Hydrogarnet

Garnet ferrites, 11 55, 56t, 57 Gamet-hydrogarnet solid solution series, 5 479t... 

Basso R. (1985). Crystal chemical and crystallographic properties of componnds with garnet or hydrogarnet structure. Neues Jahrb. Miner. Monat., 3 108-114. 

CjAHg is the only stable ternary phase in the CaO-AUOj H,0 system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. CjAHg is formed in the conversion reaction of hydrated calcium aluminate cements (Section 10.1). 

Table 6.4 Calculated values of the cell parameter a and ratio of intensities of the 022 to the 116 XRD powder reflections for some hydrogarnet phases (T5)...

Passaglia and Rinaldi (P23) discussed IR spectra and TG curves for C3AHg and other hydrogarnet phases. The TG curve of C3AHg shows major loss at 250-310°C and further loss at 450-550 C, but in that of a katoite specimen the two steps were barely distinguishable. Majumdar and Roy (M63) reported DTA and IR data for CjAH. The refractive indices of C3AS3 and CjAHg are 1.734 and 1.604, respectively those of the solid solutions are linearly related to the composition (P23). 

Copeland et al. (C38) observed that the XRD patterns of many pastes included peaks of a hydrogarnet in addition to those of other hydrated aluminate phases, and obtained evidence that this had formed by hydrationr... 

In the first effective studies of hydrated cements by DTA, Kalousek and co-workers (K27,K32,K33) observed the early formation of ettringite and its subsequent replacement by what was termed a solid solution, but which was probably a mixture of AFm phases. They found no CjAHg or other hydrogarnet phases. Quantitative or semiquantitative determinations of gypsum and ettringite indicated that less than half the total SO3 present could be accounted for by these phases. The authors concluded that the solid solution was eventually replaced by a product which they termed Phase X , and which was possibly a gel containing all the oxide components of the original cement. 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

In an ordinary Portland cement, only some two-thirds of the Fe occurs in the ferrite, the rest being contained largely in the alite and aluminate (Table 4.3). On hydration, the Fe " in these other phases probably does not enter a hydrogarnet, but goes into AFm phases or layers formed in situ. This would account for the observation by analytical electron microscopy that small amounts of Fe are present in the AFm phases. 

For any selected drying condition, a table is now completed, similar to the previous one, but with the HjO and CO2 content of each phase included and the quantities of elements expressed as weight percentages of oxide components. Table 7.3 shows results for a typical, mature cement paste thus calculated. It was assumed that the C-S-H had Ca/Si = 1.7 and Al/Ca = 0.02, that the AFm and AFt phases each had Si/Ca = 0.05 and that the hydrogarnet-type phase had the composition Ca2.95Alo.1Fe1 7sTio jMn 05-... 

Modified from Ref. T35. "Other components mainly P2O5 (0.2%) other phases mainly alkalis (adsorbed or in solution), insoluble residue and PjO (form of combination unknown). Fe HP = hydrogarnet-type product from ferrite phase. Mg HP = hydrotalcite-type phase. AFm and Mg HP include poorly crystalline materials intimately mixed with C-S-H. Discrepancies in totals arise from rounding. 

Hydrogarnet-type product from ferrite phase. 

Fig. 9.5 Si/Ca and Al/Ca atom ratios from individual X-ray microanalyscs of a 91-day-old paste of a Poriland-pfa cement (28% of a pfa low in CaO w/s = 0.5). Open circles, in situ product from alite or belitc open squares, regions near to pfa particles, and in some cases also to CafOH) full circles, undesignated product. P suggested composition of in situ product from pfa, possibly composed largely of C-S-H and hydrogarnet (HGT). Modified from Harrisson el at. (H4).

DTA (K32,025) and XRD (A29) studies indicate that cement pastes cured at temperatures of or approaching lOO C contain little or no detectable hydrated aluminate phases, the observation of hydrogarnet noted above thus being unusual. Kalousek and Adams (K32) considered that the Al , 864 and other ions were incorporated into a Phase X , which possibly included all the oxide components of the cement. In pastes cured normally, much of the AFm phase is probably poorly crystalline and intimately mixed with the C-S-H (Seetion 7.3.1). This tendency may be increased if hydration takes place more rapidly. 

Ft is doubtful whether XRD can distinguish a mixture of tobermorite and C-S-H from a uniform material of intermediate crystallinity the situation may lie between these extremes (A30). This question is discussed further in Section 11.7.4. Crystallization is probably favoured by low bulk density its extent is apparently minimal in calcium silicate bricks (P49), but considerable in aerated concretes (A30). In cement-silica materials, substantially all the AljOj appears to enter the C-S-H, which as its Ca/Si ratio decreases can accommodate increasing amounts of tetrahedrally coordinated aluminium (S70). NMR results (K34) support an early conclusion (K62) that 1.1-nm tobermorite, too, can accommodate aluminium in tetrahedral sites. Small amounts of hydrogarnet have sometimes been detected, especially in products made from raw materials high in AljOj, such as pfa or slag. Minor amounts of tricalcium silicate hydrate (jaffeite C, S2H,) have sometimes been detected (A29,K61). 

Ackermann L., Cemic L., and Langer K. (1983) Hydrogarnet substitution in pyrope a possible location for water in the mantle. Earth Planet. Sci. Lett. 62, 208-214. 

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