A preliminary assessment of conditions for the industrial manufacture of a new cementitious system based on clinker-calcined clay and limestone, developed by the authors, referred as “low carbon cement” is presented. The new cement enables the substitution of more than 50% of the mass of clinker without compromising performance. The paper presents the follow-up of an industrial trial carried out in Cuba to produce 130 tonnes of the new cement at a cement plant. The new material proved to fulfill national standards in applications such as the manufacture of hollow concrete blocks and precast concrete. No major differences either in the rheological or mechanical properties were found when compared with Portland cement. Environmental assessment of the ternary cement was made, which included comparison with other blended cements produced industrially in Cuba. The new cement has proven to contribute to the reduction of above 30% of carbon emissions on cement manufacture.
The amount of cement produced in 2011 was 3.6 billion of tonnes (
Approximately 5–9% of anthropogenic total emissions of CO2 are related to the concrete production. This is caused by the massive production and use of cement in the construction sector; 85% of concrete emissions are associated with cement manufacture; thus its contribution is approximately 3–5% of global CO2 emission (
Tackling the urgent housing problems in development countries prompts for the massive availability of construction materials. The demand of cement is projected to double by 2050; and most of this increase will come from development countries; in which infrastructure is not been developed yet; and the growth of population will put pressure on expansion of existing urban areas (
The cement manufacture process includes burning the raw materials at very high temperatures to produce clinker. Depending on the type of process used; the energy consumption can be between 3000–6500 MJ/tonne of clinker (
A strategy for a higher reduction of CO2 emissions during cement manufacture may be to reduce the amount of clinker used; for it is the main responsible of CO2 emissions. This can be attained through the use of Supplementary Cementitious Materials (SCM) as clinker substitutes. However; the current substitution rate for most cement industries is around 30%; which is not sufficient enough to drastically reduce the global carbon emissions associated with cement manufacture (
The authors of this paper have developed a new cementitious system that increases clinker substitution to 50% without significantly influencing cement performance. The new system is based on the synergy between the aluminates supplied by calcined kaolinite clay and the carbonates from limestone; which enhances the pozzolanic reaction of the calcined clay; and thus enables a higher clinker substitution rate (
The two SCMs used in the new system; calcined clay and limestone have a higher availability than other SCMs. Low grade kaolinite clay has proven to be a suitable alternative to the pure kaolinite metakaolin (MK) currently used by the industry (
This article presents the results of an industrial trial carried out at a cement plant in Cuba for the production of the new cement at industrial scale. Comparisons between industrial and lab results were made. Further; a preliminary environmental assessment of the production of the new cement was done; which included a comparison with cements produced industrially in Cuba.
Pozzolans are been extensively used as SCM to substitute clinker in cement production. They react as described below –for silica rich pozzolanic systems- (
The amount of clinker than can be substituted by pozzolans depends on the availability of calcium hydroxide produced during cement hydration. In pure silica pozzolanic systems; for substitution ratios above 15% wt.; calcium hydroxide produced during hydration is not sufficient to react with all the pozzolanic material (see
Relation between clinker substitution and portlandite availability in the blended cement
Compounds | Cement no replacement | Cement 15% replacement | Cement 20% replacement | Cement 25% replacement |
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Available calcium hydroxide | 27.0 g | 23.0 g | 21.6 g | 20.3 g |
Pozzolan | 0.0 g | 15.0 g |
20.0 g |
25.0 g |
Calcium hydroxide needed for a complete reaction |
0.0 g | 19.4 g | 25.9 g | 32.4 |
Calculated by the authors considering that 70% of pozzolan reacts.
Metakaolin (MK) (Al2Si2O7) is a highly reactive pozzolan produced through the calcination of clays rich in kaolinite mineral. The reactivity of MK depends on several factors; such as temperature; rate of heating and cooling regimen. The optimal temperature of calcination is between 700–800 °C; although the dehydroxylation of the clay is known to begin at 500 °C (
The pozzolanic reaction of MK with the calcium hydroxide (CH) in pastes is described as follows (
Metakaolin is rich in alumina; so its use as SCM increases the volume of new alumina phases in the system. If calcium carbonate is introduced in the system through an external source; for example; limestone; the alumina phases react with it to form the following compounds (
Based on this principle; it is possible to replace a mass of clinker by a similar mass of MK and calcium carbonate mixed in a 2:1 ratio; respectively; to form hydration products; which are able to fill the pore system of the cement matrix and contribute to the strength. Experimental laboratory results (
The low carbon cement developed is produced by using the ternary systems of clinker-calcined clays and limestone described above. Medium purity kaolinite clay has proven to be a good alternative to MK in this system; thus increasing the availability and reducing production cost of the cement. The limestone introduced to the system is not calcined; thus no extra CO2 is emitted to the environment (
There are already reports of such an approach for ternary Portland limestone blends with alumina rich pozzolans. De Weert et al. (
Compressive strength of blends made with different SCM and different clinker factors; results are presented for (a) 7 days (b) 28 days (
The very promising results for ternary blends were achieved only at lab scale; under strictly controlled conditions. To scale up this process other considerations have to be taken into account. Significant differences between the procedures used for clay calcination and grinding at lab and industrial scale can take place. A full scale industrial trial was carried out for the production of bulky amounts of cement under real conditions. The Cuban cement industry designated the cement factory
The target ternary cement should have clinker content around 50%. The trial included the calcination of 110 tonnes of medium grade kaolinitic clay; mixing and homogenizing of the calcined material with limestone in a 2:1 ratio; and co-grinding of the synergetic materials with clinker and gypsum.
For the trial; clay from the clay deposit Pontezuela was selected. The material is classified by the authors as a medium grade kaolinite clay; with an average content of kaolinite of 48.6%; measured by thermogravimetric analysis (TGA). Chemical composition for samples of the clay was assessed aided by X- ray fluorescence (XRF);
Chemical composition of representative samples of Pontezuela clay
Oxides | SAMPLE 1 | SAMPLE 2 | SAMPLE 3 |
---|---|---|---|
SiO2 | 54.7 | 54.2 | 55.0 |
Al2O3 | 27.8 | 28.2 | 26.0 |
Fe2O3 | 12.1 | 12.3 | 13.4 |
CaO | 1.7 | 1.7 | 1.8 |
MgO | 0.9 | 0.9 | 1.0 |
SO3 | 0.0 | 1.4 | 0.7 |
Na2O | 0.3 | 0.3 | 0.3 |
K2O | 1.5 | 1.6 | 1.6 |
TiO2 | 0.8 | 0.8 | 0.8 |
P2O5 | 0.2 | 0.1 | 0.2 |
Mn2O3 | 0.0 | 0.0 | 0.0 |
Cr2O3 | 1.6 | – | – |
LOI | 10.4 | 10.3 | 9.8 |
Humidity | 3.5 | 6.1 | 2.9 |
Calcination implied the modification of a wet process rotary kiln - regularly used for clinker production- in order to calcine the material on dry conditions (
Direct feeding of the clay to the rotatory kiln.
After the calcination 90 tonnes of calcined material were obtained; the loss of material was associated with moisture content of the original clay; as well as the chemically bound water on the structure of the clay. The calcined material was stored in five heaps.
The quality of calcination was assessed through the dehydroxylation of the calcined clay. The complete dehydroxylation is achieved when all OH− groups are released during calcination; and the peak associated with this in TGA disappears.
Thermo gravimetric analysis of the clay calcined in the rotatory kiln.
The pozzolanic reactivity of the calcined material was assessed through the compressive strength of standardized mortars; in which 30% wt. of cement is replaced by the pozzolanic material; following the protocol of Fernández and Antoni (
Compressive strength of the cements prepared with 30% wt. of calcined clay normalized to ordinary Portland cement.
Grinding was made under industrial conditions; by using a ball mill with a double chamber grinding system (see
Industrial ball mill used to produce; through co-grinding; the low carbon cement.
Grinding was completed in 8 hours. Samples of the material were taken every approximately 30 minutes.
Chemical composition of industrial low carbon cement
Chemical compounds | % |
---|---|
SiO2 | 27.3 |
Al2O3 | 4.6 |
Fe2O3 | 4.6 |
CaO | 49.8 |
MgO | 1.3 |
SO3 | 3.7 |
IR | 12.6 |
LOI | 7.1 |
CaOfree | 0.9 |
Total | 98.4 |
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Gypsum | 8.9 |
Calcined clay/Calcium carbonate | 41.1 |
Clinker | 50.0 |
The final cement was characterized following the protocol established for blended cements in Cuban standards (
Results of physical and mechanical test of the industrial low carbon cement
Material | Retained 4900 Sieve (%) | Consistency (%) | Setting time | Volume Stability (mm) | Compressive strength (MPa) | |||
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Initial (min) | Final (hr.) | 3 d | 7 d | 28 d | ||||
LCC | 12.0 | 25.0 | 135 | 2.9 | 0.3 | 11.0 | 17.5 | 30.3 |
The industrial trial enabled the technical team to sound out the reaction of the construction and manufacturing industry to this new binder. Batches of the cement produced were distributed among builders and building material manufacturers; and their use was strictly supervised by the technical team. Potential customers were asked to use the new experimental cement in the same proportions as they usually use the Portland cement.
The trial focused on two main cement applications: (
Mix proportions used in concrete manufacture
For 1 m3 | Gravimetric mix proportion (kg) | Mix proportion (m3) | ||
---|---|---|---|---|
Materials | Hollow block 150 mm | Concrete 25 MPa | Hollow block 150 mm | Concrete 25 MPa |
LCC | 300 | 360 | 1 | 1 |
Sand “El Purio” quarry | 654 | – | 1.8 | – |
Powder “Palenque” quarry | – | 780 | – | 1.6 |
Aggregates 5–13 mm “El Purio” quarry | 1302 | – | 3.5 | – |
Aggregates 19–10 mm “Palenque” quarry | – | 1034 | – | 2.4 |
Water (L) | 112 | 169 | 0.4 | 0.5 |
Superplasticizer Dynamon SX-32 (L) | – | 4.0 | – | – |
w/c batch | 0.4 | 0.5 | – | – |
w/c effective | 0.2 | 0.4 | – | – |
Designed slump | 0 | 12±3 cm | 0 | 12±3 cm |
Hollow concrete blocks were produced under standard manufacturing conditions; with a 1:1 cement substitution by the new cement. The quality of the blocks was assessed through the Cuban standard NC 247:2010 (
Results of compressive strength and % absorption of concrete and hollow blocks made with LCC
Dimensions hollow blocks (mm) | Average compressive |
Average compressive |
Performance | % absorption |
---|---|---|---|---|
500×200×150 | 3.3 | 5.9 | 2.0 | 5.6 |
Specification | 4.0 | 5.0 | – | ≤10 |
Several cubic meters of 25 MPa concrete were cast under standard manufacturing conditions; with a 1:1 cement substitution by the new cement. Quality of the precast elements produced was assessed aided by the Cuban standards (
Results of compressive strength of concrete made for prefabricated elements with OPC and LCC
Material | Cement consumption (kg/m3) | Average compressive strength at 3 d (MPa) | Average compressive strength at 7 d (MPa) | Average compressive strength at 28 d (MPa) | Cement performance |
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LCC | 360 | – | 21.0 | 31.4 | 0.9 |
OPC | 360 | 20.4 | – | 33.2 | 0.9 |
Relation between the compressive strength obtained at 28 days in kg/cm2 and the cement consumption.
Blended cements with clinker substitution up to 30% enable reduction of approximately 15–20% of the CO2 emissions. The new cement formulation presented in this paper enables to increase clinker substitution to 50% without compromising performance; this represents a reduction of around 30% of the CO2 emissions associated to the cement manufacture; as
CO2 emissions vs. clinker factor in the cement production (calculated in reference (
Phases of the productive process | Unitary value (Kg CO2/t) | Clinker factor (%) | |||
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100% Clinker | 70% Clinker | 55% Clinker | |||
Raw materials calcination (CaO y MgO) | 502.0 | 502.0 | 351.4 | 276.1 | |
Fuel | 320.0 | 320.0 | 224.0 | 176.0 | |
Additions (calcined clay, limestone) | 380.0 | 0.0 | 38.2 | 57.2 | |
Grinding | 100.0 | 100.0 | 100.0 | 100.0 | |
Others | 60.0 | 60.0 | 60.0 | 60.0 | |
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Clay calcination is foreseen in two alternative scenarios:
Short term: Production of calcined clays in using an existent wet process rotatory kiln in
Long term: Production of calcined clays in an industrial retrofitted calciner to be set in
CO2 emissions during cement manufacture were calculated by dividing the productive process in two phases: (i) clinker and clay calcination; and (ii) mixing and grinding of the clinker and additions.
During the calcination process; three main emission sources can be defined: a) Fossil fuels combustion; b) chemical decomposition of calcium carbonate (CaCO3) and magnesium (MgCO3) and c) electricity consumption. The methodology to estimate the CO2 emissions followed the recommendations of the Intergovernmental Panel for Climate Change; 2006 (IPCC) (
Where:
CO2 emissions: kg CO2/tonne CK-Calcined clay Emission factor: kg CO2/unit of energy used Consumption index: unit of energy used/tonnes of CK-Calcined clay.
The dry process clinker production operates with pet-coke and the electricity is supplied by the National Energy System (NES) as energy sources.
CO2 emissions due to the energy consumption during the clinker production in
Consumption index (energy unit/t) | Emission Factor (kg CO2/energy unit) |
CO2 emissions (kg CO2/t) |
---|---|---|
Emissions due to fuel consumption (pet-coke) | ||
100.00 (kg pet-coke) | 4.09 (kg CO2/kg pet-coke) | 409.00 |
Emissions due to electricity consumption of NES | ||
0.0399 | 744.00 (kg CO2/MWh) | 29.69 |
Total CO2 released per energy consumption to produce 1ton of CK | 438.69 |
Taken from the
For clay calcination at cement plant
CO2 emissions due to the energy consumption during clays calcination in
Scenarios | Consumption index (energy unit/t) | Emission factor (kg CO2/energy unit) | CO2 emissions (kg CO2/t) |
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Emissions due to fuel combustion (cuban crude oil) | |||
Short term (rotatory kiln-wet process) | 104.50 |
3.28 (kg CO2/kg crude oil) | 342.76 |
Long term (industrial calciner) | 57.20 |
3.28 (kg CO2/kg crude oil) | 187.62 |
Emissions due to electricity consumption of NES | |||
Short term (rotatory kiln-wet process) | 0.0399 |
744.00 (kg CO2/MWh) | 29.69 |
Long term (industrial calciner) | 0.0239 |
744.00 (kg CO2/MWh) | 17.81 |
Reduction of Cuban crude oil consumption for the calcined clay production related to clinker production are estimated in 40%, due to the decrease of the calcination temperature and a reduction of 15% of electricity consumption because clay doesn't need to be processed in the paste mill.
A reduction of 45% of the Cuban crude oil consumption and 40% of the electricity consumption are estimated related to the wet process production in the rotatory kiln, due to the reduction of the calcination temperature (the material is introduced with natural humidity) and the residence time in the calciner.
The emissions caused by the chemical decomposition of CaCO3 y MgCO3 contained in raw materials (grey limestone and clays); may be deduced through stoichiometry calculations with two different approaches: By calculating the differences in the CaO and MgO content of the raw materials before been fed to the kiln and at the exit (CK or calcined clay) ( By assessing the contribution of each component of the paste to the total CO2 emissions; through the knowledge of the chemical composition of the minerals used in the materials at the entrance of the kiln and its CaCO3 and MgCO3 content.
The use of one or other approach is determined by the data availability; which helps to describe the chemical composition of the material; before and after processing it.
The authors chose to measure the differences between the CaO and MgO content of the materials while entering or exiting the kiln. The differences in the oxides content are converted by stoichiometry on the CO2 released.
The CO2 emissions due to the decarbonation of limestone were calculated for the conditions of
Formula and data used for the calculation of CO2 released due the production of clinker by dry process
CO2 released (calcination) =[0.785*(Out CaO-In CaO) +1.092*(Out MgO-In MgO)]/100 | ||||
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Out CaO (%) | In CaO (%) | Out MgO (%) | In MgO (%) | CO2 released for decarbonatation (t CO2/t CK) |
65.09 | 0.00 | 1.63 | 0.00 | 0.53 |
The CO2 emissions in the phase of cement grinding; are associated with the electricity consumption in the ball mill; see
CO2 emissions due to energy consumption during cement grinding
Production | Consumption index (MWh/t) | Emission factor (kg CO2/ MWh) | CO2 emissions (kg CO2/t) |
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Clinker/Calcined Clay |
0.0465 | 744.00 | 34.60 |
It was considering that the grinding and mixing of the different types of cement have the same energy consumption.
The final calculation of CO2 emissions is presented in
CO2 emissions associated to LCC production with 45% of SCM and compared to thre reference cements P-35 and PP-25.
In the projected long term scenario; the total estimated emissions for low carbon cement achieve emissions reduction higher than 300 kg CO2/tonne in relation to P-35 and 200 kg CO2/tonne in relation to PP-25. This represents more than 35% of reduction compared to business as usual practice.
Relation between emissions of CO2/ton cement and associated compressive strength of P-35; PP-25 and LCC produced in industrial trial.
There are alternatives available to move the current boundaries of clinker substitution for the production of blended cements through the use of ternary systems based on clinker; calcined clays and limestone. The principle behind this proposal is the synergy between calcined clay and limestone; which allows increasing the reactivity of the SCMs and reduces clinker factor. This system is based on the use of medium grade kaolinite clay; and only small changes should be made to the production process. Reserves of medium grade clay and limestone are much higher than any other SCM. The industrial trial to produce cement with a clinker factor of 50% at the industrial scale has proven that the new system is very robust; while even in non-optimized conditions acceptable results have been achieved in terms of performance of the resulting material as cement; as well as in its applications in concrete. The new cementitious system could enable reduction on the emissions associated to the cement manufacture in the range of 25–35% related to business as usual practice. This reduction is based on replacing clinker; which is the main CO2 releaser; by a combination of materials whose emissions are negligible compared to clinker.
The authors would like to thanks the financial support from the Swiss National Science Foundation (SNSF) to this project and the Laboratoires des Matériaux de Construction (LMC) of the École Polytechnique Fédérale de Lausanne (EPFL) for offering their facilities. The authors would also like to acknowledge