absorption (MGA) with porous membranes was used in this study to enhance the mass transfer of CO2 gas into liquid absorbent for the carbonation of calcium-rich solution. The effects of sodium dodecyl sulphates (SDS) and poly(ethylene glycol) (PEG) on the properties of CaCO3 nanoparticles formed in wet carbonation were first studied. The calcite nanoparticles with particle size smaller than 25 nm were successfully produced, as proven in X-ray diffraction analysis and the transmission electron microscope images. Calcite nanoparticles with a BET surface area of 77.17 m2/g could adsorb 91.8 % of Congo red dye. The wet carbonation with surfactant-template was subsequently integrated with MGA. The pure CO2 absorption flux through the silanized polyvinylidene fluoride (PVDF)/TiO2 membrane enabled the formation of calcite nanoparticles in a calcium-rich solution containing a surfactant-template complex. The slow CO2 absorption flux of the commercial PVDF membrane with a hydrophobic surface cause failure to produce calcite with nano size.

Differentiating fluoride reservoirs in dental biofilm

Fluoride is found in dental biofilm bound to bacteria cells or or as precipitated minerals. The aim of this study was to test de dissolution rate of CaF2 in Fluoride or Calcium rich solutions. The results show that in 10 washes, CaF2 solubilizes partially in Ca or F-rich solutions.

Provenance and depositional environments of Quaternary sediments in the southern Kalahari Basin

The vast Kalahari Basin is characterized by uplifted margins, terrestrial sedimentation within semi endorheic sub-basins, subdued morphology and tectonic quiescence. This intracratonic basin has been subjected to a prolonged period of subsidence affecting its sedimentary fill by plate motion and climatic cycles. Provenance studies of Kalahari Group sediments mainly focused on the surficial deposits that represent only the last phase of sedimentation, leaving unresolved questions related to the rest of the strata.The southern Kalahari Group succession exposed along the walls of the Mamatwan Mine, Northern Cape, South Africa, reveals three main depositional environments: a bottom low-energy water-body, a middle fluvial, high-energy environment and an upper aeolian sandy unit. The entire section, which was deposited during the early to middle Quaternary, records significant environmental and depositional changes suggesting a highly dynamic landscape.The fully exposed section (55m) of the Kalahari Group at Mamatwan Mine was analysed for its mineralogy, elemental composition, Sr, Nd and Pb isotopic ratios and iron species. Isotopic fingerprints were used to determine the source rocks of the sediments. The basal part of the section was derived from the distant Angolan highlands supporting previous suggestions for the existence of a paleo, trans-Kalahari drainage system. Weathering sensitive indices show that these sediments underwent considerable chemical weathering typical to humid areas. Mineralogical assemblage and iron phases imply that a brackish and alkaline shallow water-body existed during the early-middle Pleistocene contemporaneous with relative dense hominin occupation of the area.During the Early-Middle Pleistocene transition, the lacustrine environment was rapidly filled with volcano-sedimentary Archean clasts that were derived mainly from the surrounding hills and experienced limited degree of source rock chemical weathering, but underwent subsequent groundwater alteration by iron, silica, and calcium-rich solutions and precipitation of calcrete and silcrete duricrusts. These changes in provenance and depositional environments may be related to tectonic adjustments along the southern margins of the Kalahari basin. Ultimately, a shift into an aeolian-transport dominated landscape occurred in the region. This was accompanied by reduced surface water availability and a supply of sediments from Paleoproterozoic western and north-western source areas following the establishment of the modern wind regime.

CO2 sequestration using red gypsum via pH-swing process: Effect of carbonation temperature and NH4HCO3 on the process efficiency

The main purpose of this study is to investigate the effect of reaction temperature and NH4HCO3 on the overall performance of a pH swing mineral carbonation. The overall performance of the pH swing process is investigated in terms of carbonation efficiency and product purity. Initially, 2M H2SO4 is used for red gypsum dissolution at 70°C. Then in the second stage, NH4OH is added for increasing the solution pH and removing the impurities from solutions. Finally, CO32– is introduced to calcium rich solution in the form of pure CO2 and NH4HCO3. The experimental results show that using NH4HCO3 improves carbonation efficiency and product purity. Carbonation efficiency attains a maximum value at 75°C and then decreases gradually with increasing temperature up to 300°C, with both CO2 and NH4HCO3. In this research, CaCO3 with the maximum purity of 99.05% is produced successfully when NH4HCO3 is used as a CO32– source.

Archive of bacterial community in anhydrite crystals from a deep-sea basin provides evidence of past oil-spilling in a benthic environment in the Red Sea

Abstract. In deep-sea sediment, the microbes present in anhydrite crystals are potential markers of the past environment. In the Atlantis II Deep, anhydrite veins were produced by mild mixture of calcium-rich hydrothermal solutions and sulfate in the bottom water, which had probably preserved microbial inhabitants in the past seafloor of the Red Sea. In this study, this hypothesis was tested by analyzing the metagenome of an anhydrite crystal sample from the Atlantis II Deep. The estimated age of the anhydrite layer was between 750 and 770 years, which might span the event of hydrothermal eruption into the benthic floor. The 16S/18S rRNA genes in the metagenome were assigned to bacteria, archaea, fungi and even invertebrate species. The dominant species in the crystals was an oil-degrading Alcanivorax borkumensis bacterium, which was not detected in the adjacent sediment layer. Fluorescence microscopy using 16S rRNA and marker gene probes revealed intact cells of the Alcanivorax bacterium in the crystals. A draft genome of A. borkumensis was binned from the metagenome. It contained all functional genes for alkane utilization and the reduction of nitrogen oxides. Moreover, the metagenomes of the anhydrites and control sediment contained aromatic degradation pathways, which were mostly derived from Ochrobactrum sp. Altogether, these results indicate an oxic, oil-spilling benthic environment in the Atlantis II basin of the Red Sea in approximately the 14th century. The original microbial inhabitants probably underwent a dramatic selection process via drastic environmental changes following the formation of an overlying anoxic brine pool in the basin due to hydrothermal activities.

Mineral carbonation of red gypsum via pH-swing process: Effect of CO2 pressure on the efficiency and products characteristics

The objective of this study was to investigate the feasibility of indirect carbonation of red gypsum through pH swing process. The effect of CO2 pressure as one of the major controlling parameters in carbonation process was investigated in details. The dissolution of red gypsum was performed using 2M H2SO4 at 70°C for 60min reaction time. The pH was regulated using NH4OH, which also precipitated out the impurities from the solution. Finally, the calcium rich solution was poured in an autoclave reactor and CO2 was injected into the reactor for carbonate precipitation. The experimental results showed that the Fe, the main impurity can be effectively separated prior to carbonation stage (92–95%). At low pH the Fe was separated while at high pH Ca was precipitated as carbonate. The carbonation efficiency was directly affected by the CO2 pressure and the maximum efficiency (100%) was achieved when 8bar CO2 pressure was used. In addition, carbonation experiments resulted in CaCO3 production in the form of calcite, aragonite, and vaterite with 98% purity.

Formation of alteration products during dissolution of vitrified ILW in a high-pH calcium-rich solution

To simulate the possible disposition of a vitrified intermediate-level waste (ILW) in a cementitious environment within a geological disposal facility (GDF), the durability of a laboratory simulant ILW vitrified in a borosilicate glass in a saturated Ca(OH)2 solution (pH ∼12.5) was measured. Both a low surface area to volume (SA/V) ratio (∼10m−1) Materials Characterisation Center test 1 (MCC-1) and a high SA/V ratio (∼10,000m−1) product consistency test type B (PCT-B) were used at 50°C for up to 170days. The formation of alteration layers and products was followed. The surfaces of the monoliths were analysed using SEM/EDX and showed the formation of magnesium-rich precipitates and distinct calcium silicate hydrate (CSH) precipitates. Cross sections showed the development of a calcium-rich alteration layer, which was observed from 14days. The altered layer was up to 5μm thick after 170days and showed accumulation of zirconium, iron and magnesium and to a lesser extent aluminium, along with calcium and silicon. Based on comparison of the rate data, it is suggested that the presence of this layer may offer some protection to the underlying glass. However, the high SA/V ratio experiments showed resumed alteration after 56days, indicating that the altered layer may not be protective in the long term (under accelerated conditions). The formation of a magnesium-containing smectite clay (likely saponite) in addition to CSH(II), a jennite-like CSH phase, were identified in the high SA/V experiment by X-ray diffraction after 170days. These results suggest that calcium and magnesium have important roles in both the long and shorter-term durability of vitrified wastes exposed to high pH.

Antagonist effects of calcium on borosilicate glass alteration

Numerous studies have been conducted on glass and cement durability in contact with water, but very little work to date has focused directly on interactions between the two materials. These interactions are mostly controlled by silicon–calcium reactivity. However, the physical and chemical processes involved remain insufficiently understood to predict the evolution of coupled glass–cement systems used in several industrial applications. Results are reported from borosilicate glass alteration in calcium-rich solutions. Our data show that four distinct behaviors can be expected according to the relative importance of three key parameters: the pH, the reaction progress (short- or long-term alteration) and the calcium concentration. Glass alteration is thus controlled by specific mechanisms depending on the solution chemistry: calcium complexation at the glass surface, precipitation of calcium silicate hydrates (C–S–H) or calcium incorporation in the altered layer. These findings highlight the impact of silicon–calcium interactions on glass durability and open the way for a better understanding of glass–cement mixing in civil engineering applications as well as in nuclear waste storage.

Production of precipitated calcium carbonate (PCC) from steelmaking slag for fixation of CO2

Producing precipitated calcium carbonate (PCC) from steelmaking slag is a technology that contributes to the reduction of carbon dioxide emissions from iron and steel industries. While the carbon dioxide emissions from the sector are large, it could benefit from this option by utilizing its own by-products, i.e. steelmaking slags for fixation of CO2. Since the calcium content of the steelmaking slag is high, a calcium carbonate precipitate can be produced with the method which we have recently developed, and, if fulfilling the requirements (e.g. purity and crystal shape), it can be utilized as PCC. Therefore, the objective of this study is to further evaluate the feasibility of this method. Calcium was extracted selectively from the slag with aqueous solution of ammonium salt (NH4NO3, CH3COONH4 or NH4Cl) in an extraction reactor. After removal of the residual slag, the calcium-rich solution reacted with CO2 in a carbonation reactor producing PCC. Based on the experimental results, the slag’s grain size has a clear effect on the calcium extraction efficiency; the smaller the steel converter slag’s grain size, the larger the surface area, and the better the mass transfer rate which in turn results in a higher extraction efficiency. Grinding to smaller sizes is therefore one strategy towards improved efficiencies and chemical conversion rates. Solid to liquid ratio is another important parameter for improving extraction efficiency. The smallest solid to liquid ratio 5g/l resulted in the maximum calcium extraction efficiency (73%) while the highest solid to liquid ratio 100g/l resulted in the lowest extraction efficiency (6%). Consequently this option will be operationally expensive because of larger reactor volumes. The PCC produced from the calcium rich solution is comparable to the PCC produced with conventional methods.

Dissolution of vitrified wastes in a high-pH calcium-rich solution

The current baseline for the conditioning of most UK intermediate-level radioactive waste (ILW) is immobilisation using cement. However, vitrification of some UK ILW is being considered as an alternative. One option for the disposal of the resulting vitrified ILW would be to place it in a geological disposal facility in a high-pH environment with cemented ILW and a cement-based backfill. Therefore, the potential effects of such a high pH (∼12.5), calcium-rich cement-based environment on the dissolution behaviour of simulant ILW glasses have been studied using the product consistency test (PCT). Three non-radioactive waste compositions were assessed: a laboratory simulant ILW vitrified in a borosilicate glass and two full-scale simulant vitrified products (a slag containing simulant plutonium-contaminated material and Magnox sludge; and a glass containing clinoptilolite). Powdered samples were leached in saturated Ca(OH)2 solutions for up to 42days at temperatures between 30 and 90°C.In general the rates of dissolution were lower than expected at such a high pH compared to studies in the literature under alkaline conditions. In contrast to the typical dissolution behaviour of high level waste (HLW) glasses, dissolution of the simulant borosilicate ILW glass was initially slow, followed by a period of faster boron and alkali metal release. The saturation/residual regime was not reached within experimental timescales. The rate of dissolution during the period of faster release increased with increasing temperature; the activation energy for this stage of dissolution was calculated to be 47±2kJmol−1 based on boron release. The two full-scale simulant glasses, which contained negligible boric oxide, exhibited conventional static dissolution profiles, and entered the residual rate regime after 7-14days at 50°C. The greater durability of the full-scale simulants is thought to be due to the greater content of network-forming oxides in these glasses compared to the borosilicate glass. It is also suggested that the formation of calcium borates may delay initial hydration of the borosilicate glass in a similar manner to the retardation of cement hydration by soluble borates. More generally, the formation of calcium- and magnesium-containing precipitates on the surface of the vitrified wastes, and agglomeration of the powder, appeared to act to reduce the dissolution rate. Overall these results suggest that calcium has an important role in the long-term durability of vitrified wastes at high pH.

Forward and Inverse Bio-Geochemical Modeling of Microbially Induced Calcite Precipitation in Half-Meter Column Experiments

Microbially induced calcite precipitation (MICP) offers an alternative solution to a wide range of civil engineering problems. Laboratory tests have shown that MICP can immobilize trace metals and radionuclides through co-precipitation with calcium carbonate. MICP has also been shown to improve the undrained shear response of soils and offers potential benefits over current ground improvement techniques that may pose environmental risks and suffer from low “certainty of execution.” Our objective is to identify an effective means of achieving uniform distribution of precipitate in a one-dimensional porous medium. Our approach involves column experiments and numerical modeling of MICP in both forward and inverse senses, using a simplified reaction network, with the bacterial strain Sporoscarcina pasteurii. It was found that the stop-flow injection of a urea- and calcium-rich solution produces a more uniform calcite distribution as compared to a continuous injection method, even when both methods involve flow in opposite direction to that used for bacterial cell emplacement. Inverse modeling was conducted by coupling the reactive transport code TOUGHREACT to UCODE for estimating chemical reaction rate parameters with a good match to the experimental data. It was found, however, that the choice of parameters and data was not sufficient to determine a unique solution, and our findings suggest that additional time and space-varying analytical data of aqueous species would improve the accuracy of numerical modeling of MICP.

Glass–water interphase reactivity with calcium rich solutions

The effect of calcium on synthetic glass alteration mechanisms has been studied. It is known that the higher the calcium content in the glass, the higher the forward rate. However, in a confined medium reaching apparent saturation state and a pH 90°C around 9, synthetic calcium-bearing glasses are those with the lowest alteration rates. This work brings new and fundamental evidence toward understanding the alteration mechanisms: the rate-decreasing effect of calcium exists even if the calcium comes from the solution. Calcium from solution reacts with silica network in the hydrated layer at the glass surface. The calcium effect on the alteration kinetics is explained by the condensation of a passivating reactive interphase (PRI) whose passivating properties are strongly enhanced when calcium participates in its construction. These experiments provide new evidence of the role of condensation mechanisms in glass alteration. This better understanding of the calcium effect on glass long-term behavior will be useful both for improving glass formulations and for understanding the influence of the water composition.

The effect of polycations on early cement paste

This paper studies the possibility for improving the ductility of cement based materials by means of oligocationic additives. Actually, the setting of cement is due to ionic correlation forces between highly negatively charged C–S–H nanoparticles throughout a calcium rich solution. The main drawback of this strong attraction is its very short range that results in low elastic deformation of hydrated cementitious materials. A way to enlarge the attraction range between C–S–H particles would be to add cationic oligomers that would compete with calcium ions modifying the ionic correlation forces via a bridging mechanism of longer range, which could lead to a more ductile material. The studied parameters were the polymerization degree, the separation distance between the charged monomers and the balance between oligocations and monovalent and divalent cations in the solution. The results, both experimental and numerical by Monte Carlo (MC) simulations, demonstrate that cationic oligomers can compete with calcium cations as counterions to the C–S–H surface. The cohesive forces between C–S–H surfaces, calculated by MC simulations, show an interesting behaviour where range and magnitude can be tuned with oligomer concentration, polymerization degree and line charge density. Thus, it seems possible to modulate the ductility and critical strain of cement by addition of cationic oligomers.

Rodingite with Ti- and Cr-rich vesuvianite from the Sartuohai chromium deposit, Xinjiang, China

Rodingite containing vesuvianite with the world's highest Cr2O3 and TiO2 contents was found to occur in the ultramafic body of the Early to Middle Devonian Darbut-Sartuohai ophiolite zone belonging to the Sartuohai alpine-type ophiolitic melange, west Junggar district, Xinjiang, China. Many blocks of massive and vein rodingite were located at the Sartuohai chromium deposit in the serpentinized ultramafic body along the suture zone near Karamay. The rodingite is mainly composed of vesuvianite, grossular-andradite series garnet, diopside, and chlorite. The Cr2O3 content of the rodingite masses and veins is up to 0.2 wt% and 0.1 wt%, respectively. While the Cr2O3 content (wt%) of each mineral species in rodingite is up to 0.04 for vesuvianite, 0.4 for garnet and 0.4 for diopside, respectively. Tiny sporadic inclusions of vesuvianite and garnet of 5 μm to 35 μm in diameter also disseminate along the cleavage or in the grain of diopside of the massive rodingite. The vesuvianite inclusions are characterized by a high concentration in chromium of Cr2O3 = 6.21 wt% and titanium of TiO2 = 7.43 wt% of all the reported data, in contrast to the low concentration in the host diopside. The garnet inclusions belong to the grandite series. The morimotoite and uvarovite components are 18% and 11% at most, respectively. The rodingite was considered to have been formed by two stages of metasomatism between gabbroic rock and a calcium-rich solution, which was probably produced in the process of serpentinization of peridotite under Cr- and Ti-rich, and Cr- and Ti-free environments.

Effect of calcium rich and alkaline solutions on the chemical behaviour of hemp fibres

Chemical reactivity of hemp fibres placed into a cimentitous matrix was studied by using a lime solution as a simplified cement solution. The hemp fibres contain 56.1 wt.% of cellulose, 20.1 wt.% of pectin, 10.9 wt.% of hemicellulose and 6 wt.% of lignin. These compounds are not solubilised in a basic medium rich in calcium but a part located at the fibre surface should be degraded. This treatment leads also to the decrease of the cristallinity index of the cellulose. Calcium ions adsorb in a large quantity onto the fibre surface due to the high concentration of pectin. The structure of the homogalacturonic part of pectin forms with Ca2+ a stable chelate called “egg-box”, supporting cation adsorption. The fibres are then covered by Ca(OH)2 nodules after immersion in a lime solution.