3494 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
often require compromise in the concrete mix design. With
transport distances between quarries and concrete plants
becoming extended, sand from mine tailings may become
an economically feasible alternative.
Peys et al. (2022) proposed that large volumes of mine
tailings could be converted into synthetic aggregates by
granulation with a cementitious binder in a high-intensity
mixer. Early research showed that geopolymerization may
be used for reactive tailings (Van Jaarsveld et al. 2000).
Besides the issue of transportation costs, it is worth-
while considering the challenges of using mine tailings in
construction. Most of the reviews in the field conclude that
a small portion of sand, say up to 15%, could be replaced by
tailings sand in concrete. Tailings sand often has a particle
size and particle shape distribution that reduces the work-
ability of the concrete, which increases placement time and
inhibits adoption by the market, even if there is a saving in
material cost. Moreover, tailings sand may contain excessive
slimes or clay, which further reduces workability, increases
water demand and impacts deleteriously on the effective-
ness of plasticisers. These deleterious effects of tailings sand
are usually accentuated in low-CO2 concrete containing a
high level of SCM. Some tailings containing reactive sil-
ica may cause an alkali-silica reaction (ASR) resulting in
delayed expansion of the concrete, which is usually coun-
teracted by using a high SCM binder. If the tailings con-
tain a high level of heavy metals, its use in concrete may be
prohibited, even if it can be demonstrated that the binding
phases effectively immobilize the metals (Table 2).
It is worth considering how the above obstacles could
be overcome. On the one hand, the concrete industry
should be prepared to revise its practices of concrete place-
ment. Additionally, there is a need for the development of
admixtures that can accommodate a higher portion of slime
from tailings. Recent trials by the author demonstrated that
published plasticizer structures (Lei and Plank 2014 Plank
et al. 2015 Lei et al. 2022) are not effective in controlling
the slump of concrete contaminated by clay.
In turn, the mining industry should develop the mind-
set that tailings sand is a valuable product that requires
process design and quality control, instead of leaving it to
the construction industry to deal with. For example, flo-
tation circuits may be modified to remove coarse gangue
particles upfront, especially with recent developments in
coarse particle flotation (Anzoom et al. 2024), including
the Nova CellTM developed by Graeme Jameson. Grinding
may be tailored for a desirable size distribution of the coarse
particles and not just mineral liberation. Flotation practice
may be aimed at the removal of slimes using a combina-
tion of coarse and fine flotation cells like Concorde CellTM
(another Jameson invention). The valorization of tailings
sand in a new mining project should be considered in the
early stage of a feasibility study. Construction sand has
become such a valuable commodity that in some instances
it is even shipped internationally.
LIMITATION OF CLINKER
TECHNOLOGY
Although the chemistry of clinker formation in rotary kilns
is well known (San Nicolas et al. 2019), the limitations of
this technology are less appreciated (Table 3). Limestone as
a source of CaO, sand, clay or shale as sources of SiO2 and
Al2O3, and iron-bearing minerals are combined stoichio-
metrically and ground finely to form a “raw meal” and fed
to a precalciner followed by a rotary kiln. Drying occurs
below 750°C, followed by decarbonization of the limestone
at 950°C and calcination of the clay or shale to form alumi-
nates and silica. Between 950°C and 1,350°C, calcium, sili-
con and aluminum oxides react to form calcium aluminates
and calcium silicates, including (CaO)2SiO2 which is called
belite. Between 1,350°C and 1,450°C lime reacts with
belite to form (CaO)3SiO2 which is called alite, in a process
called clinkerization. Lime also reacts with calcium alumi-
nate and ferrite to form tricalcium aluminate and tetracal-
cium aluminoferrite. Quenching of the clinker by large air
fans then solidifies and preserves the metastable alite. The
heat recovered from the cooling process is recirculated back
to the kiln or precalciner. The clinker is subsequently
ground with about 4% gypsum to produce Portland
cement (PC).
Today, clinkerization in a rotary kiln is taken for
granted, which inhibits innovation. Metal cations reduce
the melting point of aluminosilicates to below 1,350°C.
In contrast, the melting points for CaO and MgO are
2,572°C and 2,852°C respectively. In clinker formation,
the lower melting aluminosilicates form around the high
melting CaO particles, with the latter being absorbed into
Table 3. Technical limitations of rotary kilns as incumbent
cement technology
• Clinker composition restricted to a high lime to silica ratio
• Quenching rate inadequate to preserve the glassy phase of
high silica clinker
• Temperature restricted to around 1,450°C
• Temperature instead of entropy used to decrease Gibbs free
energy
• Not possible to use high-MgO limestone
• Only economical at large scale
often require compromise in the concrete mix design. With
transport distances between quarries and concrete plants
becoming extended, sand from mine tailings may become
an economically feasible alternative.
Peys et al. (2022) proposed that large volumes of mine
tailings could be converted into synthetic aggregates by
granulation with a cementitious binder in a high-intensity
mixer. Early research showed that geopolymerization may
be used for reactive tailings (Van Jaarsveld et al. 2000).
Besides the issue of transportation costs, it is worth-
while considering the challenges of using mine tailings in
construction. Most of the reviews in the field conclude that
a small portion of sand, say up to 15%, could be replaced by
tailings sand in concrete. Tailings sand often has a particle
size and particle shape distribution that reduces the work-
ability of the concrete, which increases placement time and
inhibits adoption by the market, even if there is a saving in
material cost. Moreover, tailings sand may contain excessive
slimes or clay, which further reduces workability, increases
water demand and impacts deleteriously on the effective-
ness of plasticisers. These deleterious effects of tailings sand
are usually accentuated in low-CO2 concrete containing a
high level of SCM. Some tailings containing reactive sil-
ica may cause an alkali-silica reaction (ASR) resulting in
delayed expansion of the concrete, which is usually coun-
teracted by using a high SCM binder. If the tailings con-
tain a high level of heavy metals, its use in concrete may be
prohibited, even if it can be demonstrated that the binding
phases effectively immobilize the metals (Table 2).
It is worth considering how the above obstacles could
be overcome. On the one hand, the concrete industry
should be prepared to revise its practices of concrete place-
ment. Additionally, there is a need for the development of
admixtures that can accommodate a higher portion of slime
from tailings. Recent trials by the author demonstrated that
published plasticizer structures (Lei and Plank 2014 Plank
et al. 2015 Lei et al. 2022) are not effective in controlling
the slump of concrete contaminated by clay.
In turn, the mining industry should develop the mind-
set that tailings sand is a valuable product that requires
process design and quality control, instead of leaving it to
the construction industry to deal with. For example, flo-
tation circuits may be modified to remove coarse gangue
particles upfront, especially with recent developments in
coarse particle flotation (Anzoom et al. 2024), including
the Nova CellTM developed by Graeme Jameson. Grinding
may be tailored for a desirable size distribution of the coarse
particles and not just mineral liberation. Flotation practice
may be aimed at the removal of slimes using a combina-
tion of coarse and fine flotation cells like Concorde CellTM
(another Jameson invention). The valorization of tailings
sand in a new mining project should be considered in the
early stage of a feasibility study. Construction sand has
become such a valuable commodity that in some instances
it is even shipped internationally.
LIMITATION OF CLINKER
TECHNOLOGY
Although the chemistry of clinker formation in rotary kilns
is well known (San Nicolas et al. 2019), the limitations of
this technology are less appreciated (Table 3). Limestone as
a source of CaO, sand, clay or shale as sources of SiO2 and
Al2O3, and iron-bearing minerals are combined stoichio-
metrically and ground finely to form a “raw meal” and fed
to a precalciner followed by a rotary kiln. Drying occurs
below 750°C, followed by decarbonization of the limestone
at 950°C and calcination of the clay or shale to form alumi-
nates and silica. Between 950°C and 1,350°C, calcium, sili-
con and aluminum oxides react to form calcium aluminates
and calcium silicates, including (CaO)2SiO2 which is called
belite. Between 1,350°C and 1,450°C lime reacts with
belite to form (CaO)3SiO2 which is called alite, in a process
called clinkerization. Lime also reacts with calcium alumi-
nate and ferrite to form tricalcium aluminate and tetracal-
cium aluminoferrite. Quenching of the clinker by large air
fans then solidifies and preserves the metastable alite. The
heat recovered from the cooling process is recirculated back
to the kiln or precalciner. The clinker is subsequently
ground with about 4% gypsum to produce Portland
cement (PC).
Today, clinkerization in a rotary kiln is taken for
granted, which inhibits innovation. Metal cations reduce
the melting point of aluminosilicates to below 1,350°C.
In contrast, the melting points for CaO and MgO are
2,572°C and 2,852°C respectively. In clinker formation,
the lower melting aluminosilicates form around the high
melting CaO particles, with the latter being absorbed into
Table 3. Technical limitations of rotary kilns as incumbent
cement technology
• Clinker composition restricted to a high lime to silica ratio
• Quenching rate inadequate to preserve the glassy phase of
high silica clinker
• Temperature restricted to around 1,450°C
• Temperature instead of entropy used to decrease Gibbs free
energy
• Not possible to use high-MgO limestone
• Only economical at large scale