Civil Engineering Reference
In-Depth Information
Table 4.12
Efficiency of the chemical separation methods described in Table 4.11
Mortar removed/Total mortar (%,
by mass)
Method I
Vacid/ VRCA
Method II
Vacid/ VRCA
Method III
Vacid/ VRCA
Method IV
Vacid/ VRCA
Acid ([H
+
])
concentration
(mol/ L)
2.5
5
2.5
5
2.5
5
2.5
5
1
12 ± 4
22 ± 6
27 ± 5
45 ± 8
23 ± 8
79 ± 6
37 ± 4
86 ± 7
2
23 ± 9
54 ± 8
33 ± 4
79 ± 6
35 ± 5
86 ± 5
44 ± 5
94 ± 4
3
35 ± 8
73 ± 4
57 ± 7
88 ± 2
47 ± 10
85 ± 8
42 ± 7
~100
4
43 ± 8
68 ± 2
59 ± 5
85 ± 8
44 ± 7
91 ± 4
67 ± 7
~100
5
51 ± 4
70 ± 7
55 ± 4
91 ± 4
56 ± 3
82 ± 6
77 ± 9
~100
6
48 ± 5
75 ± 7
50 ± 8
88 ± 3
55 ± 5
89 ± 3
70 ± 3
~100
increases in acid concentration were reduced considerably. This is probably
because once a sufficient amount of H
+
ions is present, acid corrosion of the
RCA is mainly controlled by the permeability of the mortar. The permeability
of the adhering mortar tends to decrease gradually with successive stages of
acid exposure as the silica and aluminosilicate gels released by C-S-H cover
the exposed surfaces of the RCA particles [36]. As shown in Table 4.12, the
efficiency of the chemical separation method may also be increased consid-
erably through the use of a suitable rotary agitation system or addition of a
washing stage to remove the previously corroded mortar from the surface
of the RCA. This is because these methods could considerably increase the
accessibility of the acid to reach additional unexposed mortar as fresh sur-
faces are then exposed for further corrosion of the RCA to take place [13].
Chemical separation using strong acids at high concentrations is generally
considered an efficient technique for use in separating adhering mortar from
RCA. However, one major problem preventing the widespread use of this tech-
nique in practice is the potential detrimental effects any residual acids remain-
ing on the RCA have on the durability of concrete. Traces of residual sulphuric
and chloride acids, if present, can considerably increase the sulphate and chlo-
ride contents of the RCA and thus lead to a decrease in durability of the RAC.
Such durability concerns could be addressed using low-concentration acids
(~0.1 molar), but this is at the expense of efficiency [35]. Table 4.12 shows
that the removal mortar rate achievable at low acid concentrations is rather
marginal. In addition, the relatively long processing time required (>24 hours)
is another major disadvantage of the chemical separation method when com-
pared to other separation methods discussed. As a result of the significant
durability concerns and long processing time required, chemical separation
at high acid concentrations has been proposed mainly as an effective method
to achieve complete removal of mortar from RCA particles for the accurate
measurement of the RCA mortar content in laboratory test samples rather
than as a bona fide separation technique in itself [13].
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