Page 102 - vol5_n2

232 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 2
Evaluation of partial clinker replacement by sugar cane bagasse ash: CO2 emission reductions
and potential for carbon credits
constituent of clinker. The second product of this reaction is CO2.
The heating of limestone represents more than half of the contribu-
tions in greenhouse gases from cement production, having in sight
that the carbon emitted is outside the natural carbon cycle.
The clinker is mixed with other admixtures such as gypsum and
pozzolans in variable proportion according to the type of cement.
This mixture then undergoes a second grinding, giving rise to Port-
land cement itself, which is bagged and stocked, and which aver-
age composition (cement CPII) is of 80% of clinker, 15% of addi-
tives and 5% of other materials [15].
It is estimated that one ton of CO2 is released for every ton of ce-
ment produced. This average value for the world production tends
to be more or less greater in different countries mainly in function
of the national energy matrix to be less or more clean. Countries
with large participation of renewable energy such as wind, solar,
biomass or hydroelectric power, such as Brazil, tend to show lower
emissions associated with the production.
An alternative to reducing emissions from cement production activ-
ity is the use of renewable fuels at the expense of mineral fuels,
both in the self-generation within the factory and in the grid, and
the combustion efficiency improvement of furnaces.
Another possibility is to reduce the share of clinker in cement
through mineral additions, such as blast furnace slag and pozzo-
lans, as the bagasse ash. This last alternative is advantageous
since it does not require adaptations of engines and furnaces to
new fuels avoiding therefore large financial and technological in-
vestments. However, for an effective reduction of CO2 it is neces-
sary to consider all emissions relating to obtaining, transporting
and preparation of the additives used, which often undermines the
replacing viability of many materials that aim at reducing emissions
and gains in carbon credits.
4. The SCBA as “clean” additive
Currently, the sugar cane biomass has served as fuel for steam
The first studies regarding the behavior of cementitious materials con-
taining bagasse residual ash dating to the late 1990. Studies of Martire-
na Hernández et al. [11], Singh et al. [12], Ganesan et al. [13] and Mo-
rales et al. [14] show that replacing cement by ash (5 to 30% by mass)
is able to improve the mechanical properties and durability of concrete.
More recently, through mechanical, rheological, calorimetric and
durability tests in concrete samples with 0 to 20% of bagasse ash,
Cordeiro et al . [5] concluded that there is considerable improve-
ment in rheological performance and durability and maintenance
of long-term compressive strength, as shown in Figure 2. The ash
used in this study was collected during the cleaning operation of
the bagasse burning boiler and it was ground to improve homog-
enization and reactivity. This work also indicated that the maximum
adiabatic temperature rise of concrete decreased by 11%. In this
way, it is possible to infer that the use of residual SCBA as a min-
eral admixture in cement is technically viable, since it is able to
improve several physical and mechanical properties of concretes.
3. Emissions from the cement production
Emissions in cement production are mainly associated with the step
of clinkerization and the combustion of fossil fuels such as coal,
mineral and diesel oil, both for electricity self-generation and for the
power generation on the grid. The clinkerization step consists in the
heating of the flour or “the raw” until temperatures around 1450°C.
With this, the limestone undergoes a thermal decomposition giving
rise to the clinker, according to the calcination reaction shown below:
(1)
2 3 CO CaO CaCO +
D
¾®
The first product of the reaction is calcium oxide, the main oxide
Figure 2 – Average compressive strength values over time for different concretes
(dispersion of results indicated by the standard deviation) [9]