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Advantages of adding GeoSilex® to concrete

 Report by the University of Granada

Addition* of GeoSilex® reduces by 30% the carbon footprint of the cement

Emission of the cement: 750 grs. CO2/kg de cemento.

 Collection of the CO2 of the GeoSilex®: 255 grs. de CO2/kg

*Proportion 1 to 1.


Additing Ca(OH)2 to concrete


Since the beginning of the use of Portland cement as a building material substituting the traditional lime based cements, it showed better mechanical performance but, other rheological properties (plasticity, workability…) were notably low. Such a wastage, according to different research projects*, could be corrected by the addition of hydrated lime, which though a decrease in resistance was observed at the beginning, in the long term, the mechanical resistance showed a steady increase, due to the capacity that the hydrated lime has to carbonise.


Such studies at beginning of the 20th century combined with other recent ones demonstrate a series of advantages in the addition of hydrated limes to Portland cements:

  1. Increase on the plasticity and workability of the fresh mixture.
  2. Gives a high PH in the portlandite phase, which contributes to the chemical stability in the long-term, contributing to its durability.
  3. The crystals of portlandite (Ca(OH)2) act as obstacles to the spread of fractures.
  4. The high capacity of water retention of the hydrated lime favors a slow dry that propitiates a better hydraulic setting.
  5. The calcium hydroxide has cementing properties since it carbonises in the presence of CO2, which favors structural stability.

* See for example Knuepfer, C.A. & Houk, L.D. 1915. Effect produced on Portland cement by the addition of hydrated lime. Amour Institute of Technology, USA.



Sustainable advantages


The traditional process of obtaining hydrated limes, means a considerable energy cost and CO2 emissions to the atmosphere. Nevertheless, 80% of calcium hydroxide that is generated by GeoSilex® is obtained by a non conventional route according to the following reactions:


1. First, the carbide of calcium is generated in an electrical arch from calcium oxide and coke (coal) to a temperature range between 2.000 - 2.500 ºC:

CaO+3C → CaC2+CO

2. Later the calcic carbide is hydrated, according to the reaction:

CaC2+2H2O → C2H2+Ca(OH)2

In the latter, acetylene (C2H2) becomes detached and CaO is generated. The calcium oxide is hydrated forming a mixture of calcium hydroxide, which is stored in pools as residue. The recycling of the above mentioned residue (lime mixture) is an added value alternative with a great potential for the capture of greenhouse gases such as CO2.

The mixtures of carbide lime contain nanometric crystals of Ca(OH)2 (portlandite), water and other impurities, which are eliminated (oxidized) with a treatment patented by Geosilex Trenza Metal S.L and the University of Granada. This treatment turns the carbide limes into an improved product, of great added value that forms the base of GeoSilex®, a material of excellent performance (rheology, molded, workability and applying facility) and enormous capacity of collecting CO2, and with great potential as a cement material, without any of the disadvantages that the carbide might initially present.

The reaction between CO2 (gas) the Ca(OH)2 in GeoSilex® a liquid state (carbonatation) facilitates the formation of CaCO3 that acts as cement. The global reaction of carbonatation of the lime can be synthesized as follows:

Ca(OH)2(s)+CO2(g) → CaCO3(s)+H2O(aq)+74kJ/mol

Which means that the carbonatation of 1 Tm of calcium hydroxide causes the collection of 0,59 Tm of atmospheric CO2.

Bearing in mind that the industry of the cement is responsible for the emissions of 5-8 % of the antropogenic CO2**, the substitution of part of the cement of a mortar or a concrete, implies a notable reduction in CO2 emissions and the energy cost of the above mentioned element of construction. But, in addition, the carbonatation of the calcium hydroxide exposed to the atmospheric CO2, helps fix the above mentioned greenhouse gas as calcic carbonate (cementing material).

** USGS Commodity Statistics and Information: Cement Gartner EM (2004) Cement Concrete Res 34:1489-1498; Damtoft JS, Lukasik J, Herfort D, Sorrentino D, Gartner EM (2008) Cement Concrete Res 38:115-127.

Materiales para arquitectura activa frente al CO2

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