Civil Engineering Reference
In-Depth Information
Water
Cement
Fine aggregates
Coarse aggregates
Cement paste
Mortar
Concrete
2.2
Schematic of the composition of cement paste (water
+
cement),
mortar (cement paste
+
fi ne aggregates) and concrete (mortar
+
coarse aggregates).
Aggregates with diameters larger than 4.75 mm (No. 4 sieve) are referred
to as coarse, whereas particles with diameter in the range of 75
m-4.75 mm
(No. 200 to No. 4 sieve) are referred to as fi ne (Mehta and Monteiro, 2006).
When fi ne aggregates are added to the initial mix, we term the resulting
product as mortar (cement
μ
fi ne aggregates), whereas concrete is
created with the further addition of coarse aggregates (cement
+
water
+
+
water
+
fi ne/coarse aggregates), as illustrated in Fig. 2.2.
2.1.2 Nanoscale: C-S-H
Cementitious materials are the product of complex chemical reactions that
take place when cement (primarily tricalcium and dicalcium silicates) reacts
with water to form various hydration products with nanoscale features that
are arranged in a multi-scale fashion in a three-dimensional space (Feldman
and Sereda, 1968; Taylor, 1990, 1993; Nonat, 2004; Richardson, 2008). As
alite and belite (C
3
S and C
2
S) comprise over 80% of most cements, their
hydration products dominate in terms of volume. Both silicate phases react
with water to form a hydrated version of calcium silicates (C-S-H) and
calcium hydroxide (CH or Portlandite):
C S C S
+→
H
C-S-H
+
CH
[2.1]
3
2
The main constituent phase, C-S-H, which dominates in terms of volumetric
proportions (
70%) and as a consequence governs the macroscopic
response, manifests itself in the nm to
>
m length scale (Nonat, 2004;
Jennings, 2008; Richardson, 2008). The poorly crystalline, highly porous and
non-conductive nature of the material makes it diffi cult to study and as a
result the actual mechanisms that govern the formation and its properties
remain unidentifi ed to date. In fact, the hyphens in C-S-H refl ect its uncer-
μ
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