Civil Engineering Reference
In-Depth Information
cement.
If ground together with Portland clinker, limestone will predominate in the fine
fractions, as it is significantly softer than the clinker. In a fresh paste made from such a
cement, the fine limestone particles will fill the spaces between the coarser particles of
clinker, and consequently the amount of water needed to attain a given consistency will
decline, up to an optimum limestone addition. This amount of limestone may vary in
different cements, up to as much as 25 wt%.
A limited substitution of Portland clinker by limestone may result in a moderate
increase of strength of the resultant cement (at a constant consistency of the fresh mix).
This is brought about mainly by the water-reducing effect of limestone.
The added limestone also exhibits an accelerating effect on the hydration process, and
mainly on the hydration of alite, as its particles can act as nuclei for the crystallization of
portlandite. This effect may also positively affect the short-term strength of the cement.
In cements with a sufficiently high tricalcium aluminate content the calcium carbonate
of limestone may react with this phase, yielding calcium carboaluminate hydrate (C
4
A
H
11
). Up to about 6 wt% of calcium carbonate may be consumed in this reaction within
28 days (Ingram and Daugherty, 1992). Whether the formation of calcium
carboaluminate also contributes to a strength increase of the hardened cement paste
remains uncertain.
At smaller limestone additions the texture of the hardened paste does not differ
significantly from that of plain Portland cement. At higher limestone additions, however,
limestone may appear in the cement matrix in the form of crystalline particles evenly
distributed within the paste.
As tricalcium aluminate may react to a limited extent with calcium carbonate, the
limestone that is present may partially replace the gypsum present in the cement to
control setting. As, however, such substitution may adversely affect the mechanical
properties of the hardened paste, it should be avoided, except in cases of a limited
availability of gypsum.
In assessing the durability of concrete made with limestone-modified Portland cement
one has to bear in mind that—at a constant water/cement ratio—the porosity of the paste,
and along with it its permeability, increases with increasing limestone content in cement.
Nevertheless, adequate durability may be preserved, as long as the amount of added
limestone is not excessive (Tezuka
et al.,
1992).
The European specification ENV197-1 permits an addition of maximum 5 wt% of
limestone in Portland cement. A Portland limestone cement may contain up to 20 wt%
(notation II/A-L) or even up to 35 wt% (notation II/B-L) of limestone. No addition of
limestone is permitted in the American standards (C150-95 and C595M-95).
2.15
PORTLAND CEMENTS MODIFIED WITH CHEMICAL AGENTS
Some chemical agents, introduced in small amounts, can facilitate the production process,
or modify the properties of the resulting cement or those of the fresh concrete mix and
hardened concrete. They include
