Testing methods of pozzolanas
Practical Action
The curing is carried out over an accelerated period of 7 days. The rate of strength
development is sensitive to temperature and results of samples cured at elevated
temperature have been found to correlate well with medium and long-term strength
development. The samples are cured in the following manner:
First 48 hours
48 - 96 hours
96 hours to 1 week
Simple curing in moulds at 20oC in air
Samples cured under water at 50oC
Samples cured under water at 20oC
A control sample is also prepared which is not subjected to elevated temperature
during curing.
3. Type of lime and lime-pozzolana ratio
High-calcium hydrated lime is used, satisfying BS 890. A lime pozzolana ratio of 1:2
is used. This uses slightly more lime than usual for obtaining optimum strength but
this allows for incomplete mixing.
4. Type of aggregate and aggregate cement ratio
A graded sand satisfying BS 4551 is used with an aggregate: cement ratio of 1:3.
5. Water content
The effect of water content on compressive strength is significant and therefore the
water content of the sample has to be carefully controlled. To standardise samples,
the workability of the sample is measured using the flow test outlined in ASTM C593
- 4.
6. Mixing procedure, casting and compaction procedure and method of performing
compression test
The mixing procedure is given below:
Dry mix lime and pozzolana
2 minutes
Add estimated water content
2 minutes
Add sand and mix
1 minute
Clean sides of mixing bowl and mix
1 minute
Add any additional water needed (to obtain
required flow) and mix
30 seconds
Moulds are filled in three layers, each layer tamped 30 times with a 1 cm2 rod and the
surface levelled with a trowel. Compression testing is carried out on a strain controlled testing
machine at a constant strain rate of 1mm per minute and at least 3 cubes are crushed for
each plotted point. A portable machine has been developed for field compression testing.
Flow experiment
As mentioned earlier, the water content of the sample cannot be fixed due to variations in
materials being used. More useful as a working parameter is the workability of the mortar mix.
To quantify the workability the ‘flow’ of the mortar is measured. A flow of between 65% and
75% is recommended in the ASTM standard C593.
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