Calcium silicate, dissolution

The Ca(OH)2 adsorption was determined by conductometry. Superposed on the neutralization reaction is a slower formation of insoluble calcium silicates which is analogous to the dissolution of silica by sodium hydroxide. 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

Most of the dissolved calcium in groundwater in northern Wisconsin is the result of silicate hydrolysis of the aquifer materials. The assumption of conservancy is accurate only because of the relatively slow rates of silicate dissolution. The presence of more soluble calcium-containing minerals, such as calcite or gypsum, would invalidate assumptions of conservancy and would lead to significant errors in solute budgets. 

Titanium Silicides. The titanium—silicon system includes Ti,Si, Ti Si, TiSi, and TiS (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkaliflnorosilicate (155) reaction of TiCl4, SiCl4, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiS (156) and, for Ti Si, reaction between titanium dioxide and calcium silicide at ca 1200°C, followed by dissolution of excess lime and calcium silicate in acetic acid. 

For minerals that dissolve incongmently, the determination of reaction rate depends upon which component released to solution is used in Equation (5). Due to preferential release of cations such as calcium and magnesium during inosUicate dissolution, for example, dissolution rates for these phases are usually calculated from observed silicon release (Brantley and Chen, 1995). Here, we report silicate dissolution rates based upon silicon release, but we normalize by the stoichiometry of the mineral and report as mol mineral per unit surface area per unit time. It is important to note that dissolution rates reported on this basis depend upon both the formula unit and the monitored solute. 

Cement-equilibrated water derived from the repository will react with the surrounding rock and form an alkaline-disturbed zone (ADZ) around the repository. The main reactions that are expected to occur within the ADZ are dissolution of primary silicates, and precipitation of hydrated calcium silicates (CSH phases) and possibly zeolites (Rochelle et al. 1992). As such water-rock interaction proceeds, the pH of the repository-derived water will be buffered toward lower values and will eventually reach the nearneutral values typical of unperturbed far-field groundwaters. It is likely that the hydrogeological and radionuclide retardation properties of the ADZ will be different from those of the unperturbed geosphere. The NSARP therefore includes work to evaluate the size of the ADZ and the extent to which its properties will be perturbed. A summary of the ADZ sorption programme is presented below. 

Predictions involving the reaction of quartz with the evolved (Fig. 4) fluid indicated reduced quartz dissolution compared to the simulation with the young fluid. The calcium silicate hydrate (CSH) product phases, were predicted to form a succession down the column, with hillebrandite precipitating in the first 60 mm, foshagite between 60-90 mm, and tobermorite between 90-250 mm. There was a net reduction of porosity along much of the profile with particularly large changes in the first 60 mm where hillebrandite formed. 

Alite in a-TCP has some roles in the setting properties of AC. Silicate ion is known to promote apatite nncleation in certain bioactive glasses and ceramics [6,8]. In this study, silicate ion from alite may also promote the nncleation of apatite and entanglement of the crystals as well. Thus, final DTS of set ACs with 2.5-7 5 wt% alite were improved (Table 1). Another reason may be the formation of calcium silicate hydrogel, which could provide core-matrix stmcture in the apatite formation (however, the present of silicate hydrogel is not yet proven in our study). Unfortunately, dissolution of alite produced an alkaline circumstance and... 

Figure 4.3 XRD scans showing the effect of selective dissolution treatments for a port-land cement. Top residues of SAM middle residues of KOSH treatment bottom original untreated portland cement. The KOSH treatment dissolves the aluminate, ferrite, sulfate and most minor phases, leaving only the calcium silicate phases. In contrast, the SAM treatment dissolves the calcium silicate phases and thus concentrates the aluminate, ferrite and minor phases in the residue. In this particular example, the SAM treatment led to the clear identification of goergeyite (K2Ca5(S04)6 H20) as a minor phase in the cement.

Dissolution of CaCOs is a congruent reaction the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of CO2 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach... 

Both humic acids and fulvic acids have a strong affinity for particulate and crystalline substances possessing oxygen atoms at their surfaces and they have been reported to bring about the dissolution of iron phosphate, calcium phosphate (61), uranium dioxide (65), hydrated magnesium alumino-silicates (66) and limonite, a complex mixture of hydrated ferric oxides (67). 

In contrast to calcium carbonate, all seawater is undersaturated with respect to BSi. As shown in Table 16.1, the imdersaturation is very large and increases with depth because the solubility of BSi increases with pressure. Thus, all siliceous hard parts are subject to dissolution. Nevertheless, about 25% of the BSi created in the surfece waters survives the trip to the seafloor via pelagic sedimentation. Direct observations of this transport... 

Cooper A.R. and Kingery W.D. (1963) Dissolution in ceramic systems, 1 molecular diffusion, natural convection, and forced convection studies of sapphire dissolution in calcium aluminum silicate. /. Am. Ceram. Soc. 47, 37-43. 

Diatoms are unicellular, photosynthetic microalgae that are abundant in the world s oceans and fresh waters. It is estimated that several tens of thousands of different species exist sizes typically range from ca 5 to 400 pm, and most contain an outer wall of amorphous hydrated silica. These outer walls (named frustules ) are intricately shaped and fenestrated in species-specific (genetically inherited) patterns5,6. The intricacy of these structures in many cases exceeds our present capability for nanoscale structural control. In this respect, the diatoms resemble another group of armored unicellular microalgae, the coccolithophorids, that produce intricately structured shells of calcium carbonate. The silica wall of each diatom is formed in sections by polycondensation of silicic acid or as-yet unidentified derivatives (see below) within a membrane-enclosed silica deposition vesicle 1,7,8. In this vesicle, the silica is coated with specific proteins that act like a coat of varnish to protect the silica from dissolution (see below). The silica is then extruded through the cell membrane and cell wall (lipid- and polysaccharide-based boundary layers, respectively) to the periphery of the cell. 

Interestingly, impurities such as aluminum, calcium, magnesium or zinc were reported to reduce both the rate of dissolution and the solubility of silica at equilibrium. Nitric acid-cleaned silica was immersed in solutions of Al, Be, Fe, Ga or Gd ions at pH from 2 to 9. A drastic reduction of the solubility of silica was observed. In particular, the introduction of Al ions rendered silica insoluble at pH 9. This effect was attributed to the formation of a monolayer of insoluble silicate which lowered the silica solubility to that of the surface compound [17]. Seemingly, silicates involving metal ions are formed even at pH values apparently non-aggressive for silica. However, the amount of metal silicate is not specified and the formation of only a silicate monolayer is purely speculative. 

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