Page 213 - New Trends in Eco efficient and Recycled Concrete

Steel slags 183

The different residues obtained from the steelmaking process present a composi-
tion similar to cement, consisting mainly of CaO and SiO 2 , and some percentages
of Fe 2 O 3 and Al 2 O 3 (Niklio´ c et al., 2016). BF slag is a kind of SS and is considered
a pozzolanic material because presents cementing properties, being the most feasi-
ble material from steel waste for use in the production of cement.

7.4.1 Principles of geopolymers
Professor Davidovits invented and developed geopolymers in the 1970s (IUPAC
International Symposium on Macromolecules, 1976). This researcher discovered
that the same occurrence in OPC, which normally gains cementing capacity by
developing a C S H gel, could be developed in other by-products. He developed
a new generation of cements based on by-products with high contents in calcium
(Ca), silica (Si), aluminium (Al) and alkaline liquids, which are the main constitu-
ents of geopolymers (Geopolymers of the first generation, 1988). He discovered
that resistance could be obtained in geopolymers, mainly by means of silicon and
aluminium polycondensation.
Over the past three decades, different types of waste and by-products have been
used for the formation of geopolymers, such as BF slag, silica fume, and coal fly
ash, etc. The use of this waste presents a very positive environmental component,
because their use helps to minimise environmental problems arising from dumping
(Scientific Research Institute on Binders and Materials Kiev State Technical
University, 1994). Geopolymers are inorganic compounds formed on the basis of
aluminium silicate which are formed from aluminium silicates or industrial waste
and mixed with an alkali silicate solution under highly alkaline conditions (Karthik
et al., 2017). The main difference between cement and geopolymers is that cements
are hydraulic binders, which produce agglomerated materials through hydration
processes, and geopolymers use chemical processes to bind materials.
Three types of structures are established in geopolymers from the aluminosilicate
network: sialate ( Si O Al O ), sialatesiloxo (Si O Al O Si O) and
sialate-disiloxo (Si O Al O Si O Si O) (Saha and Rajasekaran, 2017). The
formation of these systems are summarised here (Saha and Rajasekaran, 2017):
For the formation of geopolymers, a reactive aluminosilicate material called the
precursor and an alkaline solution called the activator is needed. These have been
used as precursors to coal fly ash, palm oil fuel ash, BF slag and metakaolin, etc.
These waste and by-products were used in the first years of development of geopoly-
mers and they were very suitable for the development of polymerisation reactions
providing an appropriate mechanical behaviour in mixtures (Ravikumar et al., 2010).

7.4.2 Possible use of steel slags in the manufacturing of
geopolymers

The formation of geopolymers are usually obtained from residues and by-products
by alkaline activation, which present compositions of the type SiO 2 Al 2 O 3 or