Can We Make Concrete
An abundant resource offers a solution
by Mehrdad Mahoutian, Chris Stern, and Yixin Shao
For centuries, cement has been used in the production of
concrete to bind aggregates and other ingredients
together. But the production of cement is an energyintensive,
carbon-emitting process1 with the cement industry
responsible for 8% of global CO2 emissions.2 Additionally,
cement production intensively consumes energy and natural
resources. Cement plants in Canada, for example, consume
energy at the rate 4.5 GJ to produce 1.0 tonne (1.1 tons) of
cement.3 For every tonne of cement, 1.5 tonnes (1.7 tons) of
limestone and 0.30 tonnes (0.33 tons) of clay are extracted
from the earth.4 Given the current rate of annual cement
production, 6000 million tonnes (6600 million tons) of
limestone are consumed every year.
Efforts have been made in the past few years to reduce
concrete’s cement consumption and to produce low-energy,
low-CO2 footprint cements5,6; however, significant energy and
natural resources are still required.
It is possible, however, to make cement-free concrete by
replacing portland cement with a by-product of the steelmaking
process that can be carbon-activated. The by-product, steel
furnace slag, is reactive with CO2 in the presence of water.7
While steel furnace slag has not been used widely in concrete as
a supplementary cementitious material because it does not show
significant cementitious or pozzolanic behavior, it does gain
strength when exposed to CO2 and therefore can be used in
place of cement as the binder in concrete.
Steel mills produce 90 to 150 kg (42 to 331 lb) of steel
furnace slag per tonne of steel, mostly in electric arc furnaces
(EAF), basic oxygen furnaces (BOF), and ladle vessels, and
this leads to an annual production of 250 million tonnes
(276 million tons) of slag worldwide. Currently, the by-products
from steel mills are mainly marketed as aggregates for
construction—in asphalt pavement mixtures,8,9 roadbed
48 JANUARY 2020 | Ci | www.concreteinternational.com
construction,10 and concrete pavement11—as well as a flux
material in steel production.12
To use steel furnace slag as the binder in concrete, the slag
must have the proper chemical compositions, morphology,
structure, and fineness. The slag particles must be finely graded
to properly form a strong material when exposed to CO2. That
means that slags collected from steel mills must be sorted and
ground to increase the specific surface area of the binder.
Further, because the activation step requires that molded
elements are exposed to CO2 in a closed and controlled
environment, cement-free concrete is most appropriate for
precast concrete products. The three main ingredients in the
cement-free mixture are the slag binder, aggregate, and water.
After forming, compaction, and conditioning, the mixture is
exposed to CO2. As with cement-based concrete, construction
aggregates are used as fillers, and water can be potable.
Our company, Carbicrete, has demonstrated the technology
for the production of cement-free concrete masonry units
(CMUs). In a standard mobile concrete mixer, we dry-mixed a
granite aggregate with steel furnace slag powder. Water was
then added to the mixture, followed by additional mixing. A
commercial block-making machine was used to form,
compact, and vibrate the fresh concrete to form CMUs (Fig. 1).
The CMUs were then conditioned using fans. A
combination of fans and heaters may also be used.
Conditioning is an essential step that reduces the moisture
content in the concrete and opens voids that ease penetration
and diffusion of CO2 into concrete during the curing process.
During the conditioning stage, formed units are fragile and
must be handled with care.
As previously noted, carbonation curing promotes strength