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Quantification of pozzolanic activity of natural clay minerals


Clays and cements are among the most widely-used materials in many geotechnical and environmental engineering applications and are often applied as a composite material. In the field of geotechnical engineering, injection grouting, slurry wall and soil stabilisation techniques have to be mentioned. In the environmental geotechnical field, applications include engineered barrier systems for landfill, contaminated land and nuclear repositories.

From cement chemistry it is known that clays are potential pozzolanic materials. The high pH value during cement setting in the presence of calcium hydroxide (portlandite, CH) destabilises clay minerals and leads to similar reaction products as in hydrated cement. Mainly calcined clays are used as blendings in pozzolanic cements. Natural clays have technological disadvantages for common concrete applications: (i) high water demand, resulting in lower density and lower strength of the building product, (ii) stable crystal structure, resulting in reduced chemical (pozzolanic) activity and (iii) high cleavage which has a negative impact on the strength of the mortars or concrete. This might explain the scarcity of publications on the pozzolanic activity of natural clays which however is essential for numerous geotechnical applications (e.g. HERZOG & MITCHELL 1962, DIAMOND et al. 1964, MITCHELL & DERMATAS 1992). Based on knowledge on the extent of pozzolanic reaction, it is possible to compare and estimate mechanical and sorptive behaviour of geotechnical composites of known CH and clay mineral content.

The present study focuses on the quantification of pozzolanic reaction between CH and natural clay minerals, thereby extracting consumption rates for CH and clay minerals (kaolinite, illite and montmorillonite) and formation rates of products. The loss of CH and clay minerals is quantified by full profile fitting of measured X-ray diffractograms (RIETVELD analysis with internal standard). Calibration measurements for CH have indicated a low relative deviation of 5 % compared to the gravimetric content. The problem of different water contents of the solids before and after reaction is overcome by attributing CH and clay mineral contents to the ignited mass as is usually done in cement chemistry. Water contents are measured by thermoanalysis. Together with results of water chemical analyses it is possible to do mass balance calculations and extract reaction extent for CH and clay minerals. The average Ca/(Si+Al) ratio of the reaction products is calculated. In the case of an observed binary reaction product mixture of calcium silicate (C-S-H) and calcium aluminate phases, the fraction of the two as a function of C/S mole ratio in C-S-H is approximated. Results show clear differences of the three investigated clay minerals according to extent of the pozzolanic reaction and fraction of reaction products.

The C-S-H phases are the strength given minerals, the aluminate phases the ones with a high anion sorption behaviour in contrast to the remaining clay minerals with a high cation sorption behaviour. In a later stage of the project, strength and elasticity of such composites will be optimised with respect to cut-off wall materials that are mostly too brittle and less ductile.

Diamond, S., White Joe, L. and Dolch, W.L., 1964. Transformation of clay minerals by calcium hydroxide attack, Clays and clay minerals, 12th Natl. Conf., Atlanta, Ga., 1963, Proc, pp. 359379.

Herzog, A. and Mitchell, J.K., 1962. X-ray evidence for cement-clay interactions. Nature, 195: 989990.

Mitchell, J.K. and Dermatas, D., 1992. Clay soil heave caused by lime-sulfate reactions, Innovations and Uses of Lime. ASTM STP 1135, D.D. Walker, Jr., T.B. Hardy, D.C. Hoffman, and D.D. Stanley, Eds. American Society for Testing and Materials, Philadelphia: 41-64.


Müller, Christian and Hermanns Stengele, Rita

Index Terms:

pozzolanic; Clay; ClayGroup; Rietveld

Further Information:

Date published: 12.05.2003