Tobermorit formation

The effect of aluminum ions on the transformation of C-S-H into 1.1 nm tobermorite was investigated by Mitsuda and Taylor [184], The solid solution of aluminum ions in tobermorite enlarges also the stability field of this phase in relation to xonothte [185], Diamond [186] found the maximum Al/(Si+Al) ratio in tobermorite to be 0.15 above this value the hydrogamet phase is appearing. Aluminum is tetrahedrally coordinated and accelerates the formation of 1.1 nm tobermorite. Finally, Mitsuda and Taylor [184] revealed that alkalis (l%Na20+1.7%K20) are activating the tobermorite formation from zeolite and calcium hydroxide. 

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. 

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 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,... 

On the basis of the data obtained, the authors conclude that mechanical activation of the mentioned mixtures leads either to partial interaction (in the case of anhydrous oxides) or to profound interaction (in the case of hydrated oxides). In the second case, the reaction completes with the formation of X-ray amorphous calcium hydrosilicates belonging to tobermorite group and being crystallized under heating in the form of vollastonite p-CaSi03. 

It was stated that weakly crystallized tobermorite was synthesized in a mixture of calcium hydroxide and silica gel. The reaction is most efficient at the reagent ratio CaO Si02=5/6 and water-to-solid ratio 4 1. At short activation time, the formation of poor crystallized calcium silicates of B type with tobermorite-like structure was observed, Mechanochemically obtained tobermorite is stable till 800°C. Above this temperature, it... 

Figure 2.53. The crystal structure of tobermorite, viewed along the be plane. The amorphous gel produced during cement formation likely contains defects such as missing/disordered silicate tetrahedra and/or water sites. Reprinted from Chem. Geol. 2000, 167,129, Copyright 2000, with permission from Elsevier.

Shaw S, Clark SM, Henderson CMB (2000a) Hydrothermal formation of the calcium silicate hydrates, tobermorite (Ca5Si60i6(0H)24H20) and xonotlite (Ca6Si60 7 (OH)2) an in situ synchrotron study. Chem Geol 167 129-140... 

The silicate group Si207, ordered along one direction, and having the possibility of wollastonite chains formation, is the basic element of the stracture of these silicates. The lack of some individual SiO in this chain does not change the basic structure. This is the source of variable C/S molar ratio not only in the amorphous calcium silicate hydrates but also in the relatively well crystallized tobermorite in which the tendency for increasing C/S ratio is observed [160],... 

The sequence of the metastable, intermediate phases formation is strongly related to the reactivity of the mixture components, particularly of silica [165, 217, 218]. At higher reactivity of lime the C-S-H (II) appears as a first product, then the C-S-H (I) and tobermorite, irrespectively of the C/S ratio. 

The transformation into tobermorite will be also occurring, which will have characteristic plate-morphology, sometimes as elongated crystals (Figs. 8.11 and 8.12). The formation of gyrolite and trascotite is also possible the composition of these phases is shown in Table. 4.4. 

It has also been reported that the second derivative DTA is more useful to examine the products formed during the autoclaving of cement-quartz-metakaolin mixtures. Klimesch and Rayt ] subjected a mixture of quartz (38.5%) and cement (61.5%) containing different amounts of metakaolin and autoclaved them for 8 hrs at 180°C. It was found that the second derivative differential thermal curve provided a more detailed information, particularly in temperatures of 800-1000°C. In Fig. 35, DTA and second derivative curves for cement-quartz-metakaolin pastes are compared. The exotherms occur at 840, 903, and 960°C due to the formation of wollastonite from C-S-H, aluminum-substituted tobermorite, and anorthite from the hydrogamet residue respectively. The small endot-herm at 828°C preceding the first exotherm is probably caused by well crystallized calcite. 

The carbonation effect on synthesized tobermorite has been examined by XRD / DTA.I ] An endothermal effect at about 180°C indicated the dehydration of tobermorite and an exothermal effect at about 800°C was caused by the formation of wollastonite. The carbonated samples exhibited a broad endothermal effect in the range of 450-600°C and a sharp peak at about 600-650°C. The former indicated the presence of vaterite and the latter, calcite. The carbonation decreased the peak due to wollastonite. Thermal methods can also be used to estimate the amount of carbonation that has occurred at different depths in concrete. These techniques estimate the amount of calcium carbonate formed and also the amount of calcium hydroxide remaining in an uncarbonated form. 

Additions of 5 to 10% Al O and 5% CaSO -2H20 to the 0.5 CaO/SiO mixture helped the formation of a compact structure made up of tobermorite gel, and contributed to strength development. ... 

Sakiyama,M. and Mitsuda,T., Effect of Al on the formation of tobermorite ( in Japanese ),Semento Gijutsu Nenpo,31,pp. 46-49. 

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