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What is fly ash and why is it used in cement?

Fly ash is used as an ingredient in cement

One of the core ingredients of cement is fly ash. This is a byproduct from the burning of coal in power stations. Fly ash is a fine powder that is light grey in colour and is made up of tiny spherical particles. Fly ash also contains pozzolanic materials, similar to volcanic ash, that react with lime to form cement.

The substance is widely used in the manufacture of cement, including at LafargeHolcim Tanzania’s Tembo cement facility in Mbeya. Fly ash has a number of physical properties that make it perfectly suited to creating cement, which we will outline below:

Physical properties of fly ash

Fly ash is a super fine material. The particle sizes average 45 microns in diameter – that’s 0.045mm. The smallest particles can be just 10 microns thick. This helps to give cement it’s fine, powdery texture which is what makes it a good bonding agent when added to water and aggregates.

These particles are usually spherical in shape and have a glassy texture when viewed under a microscope. The colour of fly ash can differ, depending on whether other chemicals and minerals are present. Lime particles in fly ash gives it a lighter colour, whereas iron gives the ash a brown hue. A dark grey, almost black, colour is usually caused by unburned coal in the power station furnace.

Ordinary Portland cement is made up of about 50% tricalcium silicate – this is the mineral that binds with other materials when hydrated with water. The fly ash in cement combines with the wet tricalcium silicate and begins to fill up empty spaces within the wet concrete mixture. This gives concrete its impermeability and strength once it has hardened and cured.

Other uses of fly ash

This pozzolanic substance has a range of uses in construction, including the following:

  • Used in the manufacture of Portland cement.
  • Used for the construction of embankments.
  • Used as a soil-stabilisation material.
  • Used as a filler mineral in asphalt roads, helping to fill voids in the tar.
  • Used as a component in geopolymers, such as ceramics.
  • Used in roller-compacted concrete dams.
  • Used in the manufacture of fly ash bricks.


LafargeHolcim is a leading building materials and solutions company that has been operating in international markets for decades. We produce cement and aggregates for construction projects, ranging from small affordable housing developments to large-scale infrastructure projects such as high-rise buildings, dams and bridges.


LafargeHolcim Tanzania has been supplying the country and neighbouring countries with our world-class Tembo cement brand for over 30 years. Our head office and fully-integrated plant are located in Mbeya, Southwest Tanzania.


At LafargeHolcim Tanzania, we believe customers come first. We listen to your specific requirements to supply and develop the best solutions for your needs. As the new leader in building materials, you can also rely on our cutting-edge research and development capabilities that have resulted in the finest materials for your construction projects, whether large or small.


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