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TECHNICAL PAPER COLLECTOR’S EDITION

The potential to form space-filling hydrates is the insoluble hydrates, but because their mobility in
key to a hydraulic cement. First, the anhydrous the alkaline pore solution of cements is very low,
cement is mixed with water. The solid content of these hydrates are almost exclusively deposited
the resulting mixture cannot exceed around 50% within the boundaries of the original grains and do
(W/C ~ 0.3) if the mixture is to be flowable. Then the not contribute to filling the space initially occupied
anhydrous material must dissolve and precipitate by water or to binding the grains together. Even
new solids incorporating water with a higher solid in very old cement pastes (even > 100 years [6]),
we see bright areas corresponding to the original
volume than the original anhydrous material. The
ferrite phase in the cement, as in Figure 3 (a). In
phases present in Portland cement clinker are ideal
slag cements, the magnesium component of the
in this respect as there is roughly a doubling in
slag remains within the boundaries of the original
solid volume, which enables most of space initially
slag grains and does not contribute to binding the
occupied by water to be filled, yielding a strong
materials together (Figure 3 (b)). This brings us
solid with low porosity.
back to the three oxides that dominate Portland
cement – lime, silica and alumina.
The alkali oxides retained from Figure 2, Na O
2
and K O, cannot produce hydraulic compounds Strategies to lower the environmental impact of
2
as these have a very high solubility and will not cement manufacturing are already applied by
deposit hydrates. Nevertheless, small amounts the industry and their future potential has been
of these compounds play an important role in evaluated in detail in by the IEA (Internatonal
Portland cement, where they nearly all end up in Energy Agency) for the Cement Sustainability
the pore solution, conferring the concrete with the Initiative (CSI) study of the World Business Council
high pH needed to protect reinforcement. Iron for Sustainable Development (WBCSD), from which
and magnesium oxide, on the other hand, lead Figure 4 is adapted [7]. Today the three most
to the opposite problem. These oxides do form important approaches are:

134 The Indian Concrete Journal | November 2018