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TECHNICAL PAPER

unaltered or remaining phase is called as ‘resistate’ . Alkaline
[9]
elements possess high dissolution kinetics; whereas, resistate
endures as sediment. Examples for some typical resistate
are quartz, potash feldspar, and magnetite. Usually, the end
products of chemical weathering are clay minerals. The basic
clay minerals formed may be kaolinite, illite, and montmorillonite
[9]
(Table 3) . These hydrolysis reactions to form clay minerals are
very complex . The hydrolysate cations present in the minerals
[9]
play a significant role in the sequential process of chemical
weathering. In feldspar mineral, these ions may include K ,
+
Na , and Ca . In ferromagnesian mineral olivine, Mg is the
2+
2+
+
susceptible hydrolysate cation. Pyroxene group minerals have
Ca , and Mg as hydrolysate cations. Amphibole group of
2+
2+
+
2+
2+
minerals are composed of K , Na , Ca , and Mg with a great
+
Figure 11: Physical appearance of vermiculite (Rashad, 2016) [29] affinity for hydrolysis reactions. The minerals with chemical
weathering may exhibit a dull or drab appearance which
[9]
power plants [28] . Other igneous, sedimentary, or metamorphic obscures the diagnostic properties during their identification .
[9]
rocks (may be siliceous, calcareous and silico-calcareous) with The observations of Goldich (1938) [31] as cited by Perkins
normal specific gravity ranges (2.5 to 3) are commonly used in are relevant to aggregate chemical weathering. He found
concrete as aggregates. There are exceptional cases of highly that the minerals formed from molten magma at a relatively
porous aggregates such as vermiculite shown in Figure 11 [29] . high temperatures (deficient in Si, and O) are more prone to
It is an altered product of biotite as a result of hydrothermal chemical weathering compared to the minerals crystallized at
phenomena. If vermiculite gets heated up to 650°C to 950°C, it low temperatures. On the contrary, the ferromagnesian minerals
expands continuously, and imparts a bulk density 10 to 11 less of pyroxene or olivine groups, calcium rich feldspar, and other
than the preheated density [30] . The product can be used as water-soluble minerals exhibited high weathering potentials. In
aggregate to produce low density or light weight concrete. This general, minerals with more Si-O bonds possess more resistant
exfoliated, and porous aggregate also possesses low thermal to chemical weathering. Based on these observations, Goldich
conductivity, and excellent acoustics. The melting temperature (1938) developed a dissolution series for minerals . Figure 12
[9]
of vermiculite is found to be varied from 1240 to 1430°C, and it proposes a chemical weathering chart for aggregate minerals
does not show any chemical changes during thermal expansion based on Goldich’s dissolution series [9,31] . This modified chart
(29 and 30). Aggregates with large, and round open pores of size also illustrates the possible hydrolysate cations in each mineral
greater than 1 µm can also help in resisting freeze-thaw damage (or groups) and explains major resistate in each group of
and salt crystallization damage in the materials [23] . minerals. In conjunction with Tables 1, 2 and 3, this diagram
(Figure 12) can be indicatively used for selecting suitable
5.3 Weathering potential of aggregate minerals aggregates for concrete in different environments (chemically or
physically triggered exposures).
The entries to column 7 of Table 3 provide indicative weathering
potential of minerals in the environment. It is imperative to
focus on the weathering potential of minerals in aggregate,
which may have an influence on the durability of concrete.
The possible physical weathering is abrasion, or any other
mechanical, or physical triggering. The extent of erosion, or
physical weathering of minerals generally follows the relative
hardness based on Mohs’ number listed in Table (3). The extend
of physical alteration may also be affected by the environment
or medium in which the weathering is progressing. However,
the chemical weathering of minerals is much significant than
physical weathering .
[9]
Chemical weathering occurs by the action of water, and is
referred as dissolution. The dissolved product by the action of Figure 12: A modified chemical weathering chart with hydrolysate
chemical weathering is termed as ‘hydrolysate’; whereas, the cations for aggregate minerals based on Goldich’s dissolution series [9,31]

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