Civil Engineering Reference
In-Depth Information
the case for most of the supplementary cementitious materials (SCM) and
specifically to fly ash and blast furnace slags that are by-products from other
industries. the question of the value of the environmental load for SCM has
been emphasized by many authors and has not yet been solved. the current
practice is to consider them as waste and with that assumption to assign a
null environmental impact to them, but this situation will evolve toward
a practice where the different industrial sectors will have to share these
environmental loads. Particularly in europe where, a recent european Union
Directive (EU, 2008) notes that a waste may be regarded as by-product if
the following conditions are met
:
∑ Condition (a) further use of the substance or object is certain;
∑ Condition (b) the substance or object is produced as an integral part of
a production process;
∑ Condition (c) the substance or object can be used directly without any
further processing other than normal industrial practice;
∑ Condition (d) further use is lawful, i.e. the substance or object fulfils
all relevant product, environmental and health protection requirements
for the specific use and will not lead to overall adverse environmental
or human health impacts.
this Directive is very relevant to the use of SCMs such as blast furnace
slags and fly ash. Therefore in Europe these two materials can no longer be
considered as waste but instead as by-products. Hence, the question is what
is the environmental cost of these by-products?
Different scenarios can be promoted. Dividing the impact by the relative
mass value of the different products (steel/slags) induces too high an
environmental load for by-products and a division in relation to the relative
economic value of the products seems more in accordance with the perception
of what could be the environmental load of these SCMs. the main problem
with the economic allocation is the question of price variability as illustrated
in Fig. 1.3 where blast furnace slag is varying between 40 and 90
7
/t and
steel price is varying between 150 and 1500
7
/t. For further reading on this
subject see Van den Heede and De belie (2012).
among alternative cementitious materials, it should also be pointed out that
rather than partly substituting the clinker, it is also possible to develop new
binders. Chapters 16 to 18 focus on these new chemistries. the environmental
concerns associated with these binders are similar to the SCMs as they are
often based on the use of waste/by-product which induces allocation problems.
If raw materials are used, the energy for heating is most of the time lower
than for clinker except for the production of alkaline solution which can
change the environmental balance depending on the amount that is needed
(Habert et al., 2011; McLellan et al., 2011).
