Civil Engineering Reference
In-Depth Information
pulverized fly ash. Unfortunately, high alkalinity of these systems may lead
to high corrosion rates with reactive metals within such wastes releasing
hydrogen and forming expansive corrosion products. CSA cements are
being tested as alternative encapsulation binders mainly due to their highly
dense matrixes and lower alkalinities (Zhou et al., 2006). Hayes and Godfrey
(2007) used commercial CSA (rockfast, see Table 18.3) which theoretically
would need 37 wt% of gypsum and 0.54 water-to-cement mass ratio for
full hydration. These authors varied the water-to-binder mass ratio from
0.50 to 0.65, and different gypsum and pulverized fly ash contents with
very encouraging results. Finally, oPC/CSA blend cements have also been
tested, since the setting time is shortened which is useful for nuclear waste
encapsulation (Cau dit Coumes et al., 2009).
CSA cements have been also investigated to solidify and stabilize different
type of wastes. We can underline three examples: (i) wastes containing
large amounts of soluble zinc chloride, which is a strong inhibitor of oPC
cement hydration (Berger et al., 2011b, 2011c); (ii) galvanic sludge with
high amounts of chromium which was stabilized in blended CSA-bottom
ash cement (luz et al., 2006, 2009); and (iii) heavy metals in general, Cr,
Pb, Zn, Cd (Peysson et al., 2005). The valorization of phosphogypsum (a
side-product in the fabrication process of phosphoric acid) as a binder by
adding CSA has also been studied (Kuryatnyk et al., 2008).
18.4.5 Plastic/rheological properties of mortars and
concretes
High mechanical strength, dimensional stability and durability are the most
important final performances of cement-based materials. In order to achieve
these performances, the control of the rheological properties of the fresh
products is needed. rheology of fresh cement is often characterized by
its workability. The workability of cement pastes is a performance index
representing how easily it is mixed or transported, and also showing the
ability of filling voids. Compactibility, finishability and resistance against
segregation are also important parameters (Hanehara and Yamada, 2008). In
yeelimite-containing systems, the first hydration hour(s) are very important
as large amounts of ettringite are formed and plasticity/workability strongly
depends on this reaction. There are few studies concerning how to improve
workability of CSA cements. Additives to enhance workability will act by
slowing down the hydration rate and dispersing CSA particles over a longer
time to enlarge setting time (Chang et al., 2009). Moreover, CSA pastes
with additives may present excellent dispersibility at low W/C ratios and,
consequently, with improved mechanical performances (García-Maté et al.,
2012). Accelerating admixtures may also affect, in addition to the setting time,
to the compressive strength development and to the rheological properties.
