Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES
In present study different hydrogarnet compositions (Ca3(Al,Fe)2(SiO4)3-y(OH)4y); y ≤ 1)
containing various amounts of aluminum, iron, OH - and SiO4 4- have been investigated.
The solids were characterized by X-ray powered diffraction, thermogravimetric analysis
and scanning electron microscopy. Synchrotron diffraction was used to study the
structure of Fe siliceous hydrogarnet. The solubility products of the different
hydrogarnets was calculated based on the measured concentrations.
3.5.2. Al-Katoite, C3AH6
Several studies have been carried out to date to determine the stability of CaO-Al2O3-
H2O at different temperatures [7, 111-116]. C3AH6 is thermodynamically stable in the
CaO-Al2O3-H2O system over the temperature range from 5 to 250 °C [7, 115, 116]. In
many cases OH-AFm (C4AHx) precipitated in the initial stage, which generally converted
to C3AH6 with time. Note that C4AHx is metastable with respect to C3AH6 at 20 °C and
above. Solubility data for C4AHx have been determined from 1 to 90 °C [7, 113-116]. In
the present study the formation of C3AH6 at 20, 50 and 80 °C was determined in 0.1 M
KOH solutions (pH values ~ 13 at 20 °C).
The XRD pattern shows that C3AH6 together with a small amount of C4AHx was already
observed after 1 week (Fig. 48). The strong reflection assigned to C3AH6 at 2θ = 17.27°
corresponds to a basal spacing d = 5.13 Å. Minor peaks with basal spacing d = 7.69 Å (2θ
= 11.49°) were observed, which could be assigned to C4AHx. After three years of
equilibration, a weak reflection was found with basal spacing d = 8.11 Å (2θ = 10.90°)
indicating the presence of C4Ac0.5H12 due to CO2 contamination of the sample with time.
At 50 and 80 °C, C3AH6 formed rapidly. The C4AHx present initially at 50 °C was
replaced by hemi/mono-carbonate due to CO2 contamination at later ages (see Table 26).
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