Silicate solutions composition

Basic Silicate Solutions Composition. In Table III the compositions of some TEAOH and TPAOH silicate solutions in the presence and absence of DMSO as a function of the OH/Si ratio (pH) are compared. DMSO was added since it is known to promote the amount of... 

The initial transition of dissolved silicate molecules into solid nanoparticles is perhaps the least explored step in the synthesis of zeolites. One impediment to understanding this mysterious step is the poorly elucidated molecular composition of dissolved particles. The major mechanistic ideas for the formation of zeolites approach these structures differently i) many researchers believe that secondary building units (SBU) must be present to form initial nanoslabs [1,2] ii) some others prioritize the role of monomers to feed artificially introduced crystal nuclei or assume that even these nuclei form via appropriate aggregation of monomers [3] iii) silicate solutions are also frequently viewed as random mixtures of various siloxane polymers which condense first into an irregular gel configuration which can rearrange subsequently into a desired crystal nucleus at appropriate conditions [4,5],... 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

Pozzolanic S/S systems use portland cement and pozzolan materials (e.g., fly ash) to produce a strucmrally stronger waste/concrete composite. The waste is contained in the concrete matrix by microencapsulation (physical entrapment). It is a chemical treatment that uses commercially available soluble silicate solutions and various cementious materials such as cement, lime, poz-zolans, and fly ash. By addition of these reagents and rigorous mixing, the waste is fixed or stabilized. Contaminant mobility is reduced through the binding of contaminants within a solid matrix, which reduces permeability and the amount of surface area available for the release of toxic components. 

The addition of an acid to a sodium silicate solution causes a separation of silicic acid which appears as a jelly-like substance. Orthosilicic acid has the composition HtSiCh, metasilicic acid, H2Si03 the acid corresponding to the sodium salt of the above formula, H2Si409. Suspended in water these different silicic acids are more or less easily interchangeable one into another, but, if silicic acid is heated, it loses all its water and becomes the anhydride. The anhydride practically will not take on water again to form acids. 

The composition and dynamic properties of basic silicate solutions and the implications derived therefrom as to the possibility of positively identifying zeolite precursor species. 

Composition of various TEA and TPA silicate solutions C[SI]—2) as a function of the OH/Si ratio. Absoluts molar percentages are given for the polymeric species and relative ones for the rest. -A- Mono/dimer ... 

Si—NMR spectrum of a silicate solution of molar composition 1 Si(>2 1 hexamethonium hydroxide ... 

First, although the use of bulky organic bases clearly shifts the silicate equilibrium to the DnR species, there may be a large amount (up to more than 90%) of polymeric species present in silicate solutions. This is true especially at low OH/Si ratios (<0.5) or high Si concentrations (>2), i.e., normal values for a zeolite synthesis composition. This range of polymeric silicates cannot at present be characterized satisfactorily, and the presence of zeolite precursor species other than DnR silicates in this range cannot be excluded. 

On the basis of the composition and dynamics of silicate solutions and zeolite synthesis mixtures only, a precursor role for the D5R silicates during both stages cannot be excluded since these species are present in relatively high concentrations during the zeolite synthesis process. 

Figure 3 shows what happens for compositions e - 8e as more alkali is added to the silicate solution before mixing with the aluminate. The upper curve corresponds to the line marked "X" in Figure 2, and the others show the effect of progressively increasing the amount of alkali premixed with the silicate solution. The gel times for the least concentrated solutions decrease almost to zero, but as the total amount of silica increases, the change becomes less pronounced until solution 8e is virtually unaffected. 

Why is this Adding more alkali to the silicate solution increases the proportion of small species, particularly monomer (2.3). The compositions with the shortest gel times were thus made using silicate solutions that contained virtually all monomer. The silicate solutions used to prepare the others, which took longer to gel, would have contained an appreciable amount of more highly polymerised species. These points are discussed in more detail later. 

Fig. 8 (a) A 29 i NMR spectrum of a monomeric Na silicate solution of the composition 1.0 mol% SiC>2, R = 0.1 and an 27A1 spectrum of a monomeric Na aluminate solution of the composition 1 M NaAlC>2 (b)-(d) 29si and 27A1 spectra of aluminosilicate solutions with increasing AI concentration. 29si spectral frequencies are referenced to Si(OH)4 and Al spectral frequencies are referenced to the octahedral Al + ion in an aqueous solution of AICI3. 

The ionic composition of dilute sodium silicate is a very complex problem involving Na20/SiC>2 ratio, water content, and even trace impurities. Equilibration seems to be very slow at ordinary temperatures. As shown in Figure 5, Harris, et.al, were able to identify a wide variety of structures in potassium silicate solutions which bear a striking resemblence to the secondary building units proposed for zeolite... 

An acid silicate, of composition Ni0.5Si02.lH20, is described8 as resulting by the interaction of sodium silicate and a nickel salt in aqueous solution. 

Several studies on the quaternary systems of CaO-SiOj-HiO with NajO or K,0 have been reported (K19,S52,M52). Alkali greatly lowers the concentrations of CaO in the solution and raises those of Si02. The solid phase compositions are difficult to study. Determinations based on changes in concentration on adding CH to alkali silicate solutions are subject to considerable experimental errors, while direct analyses of the solid are difficult to interpret because the alkali cations are easily removed by washing. Suzuki ei al. (S52) considered that they were adsorbed. Macpheeef /. (M52) reported TEM analyses of the C-S-H in washed preparations obtained by reaction ofCjS (lOg) in water or NaOH solutions (250 ml). The C-S-H obtained with water had a mean Ca/Si ratio of 1.77 that obtained with 0.8 M NaOH had a mean Ca/Si ratio of 1.5 and a mean NujO/SiOj ratio of 0.5. These results do not appear to be directly relevant to cement pastes. The pore solutions of the latter may be 0.8 M or even higher in alkali... 

Silicate solutions of equivalent composition may exhibit different physical properties and chemical reactivities because of differences in the distributions of polymer silicate species. This effect is keenly observed in commercial alkali silicate solutions with compositions that lie in the metastable region near the solubility limit of amorphous silica. Experimental studies have shown that the precipitation boundaries of sodium silicate solutions expand as a function of time, depending on the concentration of metal salts (29,58). Apparendy, the high viscosity of concentrated alkali silicate solutions contributes to the slow approach to equilibrium. 

Weathering rates are dependent on a number of factors that can be classified as either intrinsic or extrinsic to a specific mineral (White and Brantley, 2003). Intrinsic properties are physical or chemical characteristics such as mineral composition, surface area, and defect densities. If intrinsic properties dominate weathering, such characteristics should be transferable between environments, e.g., laboratory and field rates of the same mineral should be comparable. Extrinsic feamres reflect environmental conditions external to the silicate phase that impact chemical weathering such as solution composition, climate, and biological activity. These processes are dependent on external environmental conditions that are difficult to recreate fully under laboratory simulations. 

Siliceous MCM-41, aluminosilicate MCM-41, and mesitylene based materials have also been reported by Beck et al.18 N-Brand sodium silicate solutions were added to acidic soiutions with the subsequent addition of surfactant and generation of a gel. Siliceous MCM-41 materials resulted by mixing these gels with water and heating the mixture to temperatures of 100°C for 6 days. Similar materials with different elemental compositions were prepared by using Ci2H25(CH3)3NOH/Ci surfactant solutions with sodium aluminate solutions. Ultrasil silica. 

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