Fly ash is a supplementary cementitious material (SCM) in the production of portland cement concrete. When used with portland cement, SCMs contribute to the properties of the hardened concrete through hydraulic or pozzolanic activity or both.
Pozzolan is a siliceous and aluminous material that possesses cementitious value when finely divided and in the presence of moisture with calcium hydroxide at ordinary temperatures to form compounds with cementitious properties.
The use of fly ash in concrete has exponentially increased over the last 50 years, with almost 15 million tons used in the US for concrete or concrete products since 2005.
For any given situation, there is an optimal amount of fly ash for specific concrete mixtures that maximize its technical, environmental, and economic benefits, without seriously impacting the rate of construction or the finished product’s long-term performance.
The optimal amount of fly ash varies on several factors:
- The application of the fly ash
- The composition and proportions of all materials in the mixture, mainly the fly ash
- The conditions during placement, especially the temperature
- The construction practices (finishing and curing)
- The exposure conditions
Overall, the optimum content of fly ash varies by usage. For example, fly ash contents of up to 50% may be suitable for most applications, provided the early-age strength requirements of the project and adequate moist-curing standards are met.
As a natural pozzolan, zeolite acts as a suitable replacement for a certain percentage of Portland cement production and usage. As per Raggiotti et al, “the physico-chemical characterization of the zeolite material is presented with the results of resistance and durability of concrete to which this addition has been incorporated.”
 Michael Thomas, Ph.D., P.Eng., Professor of Civil Engineering, University of New Brunswick https://www.academia.edu/7911232/Optimizing_the_Use_of_Fly_Ash_in_Concrete