Tobermorite

Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

Calcium siHcate hydrate is not only variable ia composition, but is very poody crystallised, and is generally referred to as calcium siHcate hydrate gel or tobermorite gel because of the coUoidal sizes (<0.1 fiva) of the gel particles. The calcium siHcate hydrates ate layer minerals having many similarities to the limited swelling clay minerals found ia nature. The layers are bonded together by excess lime and iatedayer water to form iadividual gel particles only 2—3 layers thick. Surface forces, and excess lime on the particle surfaces, tend to bond these particles together iato aggregations or stacks of the iadividual particles to form the porous gel stmcture. 

The important compounds in Portland cement are dicalcium silicate (CazSi04) 26%, tricalcium silicate (CasSiOj) 51%. tricalcium aluminate (Ca3Al206) 11% and the tetracalcium species Ca4Al2Fe2 Oio (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations ... 

The adhesion of the tobermorite particles to each other and to the embedded aggregates is responsible for the strength of the cement which is due, ultimately, to the formation of -Si-O-Si-O bonds. 

The form of silica in the matrix is at present unknown. In the freshly prepared cement there are appreciable amounts of silicic acid present which decline as the cement ages (Crisp, Lewis Wilson, 1976d). In the set cement silica could be present as a polymeric silicic acid, a siliceous gel or even a hydrated silicate gel, such as the tobermorite gel present in Portland cements (Taylor, 1966). 

Other papers in this series dealt with silicates which, unfortunately, were not chemically uniform. The mineral tobermorite (Ca3Si207, 2 H20) was made from Ca2Si04 under various experimental conditions180). Strangely, the yalues of nAp derived from the adsorption of nitrogen were in some instances only one-third or one-quarter those calculated from the adsorption of water vapor. When only the latter data were counted, then Qp appeared to be a roughly linear function of nAp, and Us was 388 + 30 erg/cm2. The Us for the trihydrate Ca3Si207, 3 H20 was said to be 320 70 erg/cm2. 

When a trihydrate of an identical composition was prepared by grinding in a ball mill with water181), the powder showed only one X-ray line near 3 angstroms, in contrast to the crystalline tobermorite which had two strong lines in this region. 

The Us of this powder was 415 89 erg/cm2. Presumably, these results are less trustworthy than those obtained for chemically homogeneous crystals. The most recent study182 leads to 450 erg/cm2 as the U of tobermorite. 

Synonyms Calcium hydrosilicate calsil Microcel Calflo E Florite R Marimet 45 tobermorite (crystalline form of synthetic calcium silicate) wollastonite is a naturally occurring fibrous form... 

The differences in chemical composition are accompanied by differences in the morphology of the tobermorite gel. Spicular or cigar-shaped rolled sheets are formed in the normal plain hydrated cement paste, whilst in the presence of calcium chloride, thin crumpled sheets or foils are formed. It has been suggested [16] that either the high lime content or adsorbed chloride prevents rolling of the sheets. 

When the hydraulic radii of tobermorite gel pores are examined using adsorption of water vapor, very little difference is found between plain pastes and pastes containing calcium chloride [20], as shown in Fig. 5.13. 

Fig. L Schematic representation showing two layers of 11A tobermorite adapted from Ziegler (2000) after Hamid (1981). The tetrahedra represent Si04 units, the grey and black spheres represent Ca atoms in the interlayers and main layers respectively and the open circles represent H20 molecules.

Hamid, S. A. 1981. The crystal structure of 11-A natural tobermorite Ca225[Si307i5(0H15)] H20. Zeitschrift fiir Krista llographie, 154, 189-198. 

This difference between H O and N adsorption data has been attributed to either the accessibility of water to interlayer spaces in the tobermorite gel or to the presence of ink bottle pores with narrow necks and wide bodies. The considerable increase (4-5 times) in pore surface and pore volume available to nitrogen in pastes containing calcium chloride suggests that the crumpled pore type of morphology is more open than the spicular type [20],... 

The way in which salts such as calcium chloride and calcium formate operate is not fully understood, but it is clear that the mechanism involves an acceleration of the C S and C S hydration. It has been proposed [23] that the initial products of cement hydration form a sort of membrane which acts as a restraint to the diffusion process which in turn leads to the dormancy period . It seems likely that the chloride ion, by virtue of its small size and high mobility, is able more easily to penetrate the pores of the restraining layer allowing the diffusion process to proceed more rapidly. The resultant tobermorite gel has a higher lime-silica ratio and a more open, accessible structure, based on a crumpled foil morphology rather than the usual spicular. The considerable reaction with,... 

The second part of my talk deals with the surface of a particular solid, a calcium silicate hydrate, called tobermorite. The two main constituents of Portland cements are two calcium silicates, which make up about 75% or more of a portland cement by weight, and both of these silicates produce tobermorite in their reaction with water. This tobermorite is the most important constituent of hydrated portland cement, concrete, and mortar. That is not the reason, however, for my talking about it—the reason is that it is a fascinating substance for a colloid chemist. I will discuss only two properties of the tobermorite surface the surface area and the surface energy. 

The tobermorite obtained in the hydration of tricalcium silicate (Ca3Si02), / -dicalcium silicate (/ -Ca2Si04), portland cement, and concrete is a colloid, with a specific surface area of the order of 300 sq. meters per gram. To give an idea of how the elementary particles of tobermorite look, Figure 7 is an electron micrograph of a few particles (obtained by L. E. Copeland and Edith G. Schulz at the Portland Cement Association Research and Development Laboratories). These particles look like fibers, but if you watch them closely, you see that they are very thin sheets, rolled up as one would roll up a sheet of paper. At the lower end the sheets are partly unrolled. When one prepares tobermorite by the reaction of lime and silica, one usually obtains crumpled sheets, which are not rolled up. The electron microscopists tell us that the sheets are very thin, of the order of a single unit cell in thickness. 

Tobermorite is a layer crystal, like many clay minerals—e.g., montmorillite and vermiculite. The unit cell of tobermorite is orthorhombic—i.e., a, h, and c are perpendicular to each other. The dimensions of the unit cell were determined by Heller and Taylor (17). Dimensions a and b lie within the layer, and c is perpendicular to the layer—it is the distance between two layers. 

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