Reducing the carbon footprint of concrete is an imperative. Judicious concrete
mix design is one means of achieving this, including the use of supplementary
cementitious materials, water reducing admixtures and reduced concrete
workability. However, a truly sustainable concrete should also, in addition to
having a low embodied carbon, provide long-term durability.
In the present work, many factors have been adopted to reduce the embodied
carbon dioxide in concrete. Eighteen concrete mixes were designed to provide
low-carbon concrete while maintaining durability. To achieve these requirements,
it was necessary to cast a series of concrete samples with compositions designed
to vary key components known to control embodied carbon dioxide. Based on the
statistical approach, 18 concrete mixtures were produced and designed in L18
orthogonal array with six design variables. The work is divided into three
complementary parts as follows:
Part one characterizes of the constituent concrete materials, including Portland
cement and three different supplementary cementitious materials (pulverized fuel
ash, plus course and fine ground granulated blast furnace slag) at different
replacement levels (0%, 15%, 30% of the total binder content by mass). A
superplasticizer, Sika Viscocrete 25 MP, was used at either 0.2%, 0.6% or 1.0%
binder weight. Crushed aggregate of maximum size of 10 and 20mm while the
crushed fine aggregate were of 5mm maximum size were used.
Part two of the works highlights the performance of hardened state concrete
(mechanical properties) including the time-dependent deformations and
durability. These properties have been investigated by testing 100mm cubes for
compressive strength, 75 x 75 x 200 mm prisms for shrinkage, and transport
properties (sorptivity and permeability on 100mm cubes and 40mm thick x 50 mm
Ø cylinders. Durability was assessed through measuring the depth of carbonation
following accelerated carbonation of 100 mm cube samples.
Finally, in the third part, microstructural investigations were undertaken using
pastes that were prepared with the same composition as the concrete, but without
any aggregates. The pastes were investigated using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Moreover, hydration was
investigated using isothermal conduction calorimetry.
Test results proved that some mix design variables have a significant positive
impact on the performance; for example strength enhancement, improving
carbonation resistance meanwhile, satisfying the requirement of reducing carbon
dioxide emissions.
Numerically speaking a concrete containing fine slag at 15 or 30% replacement,
low w/b ratio (0.35) above and high dosage of superplasticizer (1.0%) was found
to meet the engineering performance, carbonation resistance, and environmental
sustainability. It was also found that coarse slag might be another choice of
cement replacement but with slightly decreased carbonation resistance. Slag
showed better performance than PFA. This can be attributed to the role of slag in
refining the microstructure by: (i) leading to finer porosity, increased degree of
hydration, (iii) by inference, changing the C-S-H from fibrillary to foil-like (iiii)
improving carbonation resistance.
The above significant conclusions are the result of the high fineness of slag which
in turn reduced the pore structure, high reactivity of slag which in turn accelerated
the hydration at early age and the pozzolanic reaction which improved the
microstructure at long age.
This study has considered a range of concrete mix designs known to yield
concrete with a reduced carbon footprint. It has looked at the effects of mix design
on mechanical performance, transport properties and resistance to carbonation.
Using the Taguchi method, 18 concrete mixtures are produced and designed in
L18 orthogonal array with six design variables (binder content, superplasticizer,
supplementary cementitious materials, percentage of SCMs, aggregate Size (10-
20) mm and water-to-binder ratio) set at one of three levels have been prepared.
