Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 135

following sections, some of the results presented in the available literature
on the various properties of RCAs, including the adhering mortar content,
density, water absorption, and toughness, as well as the effect various pro-
duction parameters, including the type of the crushing process, particle
size, and the strength of the parent concrete, have on these properties are
briefly reviewed.

4.3.1 Adhering mortar content
RCAs often contain a large amount of adhering mortar and cement paste.
The volume by percentage of old mortar may vary from 20% to 70%,
depending on the recycling process, size of RCA particle, and properties of
the parent concrete [12,13]. The adhering mortar content is one of the most
important properties of RCA because it will directly and indirectly affect
all the other major RCA properties [13]. The adhering mortar in RCA may
be present in three general forms, and this has been used by some research-
ers to categorise the RCAs currently produced into three general groups
(Figure 4.4). In the first group, referred to as type I RCA, the adhering
mortar is present in the form of a layer of cementitious mortar adhering
to the surface of a single, usually larger, NA particle. Type II RCA is a
conglomerate RCA particle comprising a few, usually smaller, NAs and
adhering cementitious mortar. Type III RCA is a conglomerate principally
comprised of cementitious mortar. At present, almost all RCAs produced
commercially comprise varying proportions of these three types of RCA,
and no practical method is currently available to sort the RCA produced
efficiently in accordance to types I to III [14]. Adhering cementitious mortar
has a lower density, higher water absorption, lower Los Angeles abrasion
resistance, and higher sulfate content than NAs used in typical concretes
[15,16]. With this in mind, the lower quality of RCAs compared to NAs
has been attributed to the presence of cementitious mortar in the RCA
produced [15,17,18].

(a) (b) (c)

Figure 4.4 Three different types of RCA: (a) type I, comprising one natural aggregate
surrounded fully or partially by a layer of mortar; (b) type II, comprising two
or more smaller-size natural aggregates surrounded fully or partially by a
layer of mortar; and (c) a conglomerate of cementitious mortar.