An approach for the evaluation of local raw material po-

tential for calcined clay as SCM, based on geological and

mineralogical data: Examples from German clay deposits
Matthias Maier1, Nancy Beuntner1 and Karl-Christian Thienel1
1 Bundeswehr University Munich, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany

Abstract. This study gives an overview over the geological and spatial distribu-
tion of German clay deposits visualized in a GIS map, which is based on the map
of mineral resources, the geological map of Germany and supplemented by active
clay pits. The clays are classified regarding their geological context. Representa-
tive clays for a certain geological formation are examined closely. Detailed clay
mineralogy is determined using XRD. Optimal calcination temperature is defined
using TG/DTG. The calcined clays are characterized by XRD and BET. Poz-
zolanic reactivity is assessed by R³-test and solubility of Al and Si ions in alkaline
solution. A correlation between geological origin, chemical-mineralogical com-
position and pozzolanic reactivity is discussed. The study shows, that a rough
estimation of pozzolanic reactivity based on geological data or chemical compo-
sition is possible. For a detailed assessment, an elaborate determination of min-
eralogical phase content or a direct determination of reactivity is necessary.

Keywords: calcined clay, clay deposits, clay mineralogy, pozzolanic reactivity

1 Introduction

One of the most effective attempts to lower the ecological impact of cement production
due to CO2 emissions is the partial substitution of cement clinker. Since established
supplementary cementitious materials (SCM) like fly ash and slag stagnate or even de-
crease in many industrialized countries, the demand for alternative materials will rise
in the future [1]. The probably most promising alternative materials are calcined clays.
Much research has been done in the past years, focusing primarily on metakaolin [2].
Recently, the focus shifted on calcined natural clays [3,4] which, from an economic
point of view, form the most interesting group of new SCM. The pozzolanic reactivity
in dependence of the mineralogical clay composition, primarily kaolinite content, has
been subject of many studies [5,6], which already allows drawing conclusions on the
reactivity based on the mineralogy. In order to enhance the evaluation of clay deposits
for the use as SCM, it is important to better understand the influence of mineralogy on
reaction behavior and to relate these parameters to the geological setting of the deposit.
This could help for a rough assessment of clay deposits based on geological data and
maps, which exist for many parts of the world.
In Germany, the current main application fields for clays are the brick, the ceramic
and the refractory industry. Lower quantities are used as sealing clays. Kaolin mainly
2

serves as ceramic raw material, filler for paper, plastics, rubber or colors. In 2015,
5.3 million tons of raw kaolin have been mined leading to 1.1 million tons of kaolin
products available for sale. On the other side there were 6.4 million tons of special clay
being used for the ceramic and refractory industry plus another 12 million tons of clay
used in the brick industry [7].
This study gives an overview over the clay deposits in Germany, including an eval-
uation of characteristic samples, regarding their applicability as calcined clays as SCM,
based on mineralogical characterization and assessment of pozzolanic reactivity.

2 Spatial analysis of clay deposits and selection of clays

On the basis of the maps of mineral resources [8] and Geology of Germany [9] a base
map (Fig. 1) was created using ESRI ArcGIS. ArcGIS is a geographic information sys-
tem, which allows to collect, analyze and present spatial data. Active clay pits and ce-
ment plants where added. Areas with a concentration of clay-deposits where described
geologically. From each area, at least one sample was selected for further investigation.

Fig. 1. Geological map of Germany 1:1,000,000 [9] with deposits of clay and claystone [8] and
active clay pits

Orange areas and the small orange dots represent clay and claystone, which are not
further classified, as they are provided by the map of mineral resources of Germany.
The manually added open clay pits are divided in brick clays, special clays, kaolin and
washing sludges. Brick clays are clays which are mainly used for the production of
 3

masonry bricks, roof tiles, facing bricks or clinker. Special clays represent the raw ma-
terial for refractories, acid-resistant and technical ceramics or fine-ceramics. Primary
kaolin deposits which are further processed to high-grade products are referred to as
kaolin. Washing sludges are residues from processing of other raw materials, for exam-
ple sand, gravel or also coal. Table 1 gives an overview over the selected clays with
reference to their geological origin.
The geographic location of clay deposits plays an important role regarding a use as
SCM, since the distance from the pits to potential customers defines a major part of the
costs. Beyond that, the geographical location can be referenced with geological maps,
as it was done in this study, to get a first idea about the suitability of the clays.

Table 1. Clays selected for investigations and their geological description

Short Long name Geological description

name
KT Raw Kaolin Primary kaolin deposit accrued by Eocene and Oligo-
Taunus cene weathering of Devonian chlorite-rich shale [10]
KUP Raw Kaolin Up- Primary kaolin deposit formed from Eocene until Mi-
per Palatinate ocene by weathering of Carboniferous granite [10]
FUP Fireclay Upper Sedimentary kaolin-rich clays deposited in the Mio-
Palatinate cene [11]
RKUP Recycling Kao- Secondary component of a Jurassic sandstone,
lin Upper Palati- enriched by technical wet-processing [10]
nate
AC Amaltheen Clay Early Jurassic marine sediments of a continental shelf
[12]
SW Shale Wester- Devonian slate which outcrops in Westerwald clay de-
wald posits [13]
SCW Stoneware Clay Tertiary weathering products of fine-grained Devo-
Westerwald nian rocks deposited in Eo- until Miocene [12]
CCW Coal-bearing Secondary component of Upper Carboniferous coal
Clay Westphalia beds [13]
MOSM Marl Upper Upper Eocene to the Upper Miocene sediments result-
Freshwater Mo- ing from erosion processes of the alps which were de-
lasse posited in the foreland basin [13]
SLS Shale Lower Cretaceous shale [12]
Saxony
KS Kaolin Saxony Tertiary kaolinisation of a Permian quartz-porphyry –
purified by wet-processing [11]

3 Experimental procedure

The chemical composition of the raw clays was analyzed by means of ICP-OES (Varian
ICP-OES 720 ES) on solutions of lithium metaborate flux fusions. The clay minerals
4

where identified by XRD (PANalytical Empyrean, Bragg‐ BrentanoHD monochrom-

ator, PIXcel1D linear detector) on oriented mounts of the particle fraction smaller than
2 µm following [14]. The samples where measured in air dried and in glycolated con-
dition in order to account for swellable clay minerals. Bulk mineralogy of raw and cal-
cined clays was analyzed on side loaded powder mounts in order to reduce preferred
orientation effects. The quantitative phase composition was calculated by Rietveld Re-
finement using Profex BGMN [15]. For the determination of the amorphous fraction of
the calcined samples the external standard method was applied according to [16]. Ther-
mal decomposition of the clays was investigated using TG/DTG (Netzsch STA 449 F3
Jupiter) with a heating rate of 2 K/min. The calcination temperature was defined by
adding 100 K to the offset temperature of the main dehydroxylation reaction. The clays
were calcinated for 30 minutes in a laboratory muffle furnace using platinum crucibles.
The calcined clays where ground in a vibratory disc mill with a speed of 700 min -1 for
10 minutes, using an agate grinding tool. Specific surface area (BET) was measured in
a Horiba S-9601 MP using nitrogen as absorption gas. The solubility of Al- and Si-ions
was determined by elution of the calcined material in NaOH-solution (10 %) [17]. Poz-
zolanic reactivity was assessed following the R³ calorimetry test at 40 °C [18,19].

4 Results

4.1 Chemical and mineralogical properties of the raw clays

The investigated raw clays are plotted in a ternary diagram (Fig. 2) based on their con-
tents of SiO2, Al2O3 and CaO + MgO, normalized to the sum of these components. The
relevant silica-rich region of the diagram is enlarged.

Fig. 2. Plot of the investigated raw clays in the ternary diagram based on the contents of Al 2O3,
SiO2 and CaO + MgO normalized to the sum of them
 5

The three clays in the alumina-rich right corner are sedimentary kaolinitic clays and
technically processed primary kaolin (FUP, SCW, KS). The marl (MOSM) and the cal-
cite-bearing Amaltheen clay (AC) can be differentiated by their CaO + MgO content.
The low grade kaolinitic clays plot in the silica-rich half of the SiO2-Al2O3-axis and
cannot be differentiated in this way. Ternary diagrams based on the chemical composi-
tion can help to separate the kaolinite-rich clays from the low kaolinite clays (if there
is no other major Al2O3 source) or marls from lime-free clays but are unsuitable to
differentiate between different low grade kaolinitic clays with several other clay min-
erals. Nevertheless, chemical compositions are often provided by clay-suppliers and
can be used for a first rough classification. The mineralogical composition of the dif-
ferent clays in Table 2 and is discussed together with the reactivity below point 4.3.

Table 2. Mineralogical composition of the analyzed clays: Qtz=Quartz, Kmd=moderately disor-

dered kaolinite, Khd=highly disordered kaolinite, Ill=illite, I-S=illite-smectite, Sm=smectite,
Ms=muscovite, Chl=chlorite, Cc=calcite, Dol=dolomite, Fsp=feldspar, Rt=rutile, An=anatase,
Py=pyrite, He=hematite, Goe=goethite, Sid=siderite, Am=amorphous
MOSM

RKUP
CCW

SCW
KUP
FUP

SLS
SW

AC
KT

KS
Qtz 12 21 16 38 20 18 48 33 37 18 16
Kmd - 7 - 20 - 8 17 26 7 - 25
Khd 74 - 10 - 23 - 28 - - 40 59
Ill 3 - 11 - - - - - - - -
I-S 8 - 24 - 32 - - - 40 20 -
Sm - - - - - 25 - - - - -
Ms - 51 - 36 5 19 3 36 15 18 -
Chl - 19 1 - 6 5 - - - - -
Cc - - <1 - 7 7 - - - - -
Dol - - 1 - 1 10 - - - - -
Fsp - - - - 4 7 - - - - -
Rt 1 1 - 1 <1 1 - 1 - 2 -
An 1 - 0 - 2 - <1 - 1 - 1
Py - - - - 1 - - - - - -
He - - <1 - - - <1 - - 2 -
Goe - - - 5 - - 3 4 - 1 -
Sid - - 1 - - - - - - - -
Am - - 35 - - - - - - - -

4.2 Characterization of calcined clays

Table 3 shows the X-ray-amorphous content after calcination, the specific surface area
after grinding and the calcination temperatures that were defined based on the end set
temperatures of the main dehydroxylation reaction in the DTG data.
6

Table 3. Calcination temperatures, amorphous content and BET after calcination and grinding

MOSM

RKUP
CCW

SCW
KUP
FUP

SLS
SW

AC
KT

KS
Tcalc [°C] 650 810 660 800 680 700 680 620 740 620 650
Am. [wt%] 76 37 49 19 43 31 33 23 36 56 81
BET [m²/g] 44.1 9.1 40.2 10.0 30.2 17.3 8.4 8.7 16.5 32.2 n.d.

4.3 Reactivity
The heat release of the different calcined clays during the R³ calorimetry test is shown
in Fig. 3. The first 1.2 h are cut off, according to Li et al. [19]. Quartz powder was used
as an inert reference. The investigated materials show a broad variation, regarding the
quantitative heat release as well as the reaction kinetics. The high grade kaolinitic clays
cause two maxima which are differently pronounced, referring to the silicate and alu-
minate reaction. The sedimentary kaolinitic clays (FUP & SCW) provide a fast reaction
with a first heat flow maximum below 8 h, which probably results from a faster release
of especially Al ions due to the high amount of disordered kaolinite and a high specific
surface area. The primary kaolins (KS, KT & KUP) and the recycling kaolin (RKUP),
which show a higher degree of order and therefore also a lower surface area provide a
slower reaction and the two maxima merge to one. The clays with lower amount of
kaolinite react clearly slower with one broad reaction maximum, which is due to the
lower release of Al ions.

Fig. 3. Development of the heat release normalized per gram of calcined clay during
R³ calorimetry test (left) and cumulative heat (right) from 1.2 to 72 hours

The correlation between Al2O3 content and reactivity (Fig. 4) is good for high kao-
lintite contents, where other clay minerals do not play an important role. Clays with
lower kaolinite content do not differ significantly in Al 2O3 content which is why this
 7

parameter cannot be used to assess them. As has been shown before [5,6], the decisive
criteria for reactivity is the overall kaolinite content of the raw clay. If it exceeds about
40 wt%, the contribution of other components to the heat flow is nearly negligible. In
the area of lower kaolinite contents, the role of the other clay minerals gains signifi-
cance. This is shown by the clays with a kaolinite content below 30 wt%. Here, clays
containing significant amounts of illite, smectite or illite-smectite mixed layer minerals
provide a clearly higher heat development during the R³ calorimetry test than those
containing mainly mica and quartz. This is in good consistency with investigations on
reactivity of single phyllosilicates [20].

Fig. 4. Influence of Al2O3 (top) and kaolinite (bottom) content on heat of hydration during
R³ calorimetry test (left) and on solubility of Si and Al ions (right)

5 Conclusion

Geological data can allow conclusions on the type of clay deposits and therefore help
for a rough estimation of suitability. In order to derive a more precise assessment, a
comprehensive mineralogical analysis is requisite. Particularly for low-grade kaolinitic
clays, the impact of other clay minerals is significant. An evaluation based on the chem-
ical composition does not work for low-grade kaolinitic clays, since the difference in
8

Al2O3 content is not significant. For the assessment of pozzolanic reactivity, the R³
calorimetry test and the solubility of Al and Si ions show very good consistency.

References
1. Snellings, R., Mertens, G., Elsen, J.: Supplementary Cementitious Materials. Reviews in
Mineralogy & Geochemistry 74, 211–278 (2012).
2. Sabir, B. B., Wild, S., Bai, J.: Metakaolin and calcined clays as pozzolans for concrete: a
review. Cement and Concrete Composites 23(6), 441–454 (2001).
3. Danner, T., Norden, G., Justnes, H.: Characterisation of calcined raw clays suitable as sup-
plementary cementitious materials. Applied Clay Science 162, 391-402 (2018)
4. Beuntner, N., Thienel, K.-C.: Properties of Calcined Lias Delta Clay - Technological Ef-
fects, Physical Characteristics and Reactivity in Cement. In: Scrivener, K., Favier, A. (eds.)
1ˢᵗ Int. Conf. Calcined Clays for Sustainable Concrete, 43-50. Springer, Netherlands (2015).
5. Tironi, A., Trezza, M. A., Scian, A. N., Irassar, E. F.: Kaolinitic calcined clays: Factors
affecting its performance as pozzolans. Construction and Building Materials, 28(1), 276-281
(2012).
6. Avet, F., Scrivener, K.: Investigation of the calcined kaolinite content on the hydration of
Limestone Calcined Clay Cement (LC3). CCR 107, 124-135 (2018).
7. Lorenz, W., Gwosdz, W.: Handbuch zur geologisch-technischen Bewertung von minerali-
schen Baurohstoffen. Geologisches Jahrbuch Sonderhefte. Bundesanstalt für Geowissen-
schaften und Rohstoffe (2003).
8. Dill, H.G., Röhling, S.: Map of Mineral Resources of Germany 1: 1 000 000 (BSK1000),
BGR, Hannover (2007).
9. Toloczyki, M. et al.: Geological Map of Germany 1:1,000,000 (GK1000), BGR, Hannover
(2006).
10. Elsner, H.: Kaolin in Deutschland. BGR, Hannover (2007).
11. Radczewski, O.-E.: Die Rohstoffe der Keramik. Springer, Heidelberg (1968).
12. Börner, A. et al.: Steine- und Erden-Rohstoffe in der Bundesrepublik Deutschland. Geolo-
gisches Jahrbuch, Sonderhefte, Heft SD 10. BGR, Hannover (2012).
13. Meschede, M.: Geologie Deutschlands. 2nd edn. Springer Spektrum, Berlin (2018).
14. Moore, D. M., Reynolds, R. C.: X-Ray Diffraction and the Identification and Analysis of
Clay Minerals. 2nd edn. Oxford University Press,
15. Döbelin, N., Kleeberg, R.: Profex: a graphical user interface for the Rietveld refinement
program BGMN, Journal of Applied Crystallography 48, 1573-1580 (2015).
16. O’Connor, B. H., Raven, M. D.: Application of the Rietveld refinement procedure in assay-
ing powdered mixtures, Powder Diffr. 3, 2–6 (1988).
17. Buchwald, A., Kriegel, R., Kaps, CH., Zellmann, H.-D.: Untersuchungen zur Reaktivität
von Metakaolinen für die Verwendung in Bindemittelsystemen. In: Proceedings Bauchemie,
Munich (2003).
18. Avet, F., Snellings, R., Diaz, A. A., Ben Haha, M., Scrivener, K.: Development of a new
rapid, relevant and reliable (R3) test method to evaluate the pozzolanic reactivity of calcined
kaolinitic clays. Cement and Concrete Research, 85, 1-11 (2016).
19. Li, X. et al.: Reactivity tests for supplementary cementitious materials: RILEM TC 267-
TRM phase 1. Materials and Structures, 51:151, 1-14 (2018).
20. Scherb, S., Beuntner, N., Thienel, K.-C.: Reaction kinetics of basic clay components present
in natural mixed clays. In: Martirena, M., Favier, A., Scrivener, S. (eds) Proc. 2nd Int. Conf.
on Calcined Clays for Sustainable Concrete, 427-433. Springer, Netherlands (2018).