This technical note summarizes a study on the thermal expansion of slate wastes from mining and processing activities. The researchers treated slate waste through a thermal expansion process using a rotary kiln to generate products for use in the cement industry. Characterization studies showed that thermal expansion decreased crystallinity and increased porosity and amorphous content. Thermally expanded slate had lower density, water absorption and higher loss on ignition compared to natural slate. Testing confirmed that thermally expanded slate exhibited pozzolanic activity and could partially replace Portland cement, making it a technically viable way to recycle slate wastes in construction.

1. Technical note
Thermal expansion of slate wastes
M.E.M.C. Silva a
, A.E.C. Peres b,*
a
Centro Tecnológico de Minas Gerais, Avenida José Cândido da Silveira, 2000, 31170-000 Belo Horizonte, MG, Brazil
b
Universidade Federal de Minas Gerais, Rua Espirito Santo, 35/206, 30160-030 Belo Horizonte, MG, Brazil
Received 25 July 2005; accepted 11 October 2005
Available online 28 November 2005
Abstract
Among the technological routes for recycling mining and beneficiation slate wastes, thermal expansion seems particularly attractive,
yielding products adequate for use in civil construction, especially as pozzolanic material for cement manufacture. The present investi-
gation aims at analysing the properties of slate before and after the thermal expansion, searching for the reasons of expansion and the
variables that affect the characteristics and properties of the products. Results of standard test procedures recommended by the construc-
tion industry are presented and discussed.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Slate waste processing; Thermal expansion; Industrial minerals
1. Introduction
Slate mining and beneficiation wastes are inadequately
disposed in piles. These wastes represent a significant
amount of material with potential for utilisation.
The proposed alternative is treating this material by
thermal expansion to generate products for utilisation in
the cement industry. Studies were performed searching
for a correlation among mineralogical, petrographic and
geochemical properties of slate with those of the thermal
expansion products with the aim to shed light onto the
causes of the expansion and the variables that affect the
characteristics and properties of the products.
Natural slate wastes do not possess pozzolanic activity.
Thermally expanded wastes, on the other hand, may
replace partially the Portland cement clinker as a pozzola-
nic additive.
2. Experimental
2.1. Materials
The following materials were used:
(i) grey slate wastes (from mining and cutting activities)
from the Slate Province of Minas Gerais (Grossi Sad
et al., 1998);
(ii) Portland cement clinker and gypsum, used in tests of
axial compression resistance for comparison with
slate as pozzolanic additive;
(iii) sand, for composing the mortar mix.
2.2. Thermal expansion
A literature review suggested the use of a rotary tubular
kiln in the thermal process of expansive clays and slates (San-
tos, 1992; Cinasita, 2001; Stalite, 2001; ESCSI, 2001). An
electrically heated kiln was manufactured. The option for
the electrically heated kiln for the laboratory experiments
was based on a better operation control, especially regarding
temperature, and a lower capital cost. At pilot and industrial
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.mineng.2005.10.008
*
Corresponding author. Tel.: +55 313 238 1717; fax: +55 313 238 1815.
E-mail address: aecperes@demet.ufmg.br (A.E.C. Peres).
This article is also available online at:
www.elsevier.com/locate/mineng
Minerals Engineering 19 (2006) 518–520

2. scales this type of kiln is not economically viable. Heating is
achieved via resistive elements of silicon carbide, disposed in
two zones. The temperature control is independent for each
zone, allowing operation up to 1400 °C.
The wastes of slate were cut in squares or rectangles (1–
4 cm) for preliminary thermal expansion tests. Later the
samples were comminuted and screened. The fraction
12.7 mm + 6.35 mm was selected, for the initial expecta-
tion was meeting the specifications for lightweight aggre-
gates. For thermal expansion tests the size range was not
relevant and the selection was based on the dimensions of
the kiln.
The main variables that may influence the thermal
expansion process are: type of the kiln; feed rate, size dis-
tribution, heating rate, time and temperature of calcina-
tion, type of slate.
Only one type of kiln was utilised, allowing a feed of
30 g per batch. The heating rate is a relevant variable. A
high heating rate may cause decrepitation, due to the high
rate of water leaving the particle, resulting in aggregates
with low mechanical resistance (Martins and Lima, 1996).
In the preliminary tests, heating rates of 20, 10, 5 °C/min
and calcination temperatures of 1100 and 1300 °C were uti-
lised, considering the operation limitations of the kiln, the
thermal behaviour of the slate samples identified by
thermo-gravimetric and thermo-differential analyses, as
well as references to other studies (Martins and Lima,
1996) and industrial application (Stalite, 2001).
The kiln consists of two adjacent heating zones. The
sample was heated up to 600 °C in the cool zone, then
the slope of the kiln was changed and the sample was
moved to the hot zone.
After the preliminary experiments, the heating rate was
set at 5 °C/min up to 600 °C, then at 15 °C/min up to the cal-
cination temperature (1170 and 1190 °C). Calcination time
at each calcination temperature was set at 5 and 10 min.
The thermal expansion products were characterised via
(i) X-ray diffraction (degree of remaining crystallinity);
(ii) chemical analyses (expressed as oxides): FeO (wet
chemistry, volummetry with K2Cr2O7); Al2O3, CaO,
Fe2O3, MgO, MnO (fusion—ICP); Na2O, K2O
(microwaves—ICP); loss on ignition—LOI (calcina-
tion at 950 °C ± 50 °C 1 h);
(iii) microscopic analyses (size and dimensions of the
pores, preferential direction of the expansion and
degree of homogeneity of the phases produced by
calcination);
(iv) determination of the specific mass, water absorption
and mechanical resistance.
2.3. Characterisation of slate as a mineral additive
in cement manufacture
Pozzolanic activities with cement and lime are standar-
dised in Brazil (NBR 5752, 1992). Cement or lime is par-
tially replaced (35% by volume) by the material under
investigation. The index of pozzolanic activity is the ratio
between the resistance to axial compression after 28 days
of the two mortars: (1) reference mortar containing the
blend clinker + gypsum; (2) 35% in volume of the reference
blend replaced by the presumed pozzolanic material.
Another method for determination of the pozzolanic activ-
ity is the chemical method (NBR 5753, 1991).
Wastes of grey slate (the most abundant type) were
selected for the determination of pozzolanic activity, at
its natural condition and after expansion. The samples
were ground to maximum 34% retained in 0.045 mm.
2.4. Cement mortar method
Specimens were prepared with the standard mix
(clinker + gypsum + normal sand) and with slate replacing
the compounds clinker and gypsum (35% in volume).
The specimens were kept under controlled temperature
(38 °C ± 2 °C) for 28 days.
2.5. Chemical method
According to this method the pozzolanic activity is eval-
uated by comparing the amount of Ca(OH)2 present in the
liquid phase in contact with hydrated cement with the
amount of Ca(OH)2 required for the saturation of the med-
ium at the same alkalinity.
3. Results and discussion
3.1. Characterisation studies
The major mineralogical components of the slate,
according to X-ray diffraction, are: white sericite mica,
quartz, chlorite, feldspar, and carbonates. Minor compo-
nents are epidote, turmaline, zircon, apatite, biotite, and
opaque minerals.
The presence of significant amounts of chlorite, mica,
carbonates, hematite and pyrite, affects the expansion of
slates, due to the role of these minerals in the characteristic
structure and texture.
Chemically, the major species detected were SiO2,
Al2O3, FeO, Fe2O3, K2O, MgO, and Na2O. The compo-
sition varies widely even for slates from the same
deposit, rendering it difficult to correlate the chemical com-
position with thermal expansion. Nevertheless, the average
composition is within the range predicted for thermal
expansion.
The average value obtained for the specific mass was 2.7
and low indexes of water absorption were determined
(<0.2%).
3.2. Thermal expansion process
The calcination temperature affects the degree of expan-
sion of the slate. The products achieved at 1250 °C present
M.E.M.C. Silva, A.E.C. Peres / Minerals Engineering 19 (2006) 518–520 519

3. lower specific mass and mechanical resistance than those at
1180 °C.
The thermal expansion products are predominantly
amorphous with only quartz and pseudo-spinel with a
low crystallinity are present as crystalline phases, according
to X-ray diffraction, indicating significant structure change
with respect to the natural slate.
Regarding chemical composition, the higher expansion
indexes of slates are associated with higher levels of loss
on ignition, due to CO2 and H2O losses. The LOI index
decreases from 3.5% in the natural slate to 0.1% after
expansion, confirming the importance of the presence of
structural water in the slate, as well as the evolution of
CO2 in the expansion.
The average value obtained for the specific mass was
0.90 for thermal expansion products and low values for
water absorption index were determined, both before and
after thermal expansion.
3.3. Characterisation of slate as mineral additive
Specifications from NBR 12653 (1992) and from ASTM
C 618 (1997) for the chemical composition of pozzolanic
class N materials are compared with the average values
obtained for the natural and the product from the thermal
expansion of grey slate:
SiO2 + Al2O3 + Fe2O3 (%min): 70 (NBR & ASTM);
81.0 (natural); 85.9 (expanded).
SO3 (%max): 4.0 (NBR & ASTM); <125 ppm (natural);
<125 ppm (expanded).
Moisture (%max): 3.0 (NBR & ASTM); <1 (natural); <1
(expanded).
LOI (%max): 10.0 (NBR & ASTM); 3.43 (natural); 0.01
(expanded).
Alkalis as total Na2O (%max): 1.5 (NBR & ASTM);
1.93 (natural); 1.70 (expanded).
Results of pozzolanic activity (chemical method) related
to natural slates did not confirm the pozzolanicity of the
mortars. On the other hand, the results confirmed pozzola-
nic activity of the sample with calcinated slate. Results of
pozzolanic activity (mortar method), also confirmed pozzo-
lanic activity of the sample with calcinated slate. Thermally
expanded slate presents pozzolanic activity level higher
than the standard minimum limit and similar to that for
the reference mortar.
The pozzolanicity of the thermally treated slate is not
dependent on its chemical composition, but whether silica
present in it (and to a lesser extent also the alumina) is in
an amorphous state that can be activated to react easily
with the Ca(OH)2 released during cement hydration to
form additional cementitious C–S–H compounds to con-
tribute to the strength development of the cement.
4. Conclusions
The calcination temperature is relevant regarding the
thermal expansion parameters: expansion degree, specific
mass and mechanical resistance of the products.
The expansion of slates occurs in the direction per-
pendicular to the cleavage, as observed in the specimens
produced, indicating a close correlation between struc-
ture, mineralogical composition and thermal expansion.
It is believed that the presence of filosilicates, parallel
to the cleavage plane, presenting inserted layers of struc-
tural water, plays a relevant role in the thermal expansion.
The sequence of structure water between the plaques
also plays a relevant role regarding the thermal expan-
sion, due to the release of water present in the slate
structure.
The chemical composition of the thermal expansion
products, as well as their low crystallinity degree, are
within the specifications for use as pozzolanic materials.
The results of pozzolanic activity, obtained by the chem-
ical method, were confirmed by the results of mechanical
resistance of mortars, showing that the products of slate
thermal expansion present pozzolanic activity.
Products from thermal expansion of slates, due to their
pozzolanic character, represent a technically viable alterna-
tive for the use of wastes from mining and beneficiation of
slates.
Natural slate does not present pozzolanic activity but its
chemical composition is compatible with its use as mineral
addition to clinker.
References
ASTM C 618, 1997. American Society for Testing and Material—ASTM
C 618. Standard Specification for Coal Fly Ash and Raw or Calcined
Natural Pozzolan for Use as a Mineral Admixture in Concrete.
Philadelphia.
Cinasita, 2001. (Cinasita S. A—Indústria e Comércio), São Paulo.
Available from: http://www.cinasita.com.br/ (in Portuguese).
ESCSI, 2001. Expanded Shale, Clay & Slate Institute (ESCSI). Available
from: http://www.escsi.org/.
Grossi Sad, J.H., Chiodi Filho, C., Chiodi, D.K., 1998. Scenario of slates
in Minas Gerais state, Brazil. COMIG, 2v., Belo Horizonte (in
Portuguese).
Martins, J., Lima, N.P., 1996. Utilisation of sized slate wastes as light
aggregates in civil construction. In: 51° Congresso Anual da ABM,
Porto Alegre, Proceedings (in Portuguese).
NBR 5752, 1992. Associação Brasileira de Normas Técnicas. Pozzolanic
materials. Rio de Janeiro (in Portuguese).
NBR 5753, 1991. Associação Brasileira de Normas Técnicas. Cements.
Rio de Janeiro (in Portuguese).
NBR 12653, 1992. Associação Brasileira de Normas Técnicas. Pozzolanic
materials. Rio de Janeiro (in Portuguese).
Santos, P.S., 1992. Science and Technology of Clays, second ed.
Edgard Blücher Ltda, São Paulo, pp. 242–302 and 438–467 (in
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Stalite, Carolina Stalite Company, 2001. Performance Lightweigh Aggre-
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520 M.E.M.C. Silva, A.E.C. Peres / Minerals Engineering 19 (2006) 518–520