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Application of alkali-activated industrial waste 403

The activator concentration. The yield stress values and plastic viscosity increase if there
is an increase in the concentration of the activator. The high values of concentration
favour the dissolution of slags and the formation of more reaction product (Ferna ´ndez-
Jime ´nez and Puertas, 2003).
Concentration and SiO 2 /Na 2 O ratio. Regardless of the nature of the activator, the rheolog-
ical behaviour of AASm and AASc was similar for low concentrations of activator
(Na 2 O) and low SiO 2 /Na 2 O ratios. When an increase in the SiO 2 /Na 2 O ratio is generated,
gel formation is delayed due to the higher surface tension in the activation solution that
makes the dissolution of the slag difficult, producing this delay in activation process.
Therefore, the direct relationship between the concentration and the SiO 2 /Na 2 O ratio is
demonstrated (Puertas et al., 2014).

13.3.3 Mechanical behaviour: compressive and flexural
strengths
Very few research works have been published, which include some studies about
the influence of RCA derived from CDW on the mechanical properties of geopoly-
mer concrete (GPC) or AASc.
Kathirvel and Kaliyaperumal (2017) studied the influence of RA content in
AASc production, under special ambient curing conditions. These authors studied
the influence of the use of RCA in AASc, testing several mechanical behaviour
properties, such us compressive strength, split tensile strength, absorption character-
istics and also their influences on the flexural behaviour of the reinforced AAS
concrete.
In this investigation, the RCA were pre-saturated to improve its workability, and
to avoid an initial drying retraction in the concrete. They were applied two alkaline
activators which were mixed, and granulated blast furnace slags (GGBFS) were
used as a precursor.
Activators were prepared using Sodium hydroxide flakes with 99% purity with a
sodium silicate solution (Na 2 O.YSiO 2 .ηH 2 O) composed of 28% SiO 2 , 11.2% Na 2 O
and 60.8% H 2 O by mass. The sodium hydroxide solution was prepared a day
before, and kept in room temperature conditions to avoid a fast setting of the pastes.
Then these activators were mixed applying the adequate alkaline concentration.
In this work, the GGBFS were first mixed with the sand, keeping the coarse
aggregates separated. Then the coarse aggregates were combined with the sand and
the slag for 3 min. Finally, the superplasticizer was poured into the alkaline solution
and stirred for 2 min to achieve the proper homogeneity. The dosages applied in
this work are summarised in Table 13.6. All mixtures were melted and cured under
ambient conditions at a relative humidity of 71% and a temperature range of
26 30 C.

It was observed that a reduction in the slump was obtained with the inclusion of
superplasticizer, at the same time that increasing amount of RCA were included in
mixtures. Slump started in around 120 mm in OPC mix, and decreased until
105 mm in 100%-RCA mix.