Reduction of Carbon Footprint in Cementitious Composites

Authors: L. Sadowski, A. Chajec

The invention relates to a method for the chemical activation of waste granite dust originating
from the process of cutting rocks of various types and origins, or from their crushing during
aggregate production, intended for use in cement composites.
There are various methods for the activation or functionalization of the surfaces of powdered
materials in construction industry. Currently, the most common methods are physical, such as
grinding and texturing, and chemical, using carbon particles, such as nanotubes, for surface
activation. Other chemicals (e.g., TiO2, microsilica, nanosilica) are also used to modify grain
surfaces with active agents that, upon contact with cement paste, enhance the bonding
process of modified grains with the cement matrix.
Cement composites significantly impact the environment, contributing 6-8% of global CO2
emissions, mainly from Portland clinker production and emissions during hydration. To
address this, methods have emerged to reduce clinker use by incorporating secondary
components that preserve the hardened composite’s properties. Research shows that direct
carbonation of particulate materials containing crystalline phases like calcite in an alkaline
environment can form calcium carbonate on their surface, which enhances the composite’s
mechanical properties, especially in the first 7 days. This approach allows for up to a 30%
reduction in Portland clinker in multi-component cement.
The technical issue addressed by this invention is due date the limited potential for utilizing
waste rock dust to replace part of the cement in a cement composite without compromising
its properties. The invention develops a new method for the chemical activation of rock dust
using a technical gas mixture. It has been done by using the chemical activation of the rock
dust, characterized by subjecting the rock dust to a direct carbonation process using technical
gases in an alkaline aqueous environment, comprising the following steps:

(1) In the first step, the rock dust is mixed with a sodium hydroxide solution to obtain a
mixture.

(2) Next, during the mixing process, the mixture is flushed with a solution of nitrogen and
carbon dioxide supplied in a volumetric ratio within the range of 1:1.5 to 1:3, until
aragonite crystals (a polymorphic form of calcium carbonate) crystallize on the surface
of the rock dust grains.

(3) In the following step, the mixture is filtered.

(4) Finally, the powder obtained after filtration is dried.

An advantage of the solution is that the carbonation process results in changes to the
morphology of the granite dust grains, a by-product of stone-cutting processes, which
enhances its bonding with the cement matrix. The use of 20% carbonated granite dust
increases the compressive strength of the hardened cement composite by 20% and reduces
CO2 emissions by over 40%. Moreover, it was also proved that substitution of 40% of cement
by such prepared dust maintain the compressive strength of hardened cementitious
composite.