Civil Engineering Reference
In-Depth Information
cement constituents have been largely hydrated, and a firm structure has already been
developed within the paste. Thus the conversion of MgO to Mg(OH)
2
creates stresses
within the hardened paste, which may cause expansion and eventually crack formation
and disintegration of the material (Ramachandran, 1980). Because of this possibility,
periclase is an undesirable constituent of Portland clinker. In this respect free magnesium
oxide behaves similarly to free calcium oxide; however, the hydration of magnesium
oxide progresses much more slowly, and thus any problems associated with its presence
will become apparent only after years of service.
The amount of magnesium oxide that may be present in ordinary Portland cement
without causing expansion, cracking, and strength loss (due to the formation of
microcracks within the structure) lies at around 4-5%. Most of it will be incorporated
into the crystalline lattices of clinker minerals; however, a limited amount of free
magnesium oxide may also be present without causing problems. Virtually all Portland
cement specifica-tions set a limit for the total amount of MgO that is allowed to be
present in the clinker or cement. Such limits vary between 2.5% and 6.5% in different
specifications.
To determine experimentally whether a hardened paste made from a cement with a
given amount of MgO will preserve its volume stability indefinitely, accelerated testing
has to be performed. In such tests the test specimens are exposed to an autoclave
treatment, and the expansion that takes place under these conditions is assessed.
In high-MgO Portland cements, the MgO content in the clinker is increased above
values commonly considered safe. Such cements are produced so that starting materials
with high MgO contents, such as dolomitic limestone, can be used. To obtain a cement
with an acceptable performance even at these elevated MgO levels, one or a combination
of several measures must be taken in the production process:
• The Fe
2
O
3
content in the raw meal, and thus the amount of the ferrite phase in the
clinker, must be increased. As the capacity of the ferrite phase to accommodate Mg
2+
ions is greatest of all clinker minerals, the fraction of MgO incorporated into the
crystalline lattices of clinker minerals is thus increased. At the same time the amount
of free periclase (the only form of MgO that may cause problems) is reduced.
• In the burning of clinker the rate of cooling must be speeded up as much as possible.
Cooling of the clinker with water, rather than in air, has been found to be particularly
effective (Sharma
et al.,
1992). Under these conditions the average size of periclase
crystals that crystallize from the melt in the course of cooling is reduced, while their
number is increased. In the hydration of such a cement the stresses caused by the
conversion of periclase to brucite are more evenly distributed within the hardened
paste, and the formation of cracks due to the presence of large MgO crystals may be
prevented or reduced. Some reduction of the size of the formed periclase crystals may
also be achieved by finer grinding of the raw meal.
• Excessive expansion and crack formation of high-MgO cement may also be prevented
or reduced by grinding the high-MgO clinker together with fly ash as well as gypsum
(Ali and Mullick, 1998). Less effective are some natural pozzolanic materials,
limestone or granulated blast furnace slag (Sharma
et al.,
1992; Hu, 1997). The
amount of these materials that has to be used to achieve effective protection against
cracking ranges up to 30%. Table 2.3 shows the effect of added fly ash and granulated
