Page 346 - Handbook of Thermal Analysis of Construction Materials

Section 6.0 - Metakaolinite 323

6.0 METAKAOLINITE

Soils and clays, in general, when calcined give off adsorbed,
interlayer, and hydrated types of water. These effects produce endothermal
peaks or loss of weight in DTA and TG, respectively. The endothermal
peaks are followed by exothermal peaks that are caused by re-crystalliza-
tion. Although many types of clay minerals such as montmorillonite, illite,
and some shales show these effects, they are not suitable as pozzolans in
concrete. Metakaolin, formed by heating kaolinite, seems to be the most
suitable additive material for cement. Heating of kaolinite involves removal
of adsorbed water at about 100°C and dehydroxylation at above 600°C,
followed by the formation of metakaolinite, an almost amorphous product.
The sequence of reactions is as follows: [60]

Al Si O (OH) (Kaolinite) at 600°C
2
2
4
5
forms Al Si O (Metakaolinite) + 2H O
2 2 7 2
At higher temperatures, metakaolinite transforms to crystalline compounds
as follows:
• Metakaolin at 925°C forms silicon spinel

• Silicon spinel at 1100°C produces pseudo-mullite

• Pseudo-mullite at 1440°C forms mullite + cristobalite
Metakaolin (MK) is not cementitious by itself, but, having a high
2
surface area of about 20 m /g, it reacts with calcium, sodium and potassium
hydroxides, and gypsum/cement. The hydrated lime + MK reaction yields
gehelinite hydrate (C ASH ) and C-S-H. The C/S ratios of the calcium
2
8
silicate hydrate vary between 0.8 and 1.5. At higher hydration temperatures,
gehelinite is the stable phase. The hydration reaction at 50°C produces a
hydrogarnet phase of approximate composition C AS H . When MK is
3 0.3 5.3
mixed with portland cement and hydrated, the lime formed as a hydration
product of cement is consumed by MK and the C-S-H product is formed. At
higher MK contents, gehlenite may form. The C/S ratio of the product may
vary between 1.0 (at 50% MK) and 1.6 (at 20% MK). [61] The addition of MK
to portland cement results in the increased strength of the product. For
example, at 10% MK addition, the compressive strengths at 1, 3, and 28
days are 25, 33, and 40 MPa respectively, compared to the corresponding
values of 21, 25.5, and 36.4 MPa for the reference samples. [62]