Page 119 - Handbook of Thermal Analysis of Construction Materials

102 Chapter 3 - Formation and Hydration

Tetracalcium Aluminoferrite. In the hydration of C AF, similar
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peaks to those of C A are indicated but the rate of hydration is slower.
3
Hexagonal and cubic phases formed from the hydration of the ferrite phase
contain Fe. Not much work has been carried out on the thermal analysis of
C AF. The thermograms of C AF hydrated for various periods up to 180
4
4
[1]
days reported by Kalousek and Adams are shown in Fig. 24. The sample
hydrated for 7 days shows the presence of the cubic phase and the hexagonal
phase through peaks at 360 and 220°C respectively. The peak effect for the
cubic phase is maximum at 60 days after which it decreases and completely
disappears at 180 days. Only the hexagonal phase is evident at this period.
It is, thus, thought that the stable phase in the hydration of C AF is the
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hexagonal phase or related phase containing some Fe O , the cubic phase
2 3
being metastable. Mossabauer spectroscopic work has indicated the pres-
ence of the hydrous iron hydroxide amongst the products of the hydration
of C AF at 72°C. [17]
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Ramachandran and Beaudoin [49] investigated the effect of tem-
perature and w/s ratios on the hydration of C AF by applying various
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measurements and techniques such as DSC, TG, conduction calorimetry,
XRD, length changes, specific surface area, porosity, microstructure, and
microhardness. The samples were hydrated at w/s ratios of 0.08, 0.13, 0.3,
0.4, 0.5, and 1.0 and temperatures of 23, 80, and 216°C. The TG technique
was used to estimate the amount of hexagonal and cubic phases formed. The
weight loss up to about 250°C was attributed to the hexagonal phases and
the loss beyond, up to 500°C was assigned the presence of the cubic phase.
Generally, the TG curves exhibited losses corresponding to the endother-
mal effects in the DSC. In Table 4, the estimates of hexagonal and cubic
phases, determined by TG, are given. The samples prepared at a w/c ratio
of 0.13, or 0.3–1.0 was hydrated for 2 days, whereas that prepared at a
w/s ratio of 0.08 was hydrated for 45 days. Of all the samples studied, those
hydrated at a w/s ratio of 0.13 at 80°C indicated the highest value for the
cubic/hexagonal phase ratio and the lowest value was obtained for the
sample formed at a w/s ratio of 0.08 and hydrated at 23°C. The results
showed that the formation of the cubic phase should not necessarily be
detrimental to strength development. The cubic phase exhibits high strengths
provided it is formed at a low w/s ratio. Maximum strengths were obtained
in autoclave-treated samples. Direct formation of the cubic phase at the
sites of the unhydrated particles may occur at higher temperatures and
lower w/s ratios. In the micrographs, C AF has a closely welded structure
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similar to a vitrified body.