XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3497
As explained above, dolomitic sources can be used in the
EESS reactors in contrast with rotary kilns that can pro-
cess limestone containing less than 3% MgO. The reason is
that the electric field of the EESS fuses high melting CaO
and MgO with lower melting aluminosilicates at a lower
ambient temperature, which then form a glassy phase that
is preserved in the shock quenching.
As outlined in Figure 2, three processing options are
possible, which gives flexibility regarding the composi-
tion of source materials and the construction application.
If there is a need to produce PC, fine limestone can be
combined with fine aluminosilicates and other materials
to stoichiometrically adjust the composition of the clinker-
like product from EESS 1. This fine product is then inter-
ground with gypsum to produce PC.
As a second option, the aluminosilicate waste material
could be processed directly in EESS 2 in Figure 2 to pro-
duce glassy SCM, similar to TerraCO2. Preferably, some
CaO source is combined with the aluminosilicate to pro-
duce SCM with enhanced reactivity. A dolomitic or high
MgO limestone source may also be used, which is not pos-
sible in existing reactors, including a rotary kiln. The shock
quenching in EESS 2 gives a glassy cementitious product
that can range in elemental composition from clinker to
GBFS to CFA, without phase separation of the CaO, MgO
and aluminosilicates. This process has the potential to con-
vert a wide range of mine waste and tailings, unlike clin-
kerization. These glassy SCMs will be used to answer key
questions in cement chemistry. At present, a small amount
of PC, say 15%, can be used with admixtures to activate a
large amount of SCM like GBFS, say 85%. Hydration of
the PC generates calcium hydroxide (portlandite) that in
turn reacts with the SCM to give binding phases (Snellings
et al. 2023). Thermodynamic models are used widely to
predict phase assemblage for such PC-SCM blends and
in theory, should give the same result for a glassy SCM of
the same stoichiometry as the PC-SCM blend. However,
in practice such glasses behave differently due to kinetics,
which is a subject on which the cement literature is silent.
Further research will show whether a small amount of PC
is still required to enhance the reactivity of the glassy SCM
produced in EESS 2.
In EESS 3 in Figure 2, clay or silt from tailings is cal-
cined at a lower temperature like 750°C to cause dehydrox-
ylation without the addition of a CaO source. The product
is typically CC and can also be produced by a flash calciner.
Such CC can be used as a minority SCM blended with PC.
Alternatively, CC or CFA can be combined with GL and
PC to give limestone-calcined-clay cement (LC3) (Snellings
et al. 2023).
PERFORMANCE-BASED STANDARDS
In several jurisdictions, the standards now allow LC3 and PC
blends. Unfortunately, several standard frameworks remain
prescriptive by specifying clinker content and which type of
SCM is allowed (Table 2). Consequently, the SCM synthe-
sized in the EESS reactors in Figure 2 is not yet allowed in
such prescriptive standards, which protects the incumbent
cement industry and presents an obstacle to innovation.
The new Australian standard for geopolymer structural
concrete overcomes this barrier to some extent (Standards
Australia Technical Specification 199:2023: Design of geo-
polymer and alkali-activated binder concrete structures). A
key development was the publication of Australian Standard
AS 3582.4:2022: Supplementary cementitious materials
(SCM) Part 4: Pozzolans – Manufactured in 2022.
If the construction industry is serious about CO2
reduction, it needs to move towards performance-based
standards (Sutter and Hooton, 2023). The need for labora-
tory test methods to quantify the durability of low-CO2
concrete requires interdisciplinary research as suggested in
Table 4. If we are serious about CO2 reduction, we need to
achieve wide acceptance of such durability methods (Van
Deventer et al. 2021).
FINAL REMARKS
The proposed EESS technology offers several reaction path-
ways to convert tailings into cementitious materials with
low CO2 emissions. Clinkerization in rotary kilns com-
bined with air quenching restricts the composition of the
product to high-alite PC. In contrast, EESS combined with
shock quenching preserves the glassy state to give products
ranging from PC to low-CaO SCM with high reactivity.
The reduction in CaO content of the products decreases
Table 4. Opportunities for interdisciplinary research
• Modification of mineral processing circuits to produce
building sand
• Modification of mineral processing circuits to yield source
material for producing synthetic SCM and cement in
modular reactors
• Effect of tailings sand on concrete properties
• Effect of high-MgO in cement from modular electric
reactors
• Effect of SCM and cement from modular electric reactors
on concrete strength and durability
• Predictive models for cementitious reactors integrating
kinetics with thermodynamic models
Cementitious reactions using low-Ca SCM from modular
electric reactors in comparison with conventional clinker-high
SCM blends
As explained above, dolomitic sources can be used in the
EESS reactors in contrast with rotary kilns that can pro-
cess limestone containing less than 3% MgO. The reason is
that the electric field of the EESS fuses high melting CaO
and MgO with lower melting aluminosilicates at a lower
ambient temperature, which then form a glassy phase that
is preserved in the shock quenching.
As outlined in Figure 2, three processing options are
possible, which gives flexibility regarding the composi-
tion of source materials and the construction application.
If there is a need to produce PC, fine limestone can be
combined with fine aluminosilicates and other materials
to stoichiometrically adjust the composition of the clinker-
like product from EESS 1. This fine product is then inter-
ground with gypsum to produce PC.
As a second option, the aluminosilicate waste material
could be processed directly in EESS 2 in Figure 2 to pro-
duce glassy SCM, similar to TerraCO2. Preferably, some
CaO source is combined with the aluminosilicate to pro-
duce SCM with enhanced reactivity. A dolomitic or high
MgO limestone source may also be used, which is not pos-
sible in existing reactors, including a rotary kiln. The shock
quenching in EESS 2 gives a glassy cementitious product
that can range in elemental composition from clinker to
GBFS to CFA, without phase separation of the CaO, MgO
and aluminosilicates. This process has the potential to con-
vert a wide range of mine waste and tailings, unlike clin-
kerization. These glassy SCMs will be used to answer key
questions in cement chemistry. At present, a small amount
of PC, say 15%, can be used with admixtures to activate a
large amount of SCM like GBFS, say 85%. Hydration of
the PC generates calcium hydroxide (portlandite) that in
turn reacts with the SCM to give binding phases (Snellings
et al. 2023). Thermodynamic models are used widely to
predict phase assemblage for such PC-SCM blends and
in theory, should give the same result for a glassy SCM of
the same stoichiometry as the PC-SCM blend. However,
in practice such glasses behave differently due to kinetics,
which is a subject on which the cement literature is silent.
Further research will show whether a small amount of PC
is still required to enhance the reactivity of the glassy SCM
produced in EESS 2.
In EESS 3 in Figure 2, clay or silt from tailings is cal-
cined at a lower temperature like 750°C to cause dehydrox-
ylation without the addition of a CaO source. The product
is typically CC and can also be produced by a flash calciner.
Such CC can be used as a minority SCM blended with PC.
Alternatively, CC or CFA can be combined with GL and
PC to give limestone-calcined-clay cement (LC3) (Snellings
et al. 2023).
PERFORMANCE-BASED STANDARDS
In several jurisdictions, the standards now allow LC3 and PC
blends. Unfortunately, several standard frameworks remain
prescriptive by specifying clinker content and which type of
SCM is allowed (Table 2). Consequently, the SCM synthe-
sized in the EESS reactors in Figure 2 is not yet allowed in
such prescriptive standards, which protects the incumbent
cement industry and presents an obstacle to innovation.
The new Australian standard for geopolymer structural
concrete overcomes this barrier to some extent (Standards
Australia Technical Specification 199:2023: Design of geo-
polymer and alkali-activated binder concrete structures). A
key development was the publication of Australian Standard
AS 3582.4:2022: Supplementary cementitious materials
(SCM) Part 4: Pozzolans – Manufactured in 2022.
If the construction industry is serious about CO2
reduction, it needs to move towards performance-based
standards (Sutter and Hooton, 2023). The need for labora-
tory test methods to quantify the durability of low-CO2
concrete requires interdisciplinary research as suggested in
Table 4. If we are serious about CO2 reduction, we need to
achieve wide acceptance of such durability methods (Van
Deventer et al. 2021).
FINAL REMARKS
The proposed EESS technology offers several reaction path-
ways to convert tailings into cementitious materials with
low CO2 emissions. Clinkerization in rotary kilns com-
bined with air quenching restricts the composition of the
product to high-alite PC. In contrast, EESS combined with
shock quenching preserves the glassy state to give products
ranging from PC to low-CaO SCM with high reactivity.
The reduction in CaO content of the products decreases
Table 4. Opportunities for interdisciplinary research
• Modification of mineral processing circuits to produce
building sand
• Modification of mineral processing circuits to yield source
material for producing synthetic SCM and cement in
modular reactors
• Effect of tailings sand on concrete properties
• Effect of high-MgO in cement from modular electric
reactors
• Effect of SCM and cement from modular electric reactors
on concrete strength and durability
• Predictive models for cementitious reactors integrating
kinetics with thermodynamic models
Cementitious reactions using low-Ca SCM from modular
electric reactors in comparison with conventional clinker-high
SCM blends