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Life cycle assessment applied to recycled aggregate concrete 211

Attention is also being paid to the chromium content of cement. For instance,
the sale of cement containing more than 2 ppm of soluble Cr(VI) when hydrated, is
prohibited by European directive 2003/53/EC (European Union, 2003). Hexavalent
chromium or Cr(VI) is not stable. When dissolved, Cr(VI) can penetrate the unpro-
tected skin and be transformed into to Cr(III) which combines with epidermal
proteins to form the allergen that causes sensitivity to certain individuals. The Cr
(VI) content can originate from: (1) raw materials and fuel entering the system; (2)
magnesia-chrome refractory blocks; (3) wear metal from crushers containing chro-
mium alloys; and (4) additions of gypsum, pozzolans, ground granulated BFS, min-
eral components, CKD and set regulators (Hills and Johansen, 2007).

9.2.1.2 Supplementary cementitious materials
Commonly used supplementary cementitious materials that could partially replace
ordinary Portland cement are fly ash, blast-furnace slag and silica. All three of these
originate from industrial processes that are seemingly not directly related to the
concrete industry, that is, coal-fired electricity production, steel production and
Si-metal production. Since these materials are not the main output of those indus-
tries, their product status is somehow ambiguous. However, one could see them as
waste materials with no value which would normally be landfilled. Their broad and
well-accepted use in the concrete industry implies that they have become valuable
by-products. And indeed, in accordance with European Union directive 2008/98/EC
(2008) they meet all the necessary requirements to be identified as a by-product:
(1) further use of the substance is certain; (2) the substance or object is produced as
an integral part of a production process; (3) the substance or object can be used
directly without any further processing other than normal industrial practice; and
(4) further use is lawful (Chen et al., 2010b). Apart from meeting those four basic
criteria, fly ash, blast-furnace slag and silica fume additions are known for improv-
ing the quality of concrete in certain exposure classes. This emphasises their
by-product status even more (Van den Heede and De Belie, 2012). This shift from
the waste material to by-product status has important implications in view of the
environmental impact that is assigned to them. It determines whether both primary
production as well as subsequent basic treatment need to be considered within the
LCA (Fig. 9.1, Chen et al., 2010b; Chen, 2009). As a waste, only the impacts
related to the basic treatment needed for making them suitable as concrete addition
should be accounted for. In case of fly ash, the basic treatment before storage con-
sists of drying the ashes that have been captured from the flue gases. For blast-
furnace slag, this treatment comprises granulating, drying and grinding of the pro-
duced slags before they can be stored. After being collected, silica fume can almost
directly be stored for future use in concrete. The impacts related to basic treatment
of fly ash, blast-furnace slag and silica fume have been summarised in Table 9.2.In
terms of emissions to air (e.g., dust, SO x and NO x ) the emissions when treating 1 kg
of by-product are much lower than the ones for ordinary Portland cement
(Table 9.1). As such, partial cement replacement by fly ash, blast-furnace slag or
silica fume seems highly beneficial from an environmental viewpoint.