The evaluation of fly ash for use as a supplementary cementitious material (SCM), i.e. as a pozzolan, begins with mortar testing. Mortar is similar to concrete in that it contains cement, water, and aggregate, except that in mortar graded sand is the only aggregate present. ASTM C 311 describes the procedures used to test the fly ash in mortar. In a "standard mortar" 20% of the ordinary Portland cement (OPC) is replaced with the fly ash to be tested. The data from the fly ash mortar is compared with data from a "control" mortar without fly ash. In order to accomplish this, the mortars are prepared by varying the water content to achieve a constant workability, which is determined using mortar flow. This is done because fly ash mortar typically requires less water than an OPC mortar, which can increase the compressive strength. The most common mortar tests are described below.
The mortar flow test utilizes a specially designed table that repeatedly raises and drops a known quantity of mortar 25 times. During the test, the mortar will spread or flow to form a circular mass (shaped like a pancake), and the diameter of the mass is measured and compared to the initial size. The increase in size is expressed as a percentage of the initial size; for most mortars the required flow is 110%. The flow test is repeated, using a fresh batch of mortar each time, until the desired flow is achieved. The quantity of water needed to achieve flow is recorded, and this mortar is then tested for compressive strength.
This is probably the most relevant test to evaluate the performance of fly ash since concrete is valued mainly for its high compressive strength, and the pozzolanic reaction within the concrete produces additional cement and thus higher strengths. Once the proper flow is achieved, the mortar is placed and compacted into bronze cube-shaped molds. The surface of each cube is finished using a trowel, and the molds placed into a moist curing cabinet. After 24 hours of curing, the molds are stripped from the cube specimens. The compressive strength is then tested at specified curing intervals, usually 1 or 3 days, 7 days, 28 days, and 56 days.
Concrete that is exposed to frequent freezing and thawing contains small air bubbles that are intentionally entrained in the concrete by adding air entraining admixtures (AEAs). Most AEAs are surfactants (similar to liquid soaps) that help stabilize air bubbles that are produced within concrete during the mixing process. The air bubbles, which typically comprise 4 to 8% of the concrete volume, provide empty chambers for expanding ice crystals (since water expands during freezing) instead of damaging the concrete. A common drawback of using fly ash in concrete is that the fly ash carbon adsorbs the AEAs, which can lower the air content. Thus, a key evaluation of fly ash performance is its effect on air entrainment.
Mortar is prepared using a similar method as for compressive strength, except that coarser sand is used and an AEA is mixed with the mortar to entrain air within the mix. After mixing, the flow of the mortar is determined (by raising and dropping the table only 10 times). If the flow is within the specified range, then a portion of the mortar is placed and compacted into a brass cup of known volume (400 ml), and the mass of the cup + mortar determined. Subtracting the mass of the cup, and knowing the density of each component, the air content of the mortar is calculated. The test result reported is the quantity of AEA required to achieve a mortar air content of 18%. Fly ash will usually increase the amount of AEA needed.
The elapsed time after mixing whereupon the mortar begins to harden is the set time. This test is most commonly performed on cement paste (cement + water), but can also be conducted using mortar. The test is completed by measuring the penetration of a steel needle into the paste or mortar over the course of several hours. The needle is part of an instrument called a "Vicat apparatus". When the penetration of the needle into the material is less than 25 mm for paste or 10 mm for mortar, the material has achieved its "initial set". The time required to achieve this degree of hardening is reported as the test result.