4.11 : 火山灰

Portland cement is sometimes interground or blended with pozzolans, which are siliceous and aluminous materials.

The finely divided pozzolans, in the presence of moisture, react with the calcium hydroxide released during cement hydration to produce cementitious compounds.

Pozzolans are naturally available, such as volcanic ash and pumicite, or they are synthesized, such as fly ash, which is obtained from the flue gases of coal-fired power plants.

Pozzolans are added with cement used in roller-compacted concrete, mass concreting, and chemically resistant concrete.

When cement is replaced with pozzolan on a weight basis, the volume of pozzolan equivalent to the cement it replaces will be higher, as the specific gravity of pozzolan is less compared to cement.

The hydration of pozzolans is slower compared to that of Portland cement, resulting in a low rate of heat development, longer setting time and longer curing periods.

Lastly, the pozzolanic activity with cement can be assessed using the strength activity index, which is the ratio of the compressive strength of cement partially replaced with pozzolans to that without any replacement.