Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 156

the corroded mortar. The selection of an appropriate acid is highly impor-
tant and could significantly affect the effectiveness of the chemical separa-
tion method. Sulphuric (H 2 SO 4 ) and hydrochloric (HCl) acids have been
reported as the most efficient acids for the removal of mortar from RCA [35].
However, besides the mortar removal efficiency, another important
parameter in choosing a suitable acid for chemical separation methods is
the compatibility of acid with the NAs present in the RCA. The acid used
should have no or minimal effect on the quality of the natural stone aggre-
gates present in RCA. For instance, for RCAs with granitic embedded NAs,
HCl and H 2 SO 4 are considered the most appropriate choices because of the
considerably low solubility of the component minerals present in granitic
aggregates when exposed to these acids [36,37]. On the other hand, hydro-
fluoric acid should not be used as all the major constituents of industrial
granites, including quartz, feldspar, and mica, can be easily dissolved in it
[36,37]. In general, the chemical separation method has been suggested to
be especially suitable for RCAs consisting of NAs with high chemical resis-
tivity, such as granite [14,36].
The efficiency of the chemical separation method has been reported to
depend on the porosities of the adhering mortar and embedded NAs, acid
type and concentration, ratio between the volumes of acid and RCAs being
processed, temperature, process duration, and the container type (static vs.
dynamic). The results of a study we conducted (Tables 4.11 and 4.12) show
that, for a similar acid (sulphuric acid) and similar process duration, the
removal rate increases significantly with an increase in the acid concentra-
tion or the volumetric ratio between the acid and RCA, both leading to an
increased presence of the H + ions required for corrosion of the RCA to take
place [13]. However, it was observed that there was a particular H + con-
centration associated with each test (different concentrations and different
acid/RCA volumetric ratios) after which the beneficiation effects of further

Table 4.11 Different chemical separation methods investigated

Chemical separation methods
I Action taken II III IV

Soaking in sulfuric acid solution (for a total duration
of 24 h)

× × × ×

Continuous rotary agitation of RCA/acid container
(10 ± 1 rpm)

× ×

Washing away the corroded mortar after 8 h × ×

Replacement of the acidic solution with a fresh acidic
solution (after 8 h)

× ×

Washing and cleaning of RCA samples on 4-mm sieve
after 24 h

× × × ×

Note : Results are summarized in Table 4.12.