Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 29

(2) volcanic tuff, (3) calcined clay or shale, and (4) raw or calcined opaline
silica. As volcanic tuffs generally contain both altered and unaltered
siliceous glass, pozzolans of volcanic origin cannot be easily fitted into
groups 1 and 2. Indeed, these are the sole or primary sources of pozzolanic
activity in all groups.
The classification system adopted by the U.S. Bureau of Reclamation is
based on mineral composition and shown in Table 2.2. The exact chemical
composition of natural pozzolans will depend on the mineral composition
of the raw or processed parent material. Metha (1987), Day (1992), and
Shi (1992) summarise the chemical composition of some natural pozzolans
reported in the literature.
Although most natural pozzolans contain substantial amounts of silica,
additional elements are present, such as alumina and iron oxide, which
also react with calcium hydroxide and alkalies (sodium and potassium)
to form complex hydration compounds. This is one of the reasons why a
test of the reactivity of the pozzolan is very useful, even if the chemical
composition is known. It is certainly a useful quality control procedure
(as it is for fly ash and silica fume). An Indian system uses the strength of
lime-pozzolan mortars cured 8 days at 50°C as a means of classifying the
natural pozzolan according to its strength producing properties, as shown
in Table 2.3.
Natural pozzolans have been found to reduce permeability, diffusion, sul-
fate attack, and alkali silica reaction. However, as natural pozzolans con-
stitute a diverse group, the required performance should be determined by
appropriate testing before using. Once a particular natural pozzolan source
has been selected, it would be expected to have limited variability depend-
ing on the constancy of the geological formation. Geosilica from the ther-
mal lakes around Rotorua in New Zealand was able to be produced by
blending mined material so that the final product was remarkably consistent
Table 2.2 Bureau of reclamation classification
Activity type Essential active constituent
1 Volcanic glass
2 Opal
3a Kaolinite-type clay
3b Montmorilonite clay
3c Illite type clay
3d Mixed clay with vermiculite
3e Palygorskite
4 Zeolite
5 Hydrated oxides of aluminium
Source : After Mielenz R. C. et al., Econ. Geology , 4 6 , 3 ,
311-328, 1951.