Concrete future - Concrete Mix Design, Quality Control and Specification - page 292

amorphous aluminosilicates with alkali hydroxide and silicate solutions.
Duxson et al. (2007) has identified many other names in the literature,
such as alkali-activated cement, inorganic polymer concrete, and geoce-
ment, which have been used to describe materials synthesised using the
same chemistry.
Synthesis of a geopolymer usually involves mixing materials containing
aluminosilicates, such as metakaolin, fly ash, slag with alkali hydroxide,
and alkali silicate solution, sometimes sodium carbonate in slag-based sys-
tems (Shi et al., 2006). There are numerous publications discussing dif-
ferent properties of geopolymers synthesised from different raw materials
and activators. Therefore the term geopolymer covers a bewildering range
of potential binders that those interested in this technology must navigate.
Product data sheets, and even technical papers, on “geopolymers” might
cherry pick data obtained from different binder chemistries giving the mis-
leading impression that a specific material has been comprehensively tested
when it has not. Papers might also focus on a particular material with poor
performance to negatively characterise geopolymers. For example, the geo-
polymer concrete considered by Turner and Collins (2012) contained very
high activator levels and required steam curing so that the product had rela-
tively high embodied energy and emissions, leading to the conclusion that
there was little benefit in terms of carbon footprint compared to ordinary
Portland concrete (OPC).
One area where reference to generic geopolymer data is helpful is dura-
bility. For geopolymer concrete to be considered a suitable alternative
to Portland-cement-based concrete, the basic geopolymeric gel must be
durable. This can only be established over time. Xu et al. (2008) inves-
tigated activated slag concretes from the former Soviet Union. The slag
had been activated by carbonates and by carbonate-hydroxide mixtures.
The research found high compressive strengths that were significantly
higher than when initially cast and excellent durability over a service life
of up to 35 years. Xu et al. (2008) and Shi et al. (2006) report that the
carbonation depths were relatively low for their age and no microcracks
were observed after prolonged service. Although the performance of
each proprietary geopolymer concrete needs to be established by compre-
hensive assessment, it is comforting to know that the basic geopolymer
matrix appears to be durable and the reaction products appear stable
over time.
Until recently, geopolymers have been found in niche applications,
including fire-resistant materials, coatings, adhesives, and immobilisation
of toxic waste 6 (Provis and van Derenter, 2009). However, the main poten-
tial application for geopolymers has been in the construction industry as
an environmentally friendly concrete with reduced embodied energy and
CO 2 footprint (Gartner, 2004; Phair, 2006) compared to the traditional
Portland-cement-based concrete.