Page 442 - New Trends in Eco efficient and Recycled Concrete

394 New Trends in Eco-efficient and Recycled Concrete

1 h (WCD). Much lower strengths at 7 and 28 curing days were obtained by com-
parison to sodium silicate solution; however, similar values were observed when
CD (unused material) was compared to spent diatomite (BD, WD, WCD). An
improvement was detected when spent diatomite from wine industry was previously
calcined.
Tchakoute ´ et al. (2017b) prepared an alternative activating solution by means
SCBA. After reaction with NaOH, the suspension was filtered and the liquid frac-
tion was used for the activation of MK. Good properties of the synthetised geopoly-
mer were 32.9 MPa in compression and 7.1% water absorption. Moraes et al. (in
press) studied the use of SCSA as alternative source of silica. The activating solu-
tions were produced by means of a thermal bottle, taking advantage of dissolution
heat of NaOH pellets which favoured the dissolution of silica from SCSA. An
increase of 50% in strength was observed when the geopolymer obtained with
SCSA-NaOH suspension was compared with the case in which the SCSA was
added to the precursor. The obtained data showed similar results than those reached
for RHA.

13.2.3 Alkaline activators with total replacement of the
synthetic reagent
Two types of activators have been reported: those from industrial processes (mainly
from aluminium industry and from metal cleaning activities with caustic solutions)
and those from agro-industrial processes. In both types, alkaline elements are pres-
ent in the waste used as activator. Usually, the first type contains sodium com-
pounds and the second type contains potassium compounds.
Bayer liquor is produced from the industrial manufacture of alumina in the
Bayer process when the ores, gibbsite or boehmite, are digested in hot highly con-
centrated NaOH solutions. After filtering, a sodium aluminate slurry is obtained
and aluminium hydroxide is precipitated from it. The waste solution (Bayer liquor)
is finally discarded or recycled, which has a high content of sodium aluminate. Van
Riessen et al. (2013) studied this processed plant liquor and compared to synthetic
sodium aluminate solution with similar composition (15% Al 2 O 3 and 24% Na 2 O).
Fly ash geopolymers were prepared by using both solutions, replacing partially or
totally the NaOH content that had the reference geopolymer. The samples were
cured at 70 C for 24 h, and then left at room temperature for a period of 28 days.

SF was added in order to reach the appropriate Si/Al and Na/Al ratios.
Replacement of NaOH by processed liquor by 60% 100% let to produce similar
strength (41 43 MPa) than reference geopolymer (45 MPa), while geopolymer pre-
pared with synthetic solution yielded less strength (20 33 MPa). It was reported
(Jamieson et al., 2015) that embodied energies in geopolymers prepared with pro-
cessed Bayer-plant liquor were between 6% and 33% of that associated with OPC.
In 2016, Jamieson et al. (2016) developed geopolymers by setting and curing at
room temperature. For this purpose, FA from Collie power station was mixed with
4% 5% of Ca(OH) 2 , reaching more than 20 MPa after 28 days of curing. For FA