Silicate pastes hydrated

Fig. 5.10 Degree of hydration of a tricalcium silicate paste in the presence of calcium chloride, measured by non-evaporable water content (Odier).

The conclusion is that both the body structure and the surface structure of tobermorite are highly reproducible. Whether we use tricalcium silicate or fi-dicalcium silicate as starting solids, whether we use a water to solid ratio of 0.7 or 9.0, whether we use paste hydration or ball-mill hydration or a third type which I have not discussed (which gave the six other points on the curve), we wind up with a tobermorite having very nearly the same body structure and surface structure. 

Preparation of Hydrated Silicates. The hydrated silicate specimens used were all in the paste form—that is, mixtures of one of the calcium silicates with a limited amount of water to form a slurry, which sets and hardens as portland cement itself does. These pastes were prepared by the vacuum mixing procedure described by Powers, Copeland, Hayes, and Mann (23), adapted so that the temperature of the mix upon removal from the mixer was the temperature at which the specimen was to be hydrated. The 5° specimens were made by starting with an ice-water mixture the-50° specimens by starting with preheated water. A manostat was incorporated into the pumping system to prevent the pressure from dropping below the vapor pressure of water at the desired final temperature. This was especially important for the 50° mixes, to prevent excessive cooling. 

Young, J. F., Capillary Porosity in Hydrated Tricalcium Silicate Pastes, ... 

The calcium silicate hydrate formed on paste hydration of C3S or P-CjS is a particular variety of C-S-H, which is a generic name for any amorphous... 

CH can be observed as areas darker than the unreacted clinker phases but brighter than the other hydration products. As in calcium silicate pastes, these appear to have grown in regions initially occupied by water. Although the areas appear discrete on two-dimensional sections, they are not necessarily so in the three-dimensional material. They can engulf small cement grains. 

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

The loss above 550°C is due partly to CO2 and partly to the final stages of dehydration of C-S-H and the hydrated aluminate phases. It is not practicable to distinguish the contributions from TG evidence alone, and, unless evolved gas analysis is used, a separate determination of COj should be made. As with calcium silicate pastes, serious errors arise if TG determinations are carried out on material that has been treated with an organic liquid, e.g. to stop hydration. Losses above 550°C of more than about 3%, referred to the ignited weight, indicate serious carbonation either from this or other causes. 

Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsily- i lation (TMS) studies (L20,T12,S69,T36,L31,M43,M44) show that, as with C,S. the proportion of the silicon present as monomer decreases with age and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer (>5Si). Some results have i indicated that fully hydrated pastes of cement differ from those of CjS in that substantial proportions of the silicate occur as monomer (S69,L31), but the results of a study in which pastes of CjS, P-CjS and cement were compared (M44) suggest that the differences between the anion structures of cement and CjS pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated P-CjS was only 66% and results for cement pastes can only be considered semiquantitative. 

Other foreign ions in the anhydrous phases, and the extents to which they tend to pass into the pore solution on hydration. If the C-S-H constituent of the gel is assumed to have the same Ca/Si ratio as in calcium silicate pastes, one would expect that the ratio would be about 1.9 (Si/Ca = 0.53) in a gel with Al/Ca = 0.07. This agrees with some, but not all, of the data in Table 7.1. The hypothesis might explain the observations of Rayment and Lachowski (R29) on the bimodal distribution of Ca/Si ratio in the in situ gel and the relative constancy of its Ca/(Si + Al) ratio. It probably offers the most satisfactory explanation of the existing data but needs to be further tested. Continued studies by TEM of ion-thinned sections may be expected to yield valuable data in this respect. 

Stabilisation is a much slower process, which occurs progressively over several months, and involves the reaction of lime with the siliceous and aluminous components of the soil. The lime raises the pH to above 12, which results in the formation of calcium silicates and aluminates. These are believed to form initially as a gel, which coats the soil particles, and which subsequently crystallises as calcium silicate/aluminate hydrates. Those hydrates are cementitious products, similar in composition to those found in cement paste. The rate of crystallisation is temperature dependant and may take many months to reach completion. The resulting gain in strength (measured by the California Bearing Ratio Test [26.11]) is progressive, as illustrated in Fig. 26.2. 

Kantro, D. L., Weise, C. H., and Brunauer, S., Paste Hydration of B-Dicalcium Silicate, Tricalcium Silicate and Alite, Symp. Structure of Portland Cement Paste and Concrete, pp. 309-327, Highway Res. Board (1966)... 

Compared to the extensive thermal investigations of C3S hydration in the presence of CaCl2, only a limited amount of work has been reported on the effect of calcium chloride on the hydration of C2S. Calcium chloride accelerates the hydration of dicalcium silicate. In the thermograms of C2S paste hydrated for 1-3 months, lower amounts of calcium hydroxide are formed in the presence of calcium chloride, compared to that hydrated... 

Odler, 1., and Becker, T., Effect of Some Liquefying Agents on Properties and Hydration of Portland Cement and Tricalcium Silicate Pastes, Cement Concr. Res., 10 321-331 (1980)... 

Figure 10.19 (a) Nitrogen adsorption/desorption isotherms for C3S paste (b) cumulative pore volume versus pore width for C3S pastes hydrated for 28 days using the DFT model for slit-shaped pores. (Adapted from Costoya, M., Effect of particle size on the hydration kinetics and microstructural development of tricalcium silicate, PhD thesis, Ecole Polytechnique Federale de Lausanne, Switzerland, 2008.)... 

Concrete is made of cement aggregate and water mixed together to form a paste. The aggregate is usually a tiller material composed of inert ingredients such as sand and rocks. When water is added, the components of cement undergo a chemical reaction known as hydration. As hydration occurs, the silicates are transformed into silicate hydrates and calcium hydroxide (Ca OII 2), and the cement slowly forms a hardened paste. 

BONHOWE, I., WlELAND, E., SHEIDEGGER, A. M., Ochs, M. Kunz, D. 2003. EXAFS study of Sn(IV) immobilization by hardened cement paste and calcium silicate hydrates. Environmental Science and Technology, 37, 2184-2191. 

Richardson, I. G. Groves, G. W. 1993. The incorporation of minor and trace elements into calcium silicate hydrate (C-S-H) gel in hardened cement pastes. Cement and Concrete Research, 23, 131-138. 

Thixotropy can be obtained at fairly low loading concentrations with colloidal silica, bentonite, metallic leafing powders, and hydrated magnesium aluminum silicates. If required, thixotropic adhesive pastes may be formulated which will not flow during cure even at elevated temperatures and which are useful for bonding loose-fitting joints. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

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