Page 456 - New Trends in Eco efficient and Recycled Concrete

406 New Trends in Eco-efficient and Recycled Concrete

Surprisingly, it was found in this work that the penetration of chloride in the mix-
tures with a substitution of 75% of the natural aggregate by RCA aggregate produce
a comparable performance, even better than the mix with a 100% natural aggregate
or OPC mixes.
This work showed very interesting results, indicating that do not appear reduc-
tions of the properties of durability in AASc mixtures in which alkaline activators
were applied, while the surface of the mixture that used OPC was greatly deterio-
rated a lot in aggressive environments, the authors explain that this fact is due to
AAS such as GGBFS, which is more effective in reducing the pores in the system
with the increase in the RCA volume than OPC, when RCA are applied in concrete
manufacture. This improves the performance of AASc mixes under sulphate and
chloride environment.
In general, the results of this investigation allow to conclude that the addition of
RCA does not have a significant reduction in the AASc properties.

13.4 Concluding remarks

New SCMs have been developed in order to create new binders, namely by
Davidovits geopolymers or AAMs, to achieve a relevant decrease in the negative
environmental impact of OPC production.
The use of conventional alkaline activators can significantly increase environmen-
tal impacts and economic costs, alternative alkaline solutions have recently been used
in order to reduce the potential reduction of CO 2 emissions by up to 63%.
In the development of AAMs, it is necessary to apply two components. First, a
precursor which consists of a solid aluminosilicate material and has to be prepared
at elevated temperature to facilitate the reactivity of certain raw materials. And sec-
ond, an activator which causes the activation of the precursor and is usually pre-
pared by using three alkaline activators, such as sodium hydroxide, sodium silicate
or sodium carbonate.
Several studies have focused on AAMs, using traditional precursors such as
waste derived precursors such as blast furnace slag, coal FA or MK; but different
research groups are testing new wastes as precursors and as activation reagents.
New raw materials have been tested for preparing precursors and activating solu-
tions, mainly waste from urban, metallurgical, ceramic, construction and demolition
and agro-industry activities.
The use of waste in the preparation of an activating solution, replacing partially
or totally the commercial chemical reagents, is a powerful tool in terms of the
reduction in CO 2 -associated emission for AAMs. Many slags from metallurgical
processes are appropriate for new AAMs, some of them as alternatives to blast fur-
nace slag.
Low and high temperature ceramic-derived waste showed a high reactivity and,
taking into account their huge available amounts, they can be considered as poten-
tial raw materials, also for low-income social sectors.