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Hydraulic cements
are materials which:
(1) Set and harden after
combining with water, as a result of chemical reactions with the mixing water and,
(2) After hardening, retain strength and
stability even under water. The key requirement for this is that the
hydrates formed on immediate reaction
with water are essentially insoluble in water.
Most construction cements
today are hydraulic, and most of these are based upon Portland cement, which is
made primarily from limestone, certain clay minerals, and gypsum, in a high
temperature process that drives off carbon dioxide and chemically combines the
primary ingredients into new compounds.
Non-hydraulic cements are the cements which
must be kept dry in order
to gain strength.
These cements were amongst the most common of the
ancient cements and include materials such as non-hydraulic lime and gypsum plasters,
which must be kept dry in order to gain
strength, and oxychloride cements which have liquid components.
For example, Lime mortars
"set" only by drying out, and gains strength only very slowly by
absorption of carbon dioxide from the atmosphere to reform calcium carbonate.
Portlandite (Ca(OH)2) and gypsum are
relatively highly soluble. This is the reason why they deteriorate rapidly in
moist or wet environments.
The early Romans used to ram the wet pastes of lime
based cement mortar (cement + sand) to form a surface layer of high density.
This layer carbonated in contact with air to produce a low permeability surface
skin of calcite (CaCO3). This
protected the underlying Ca(OH)2 . Examples of Roman lime mortars
can still be seen in Hadrians Wall. Lime mortars were used in domestic
construction until relatively recently.
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