Page 425 - New Trends in Eco efficient and Recycled Concrete

378 New Trends in Eco-efficient and Recycled Concrete

Figure 13.13 Compressive strength values for RCBW activated with NaOH (Robayo et al.,
2016). RCBW, Red clay brick waste.

Zawrah et al. (2016) also studied RCBW/BFS blends for alkali activation. They
observed that mechanical, porosity and water absorption performances were
improved when blast furnace slag (BFS) was blended with RCBW. The best com-
pressive strength development was detected for 40/60 RCBW/BFS system, with
83 MPa after 90 days of curing, which was six times more than plain RCBW geo-
polymer. This improvement was confirmed by Rakhimova and Rakhimov (2015).
They observed that the conjoint grinding of both precursors obtained better mechan-
ical development of the geopolymers (Fig. 13.14). For RCBW/BFS 60/40 sample,
2
the increase of the precursor fineness to 700 m /kg let to improve the strength up
140 MPa.
Reig et al. (2016a) studied the blends of RCBW with calcium aluminate cement
(CAC, in 0% 50% replacement). Large improvement was observed when CAC
was added, especially when mortar was cured at room temperature. Thus, 100/0
RCBW/CAC yielded 10 MPa after 28 days of curing, while 90/10 and 60/40 sys-
tems yielded 35 MPa and 80 MPa respectively. Gibbsite only was detected for
100% CAC paste and C 3 AH 6 cubic was also for 100% CAC and 30% and 50%
CAC pastes cured at 65 C. Differential thermogravimetry curves (DTG)

(Fig. 13.15) displayed that main weight loss of the binding gel in RCBW/CAC

pastes was produced in the range 100 200 C, which corresponds to thermal
decomposition of C A H, alumina gel, CAH 10 and C 2 AH 8 .
Robayo et al. (2016), Robayo-Salazar et al. (2017) studied the RCBW blends
OPC. They obtained a marked improvement in compressive strength when 10% or
20% of OPC replaced RCBW in alkali-activated systems by NaOH/Na 2 SiO 3 . Thus,
the 100% RCBW system that had less than 10 MPa after 7 days, increased strength
to more than 55 MPa for 20% OPC replacement; for 28 days, these values were 60
and 100 MPa respectively. The authors calculated the global warming potential for