Calcium silicate hydrate CSH

The natural curing reactions that occur in a standard Portland cement involve the formation of calcium hydroxide (portlandite) and Ca(OH)2, as well as calcium silicate hydrate (CSH). Over time, the cement will absorb C02 from the air, converting the Ca(OH)2 and some of the CSH, to calcium carbonate, CaC03, through reactions such as those shown in Figure 15.1 (Siegers and Rouxhet, 1976 Suzuki et al., 1985 Taylor, 1990). Supercritical C02 processing carbonates the calcium hydroxide, removes water from the... 

Calcium Silicates Hydrates (CSH) is the major hydration product of Portland Cement. It has been intensively studied for several decades. CSH can be obtained by hydration of C3S and P-C2S or by precipitation fi om aqueous solutions containing a Ca salt and silicate ions. CSH has a wide range of chemical composition. Many studies [1-5] indicate that its molar ratio Ca/Si can vary fixrm 0.7 to 2 (or more) [6] and is in most cases near to 1.7 [7]. There has been indirect evidence that CSH (obtained by reaction between CaO and Silica) has a layered structure similar to that of natural Tobermorite and/or Jennite [8]. Recent solid state NMR [9-10], intfared spectroscopy [11], EXAFS [12] and thermogravimetry works [14] have... 

Dehydration/rehydration process of Tobermorite-like Calcium Silicates Hydrates (CSH) was investigated by means of High Resolution Transmission Electronic Microscopy (HRTEM) and X-Ray diffraction using a new in-situ temperature and pressure controlled experimental cell. The following points summarise the results obtained in our investigations ... 

Calculated and measured dependencies of metal solubility from pH-variations indicate (Kersten 1996 Johnson etal. 1998) that at between pH 11 and pH 7, concentrations of Pb and Cd remain below 10 mol L at pH 7, remobilization of Cd begins, whereas Pb is significantly mobilized only below pH 5. At pH 4, approximately 25% of Cu, 50% of Pb, 45% of Cd, and 70% of the total Zn-inventory has been mobilized. Therefore, the long-term potential for metal mobilization by pH decrease can be considered as significant. It has been argued that the major reason for the reduced mobilization of the metals compared with the calculated solubility for carbonate and hydroxide species might be found in the chemical incorporation of the metals in hydrated cement phases such as calcium silicate hydrate (CSH). 

The evolution of hydration and carbonation of powder samples was assessed by XRD, mercury intrusion porosimetry (MIP), FTIR, DTA/TG, EDXRF and SEM. By identifying calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH) at different ages, not only qualitatively, but also semi-quantitatively, the hydration and carbonation processes could be monitored. The mineralogical analysis was investigated by XRD with a Siemens D-500 diffractometer (40 kV/35 mA) the spectra were collected... 

Alite (tricalcium silicate) reacts with water to form calcium silicate hydrates (CSH phases) containing less lime, while calcium hydroxide is split off. Belite (dicalcium silicate) shows similar behaviour. The hydration reaction is, for example ... 

Fig. 16 Calcium silicate hydrates (CSH phases) in hardened cement paste (scanning electron micrograph)...

When the cement comes into contact with water, chemical reactions start between the clinker minerals (CaO = C, Si02=S, AI2O3 = A) and the water (abbreviated in the terminology as H ), forming colloidal, insoluble reaction products such as calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH) ... 

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. 

Published experimental studies of mineral/cal-cium hydroxide reactions show that at low temperatures (below 110°C), the chief reaction products are calcium silicate hydrate (CSH) gels, while zeolites and feldspars are formed at higher temperatures and in the presence of alkalis NaOH and KOH. The phase identifications have however often been made by low resolution or bulk methods, neither of which are ideal for such material. Published results of numerical simulations are in broad agreement with those of experimental studies of cement/ rock interaction. These models predict that CSH gels will be replaced by zeolites and maybe feldspars as plume chemistry evolves. 

Comparing metal concentrations in the BA with the amount of the different types of waste (household, industrial, etc.) only one poor correlation (R = 0.6) was observed a higher amount of waste coming from the construction sector caused higher CaO contents in the BA. It is likely that in this case Ca-rich CSH phases (calcium-silicate-hydrate) and gypsum account for a higher CaO content in the BA. 

CASH CBM CBO CBPC CC CCB CCM CCP CDB CEC CFBC CFC CFR CMM COP CSH CT Calcium aluminosilicate hydrate Coal bed methane Carbon burn-out Chemically-bonded phosphate ceramics Carbonate carbon Coal combustion byproducts Constant capacitance model Coal combustion product Citrate-dithionate-bicarbonate Cation exchange capacity Circulating fluidized bed combustion Chlorofluorocarbon Cumulative fraction Coal mine methane Coefficient of performance Calcium silicate hydrate Collision theory... 

Calcium silicate hydrate C3S2H3, designed also as CSH, has the form of very small and weakly crystallized particles which by their dimensions and irregularity resemble clay. CSH covers from 50% to 70 % of total volume of the cement paste and its properties and behaviour are complicated because they depend on several interrelated processes. Its representation... 

The gel pores form a part of CSH (calcium silicate hydrate), and may be classified as micro pores or meso pores. The principal difference between gel and capillary pores is that the former are too small to be filled by the hydration products and for capillary effects, it means that no menisci are formed. The gel pores occupy between 40% and 55 % of total pore volume, but they are not active in water permeability through cement paste and they do not influence the composite strength. Water in the gel pores is physically bonded. It is believed that gel pores are directly related to shrinkage and creep properties of the cement paste. 

The essentials of modern cement manufacture have been followed for the last 200 years (Lea 1970). A calcium source (usually limestone) and an aluminosilicate source (usually shale or clay) are mixed together and fired at high temperatures (1300 to 1500°C) to produce calcium silicates, aluminates and alumino-ferrites. The calcium silicates hydrate on contact with water to produce CSH gels which are the chief binding agents in the cement. Cements containing CSH gel (including pozzolanic cements) are termed hydraulic because they set and harden on reaction with water and once set, continue to harden if placed underwater (Taylor 1990). 

Pointeau, I. 2000. Etude Mechanistique et Moderation de la Retention de Radionucleides par les Silicates de Calcium Hydrates (CSH) des Ciments. PhD thesis. Universite de Reims Champagne-Ardenne, France. 

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. 

Similar results to those observed in quartz columns were seen in the columns reacting albite with the simple young (Na-K-Ca-OH) fluid, where the Ca Si ratios of the CSH phases varied from approximately 0.6 to 1.3, with increasing distance from the inlet of the column. The main difference in the albite columns relative to the quartz columns was that all analyses of the CSH phases showed they contained up to 1 wt% A1 (i.e. calcium aluminium silicate hydrates (CASH) phases were formed). 

CSH, tobermorite, xonotlite and related materials are being widely utilized in recent years. It is produced on autoclaved lightweight concrete, autoclaved concrete, sand-lime blocks, hydrous calcium silicate he at-insu1 ation materials, hydration products of cement and other building or construction materials. 

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