CLAY MINERALOGY IN B U I L D I N G RESEARCH*
by
J. E. GILLOTT*
Division of Building Research, National Research Council, Ottawa, Canada
EXTENDED
ABSTRACT
Clay is i m p o r t a n t in t h e construction i n d u s t r y b o t h as a building material a n d as a
foundation for structures. Buildings a n d utensils m a d e of clay date back to t h e earliest
periods of m a n ' s civilized development, a n d t h e use of clay is intimately associated w i t h
his history. Sun-baked clay structures were built during t h e pre-European period of
American history. I n t h e southwest of the U n i t e d States b y about 500 B.C. p o t t e r y was
m a d e from sun-dried clay. Bricks were m a d e from t h e s a m e material, which is durable
under semi-arid conditions, a n d great c o m m u n i t y houses were built. Bricks were commonly used in Europe at the t i m e of the early E u r o p e a n settlement of N o r t h America a n d
fired bricks were produced a t a n early date. The widely scattered and rather sparse population of this period favoured small-scale production designed to meet local demand.
As a result of the increase in the density of population a n d concomitant exploitation
of resources, extensive scientific s u r v e y s were carried out a n d available raw materials
catalogued. W i t h t h e development of u n d e r s t a n d i n g of t h e t r u e n a t u r e of clay it became
possible to correlate composition with t h e behaviour of clay on firing. B y comparing t h e
effect of different brick-maklng processes on one t y p e of clay it was shown t h a t t h e
effect of m a n u f a c t u r i n g m e t h o d s m a y be less t h a n h a d been supposed (Butterworth,
1949). Semi-dry pressed bricks were found to be only slightly more porous t h a n wirecut
or stiff-plastic pressed bricks when t h e s a m e clay was used.
Differential t h e r m a l analysis was employed as a guide n o t only to the mineralogical
composition of t h e material b u t also to indicate its ceramic properties. This was possible
as sufficient experience h a d been gained in trial r u n s in which the clay was e x a m i n e d on
the t h e r m a l analysis e q u i p m e n t a n d t h e curve t h e n correlated with its ceramic behaviour.
Certain general principles emerged f r o m t h i s work. I t was found t h a t montmorillonite
or illite confer h i g h plasticity a n d h i g h shrinkage a n d t e n d to be non-refractory, w i t h a
short vitrification range, while a l u m i n u m hydroxide or kaolinite confer refractory
properties a n d h a v e a long vitrification range (Grim a n d Rowland, 1944).
A n alternative approach to taking a n a t u r a l l y occurring clay a n d a t t e m p t i n g to
correlate its behaviour on firing with its mineralogy is to s t u d y the effects of h e a t treatm e n t on artificial mixtures. For example, studies h a v e been m a d e on co-precipitated gels
of silica a n d alumina. Of considerable practical importance to the building i n d u s t r y h a s
been t h e production of lightweight aggregate. I t was found t h a t certain clays on firing
have t h e property of swelling or "bloating", becoming light in weight yet strong a n d
durable. Much work h a s been done to produce commercially feasible lightweight aggregate in areas where t h e clays are of t h e non-bloating variety. The addition to such clays
of c o m p o u n d s of iron, alkalies, alkaline earths, carbon, a n d flowers of sulphur often
induces bloating. The essentials for success are t h a t a viscous melt m u s t form during
evolution of gas.
* Published with permission of the Director, Division of Building Research, National
Research Council, Ottawa, Canada.
296
CLAY M-INER~LOGY IN BUILDING RESEARCH
297
Great progress in materials research has been possible since the crystallography of
clay minerals became understood but the properties of clay a t normal temperatures are
determined to a great extent by the interaction of clay minerals with water. This system
is of considerable commercial importance particularly in the ceramics field, but from the
viewpoint of research oriented toward the building industry the major importance of
clay-water interaction concerns the behaviour of foundations and is closely linked to
soil mechanics. Distortion of foundations results from u p ~ k e or loss of water from the
soil and the magnitude of the volume change is largely determined b y the clay minerals.
Geological factors such as conditions at the time of deposition and post-deposltional
changes have an important influence on the properties of a sediment. The high electrolyte
concentration of the marine environment favours the more open type of packing produced
by floceulation, whereas clays deposited in fresh-water lakes are more closely packed due
to the parallel orientation of the flaky minerals (Mitchell, 1956, and Rosenqvist 1959).
Since texture of sediments has as important an i~fluence on properties as mineralogy,
a knowledge of local geology is of the utmost importance in understanding the engineering
behaviour of a clay. Canada contains a greater area of glacial deposits than any other
country in the world (Prest, 1961). The mineralogy o f the soils is in large measure controlled by the nature of the rocks eroded b y the glaciers and clay minerals are often mixed
with rock flour. The conditions of deposition~ however, differed as the end of the Pleistocene was marked by marine transgression in the St. Lawrence lowland, whereas much of
the west was occupied b y ice-dammed lakes. Isostatie uplift in the St. Lawrence has led
to leaching of the clays with removal of salt. As in the case of similar Norwegian deposits
the flocculated fabric ceases to be stable and the clays show high sensitivity (Bjerrum,
1954) and are subject to slides (Crawford, 1961). Less dramatic, but still important from
the viewpoint of the construction industry, is the large non-reversible shrinkage which
these Leda clays show on drying or being disturbed.
Chemists, physicists and mineralogists tend to interest themselves in the units of
which a clay is composed, geologists are primarily concerned with t h e history and conditions of the formation of a deposit, and engineers consider quantitative properties
such as strength and compressibility of materials, I t is to this almost instinctive approach
on the part of engineers t h a t soil mechanics owes its character. Various simple yet
ingenious tests have been devised b y which the engineering properties of a sediment can
be expressed in numerical terms. The present status and success achieved by soil mechanics is in itself a justification for such tests, which indeed arc the only ones possible in
many laboratories where the analytical equipment and background training of mineralogy
are not available. There is, however, an inherent danger t h a t the more fundamental
mineralogical approach may become neglected since it may be thought that a determination of "activity" or sensitivity t a k e s a c c o u n ~ o f t h e n a t u r e o f t h e f u n d a m e n t a l b u i l d i n g
units to the extent, a t any rate, which is required in engineering practice. This is not
necessarily the case and a knowledge of the mineralogy of a deposit is essential if one
wishes in some way to predict or modify the properties of a sediment.
The study of clays as colloids is of fundamental importance to soil mechanics while
knowledge of the crystal chemical and structural properties of clay minerals is fundamental to materials research.
REFERENCES
Bjerrum, L. (1954) Geotechnical properties of Norwegiau marine clays: Geotechnlque,
v.4, No.2.
Butterworth, B. (1949) Clay building bricks: H.M.S.O. National Building Studies,
Bulletin N o . 1.
Crawford, C. B. (1961) Engineering studies of Leda clay. In Soils i n Canada (edited by
R. F. Legget), Royal Society of Canada, Special Publications Series, No.3, pp.20029.
298 ELEVENTII NATIONAL CONFEI%ENCE ON CLAYS A N D CLAY MINERALS
Grim, 1%. E., a n d 1%owland, R. A. (1944) D.T.A. of clays a n d shales, a control and prospecting method: J. Amer. Ceramic Sot., v.27, No.3, pp.65-76.
Mitchell, J. K. (1956) The fabric of naturalclays a n d its relation to engineering properties :
Highway 1%eseareh Board. lProc. 35th Annual Meeting.
Prest, V. K. (1961) Geology of the soils of Canada. I n Soils i~ Canada, 1961 (edited by
1%. F. Legget), 1%oyal Society of Canada, Special Publications Series, No.3, pp.6-21.
l~osenqvist, I. Th. (1959) Physico-ehemical properties of soils: soil-water systems:
J. Soil 1Viechanies and Z~oundations Division, Proc., American Society of Civil
Engineers, v.85, No.SM2, Pt.1, pp.31-53.