Pozzolanas: rice husk ash and pulverised fuel ash
which is roughly two-thirds that of OPC. As with all pozzolanas, fineness is critical to the
performance of PFA, with finer pozzolanas giving faster pozzolanic reactions. Electrostatically
collected PFA will be finer than PFA collected mechanically and is therefore normally
preferred as a pozzolana. The Blaine method of measuring fineness is felt to be unreliable for
PFA and the simpler sieving technique is often better. Standards give a maximum to be
retained on a 45 micron sieve of 12.5 to 50% depending upon the country of origin.
PFA is not normally ground to produce a finer material as this will break up the rounded
particle shape which is important for its water reduction and increased workability
Unlike most other pozzolanas, PFA requires no processing before use. It is normally
transported in bulk to the cement factory or construction site where it is blended with OPC
and/or lime to form a pozzolanic cement.
Table 2. Typical percentage compositions of PFA
Utilization of RHA and PFA
The pozzolana must be blended with lime and/or OPC to produce a pozzolanic cement. This
can be accomplished by human or animal-powered methods but full homogeneity is unlikely
to be achieved and the strength and consistency of cements blended in this manner will be
Mechanical techniques, preferably intergrinding in a ball mill or, as a second option, dry
blending in a pan or concrete mixer, will give better results in terms of both strength and
Pozzolanas can be used with either lime and/or OPC. With the latter, replacement of up to
25-30% is common, although research has suggested that for non-structural purposes
replacement of up to 50% can be used. With lime pozzolana cements, mixtures of 1:1 to 1:4
lime: pozzolana) by weight are used. A small percentage (normally 5-10%) addition of OPC
will improve strength and decrease setting times. A larger percentage of OPC may be
required if only poor quality pozzolanas are available. The exact ratio of the ingredients will
depend upon the quality of the respective raw materials and on the required characteristics
of the concrete or mortar made from the cement.
Amorphous RHA is a high quality pozzolana which, when mixed with a good quality lime,
should produce a cement giving 7 and 28 days compressive strengths of mortars well in
excess of 2 and 4 mega pascals (MPa) respectively. The pozzolanic reaction of RHA is
relatively fast and, unlike most other pozzolanas, most of the strength gain of RHA-based
cements will take place during the initial 28 days.
The pozzolanic reactivity of PFA is variable. In some countries quality-assured PFA is
available and this should produce mortars and concretes to meet Standard strength
requirements, similar to those quoted above. Other PFAs should be carefully tested. PFA is
rarely used on its own to make a lime-pozzolana cement, although it has been used in
combination with other pozzolanas.
The low bulk density and rounded particle shape of PFA give it better water-reducing and
workability-enhancing properties than other pozzolanas. It is therefore ideal for blending with
OPC, particularly if there is a coal-fired power station in the vicinity and it is available at low
cost and in large quantities. PFA is also ideal for use in block and brick manufacture.