Strength Activity Index (SAI) Test (ASTM C311/C311M)

Ordinary Portland Cement (OPC) production is highly energy-intensive and contributes significantly to carbon emissions. To reduce cement consumption and improve sustainability, supplementary cementitious materials (SCMs) are incorporated into concrete. Many of these materials exhibit pozzolanic properties, meaning they react with calcium hydroxide [Ca(OH)₂], a by-product of cement hydration, to form additional calcium silicate hydrate (C–S–H) gel. This secondary C–S–H enhances the strength, durability, and long-term performance of concrete. Materials that undergo this reaction with calcium hydroxide are known as pozzolanic materials.

Use of Strength Activity Index Test

• Determines the pozzolanic reactivity of a supplementary cementitious material (SCM) by evaluating its contribution to compressive strength.
• Assesses the suitability of a material for use as a pozzolana in the manufacture of Portland Pozzolana Cement (PPC) or in blended cementitious systems.
• Provides a quantitative measure of the Strength Activity Index (SAI), which indicates the effectiveness of the pozzolanic material in enhancing mortar strength.
• Assists in the selection and quality evaluation of pozzolanic materials for concrete and cement applications.

Apparatus Required

• Weighing balance

• Non-absorbent mixing bowl and planetary mixer for mixing mortar

• Flow table with flow mould

• Cube moulds of 50 mm dimension with a non-absorbent base plate

• Scrapper

• Graduated cylinder

• Trowel

• Tamping rod of 12 × 25 mm cross-section and a length of 125–150 mm

• Compression testing machine of capacity 100 kN

Procedure

Step 1. Prepare two sets of mortar specimens:

(a) Control specimens made only with ordinary Portland cement (OPC) and

(b) Test specimens in which 20% of the cement is replaced with the supplementary cementitious material (SCM).

The comparison of compressive strengths is used to evaluate the pozzolanic reactivity of the SCM.

Step 2. Prepare the control mortar using a cement-to-graded sand ratio of 1:2.75. For one batch producing six 50 × 50 × 50 mm cubes, use 500 g OPC, 1,375 g graded sand, and 242 mL water. The water content for the control mix is fixed by the standard, whereas the water required for SCM-blended mortar is determined from the flow test.

Step 3. Determine the flow value of the control mortar by carrying out the flow table test.

Conducting Flow Table Test

Step 4. Before mixing, thoroughly clean and dry the mixing bowl. Add the required quantity of water to the bowl first.

Step 5. Add the OPC to the water and mix for 30 seconds at 140 ± 5 rpm.

Step 6. Gradually add the graded sand over the next 30 seconds while continuing mixing at 140 ± 5 rpm.

Step 7. Stop the mixer, change the speed to 285 ± 10 rpm, and continue mixing for another 30 seconds.

Note: Always stop the mixer before changing speed.

Step 8. After a total mixing time of 1 minute 30 seconds, stop the mixer and manually scrape the sides of the bowl for 15 seconds to achieve uniform mixing.

Step 9. Close the mixing bowl with its lid and allow the mortar to rest undisturbed for 1 minute 15 seconds.

Step 10. Carry out the final mixing for 1 minute at 285 ± 10 rpm. Remove the mixing paddle and scrape off all adhering mortar back into the bowl.

Step 11. Clean and dry the flow mould and position it at the centre of the flow table.

Step 12. Fill the flow mould in approximately 25 mm thick layers. Tamp each layer 20 times with the tamping rod to distribute the mortar uniformly.

Step 13. Overfill the final layer slightly, then strike off the excess mortar with a straightedge to obtain a smooth, level surface.

Step 14. Clean the flow table thoroughly and remove any excess water, particularly around the outside of the mould.

Step 15. Exactly 1 minute after filling, lift the mould vertically. Drop the flow table 10 times through a height of 12.5 mm within 6 seconds.

Step 16. Measure the spread diameter in four equally spaced directions and calculate the average. Record this value as the flow of the control mortar.

Step 17. Prepare a blended mortar by replacing 20% of the OPC with the pozzolanic material while maintaining 1,375 g of graded sand.

Step 18. Repeat the mixing and flow test using different water contents until the blended mortar achieves a flow within ±5% of the control mortar (95–105% of the control flow). Record the corresponding water content.

Step 19. Using the selected water content, prepare sufficient blended mortar (20% SCM replacement) to cast six 50 mm mortar cubes. Follow the same mixing procedure used for the control mix.

Step 20. Coat the cube moulds and base plates with a thin layer of low-viscosity mineral oil or grease and place them on a level surface.

Step 21. Fill each mould in approximately 25 mm thick layers, tamping each layer 25 times.

Step 22. Overfill the moulds slightly and strike off the excess mortar with a straightedge to produce a smooth surface.

Step 23. Cover the moulds with cover plates and keep them under wet gunny bags for 24 hours. Demould the specimens and cure them in saturated lime water until the testing age to minimise calcium hydroxide leaching.

Step 24. After 7 or 28 days of curing, remove the cubes, clean their surfaces, and remove any loose particles before testing.

Note: Place the specimen so that faces other than the cast face are subjected to loading.

Step 25. Apply the compressive load continuously at a rate corresponding to 3.5 MPa/min until failure and record the maximum load.

Step 26. Calculate the compressive strength by dividing the failure load by the loaded area. Report the strength to the nearest 0.05 MPa and use the average of three specimens.

Step 27. Determine the Strength Activity Index (SAI), also called the Pozzolanic Activity Index (PAI), by comparing the average compressive strength of the blended mortar with that of the control mortar.

Observations and Calculations

Step 1. Record the flow obtained for the control mix.

Step 2. Conduct trial mixes to determine the water required to achieve a flow within ±5% of the control mix and record the results in table as shown below.

Particulars	Trial 1	Trial 2	Trial 3	Trial 4	Trial 5
Weight of Portland cement (g)
Weight of pozzolana (g)
Weight of sand (g)
Weight of water (g)
Average flow difference (%)

Step 3. Use the selected water content for casting the SCM-blended mortar specimens.

Step 4. Determine the compressive strengths of the control and blended mortar specimens and record the results in table as shown below.

Note:

Determine the fineness of the pozzolanic material before testing, as fineness significantly influences pozzolanic reactivity and the measured strength activity index.

Compressive Strength Results

Particulars	Control Specimen 1	Control Specimen 2	Control Specimen  3	Test Specimen 1	Test Specimen 2	Test Specimen 3
Temperature
Load at failure (kN)
Stress at failure (MPa)
Average compressive strength (MPa)
SAI (%)	Strength(OPC–pozzolana mix)/Strength of control mix × 100 =
Quality of SCM
Qualified (≥75%) / Disqualified (<75%)