Quality assurance


Aggregates are small rock fragments (0.08 to 80 mm) of mineral origin. They may be sand, gravel or crushed rocks. Aggregates, mixed with water and cement, are essential for the production of concrete. Aggregates are also essential inputs in asphalt production and in road engineering works. In theory, producing aggregates simply involves crushing rock. In practice, it is much more complex, as standardized sizes and homogenous aggregates must be obtained. The aggregates produced by Lafarge Cement Zimbabwe are tested and comply with Zimbabwe Standard Specification for Aggregates for Natural resources: SAZ 233:1997.

The following are the step-by-step processes involved in the manufacture of aggregates.


Step 1: Supply

Three major sources of aggregates can be identified:

  • unconsolidated (loose) rock: sand and alluvial materials (dry river beds). This is "rolled" sand because the grains are rounded,
  • solid rock: limestone and hard rock or crushed volcanic rock. This is "crushed" sand because the grains are pointed,
  • recycled materials: often of industrial origin, from demolition, recycled concrete, railway ballast, etc.

Step 2: Extraction

Extraction is a key phase during production from solid rock, particularly because strategic choices, such as the selection of a slab for color or hardness, can make a real difference. 

Step 3: Crushing, Grinding, and Screening

Once extracted, the materials are transported to the processing site for scalping. This process involves removing unwanted materials, such as blocks, clay, etc.

The scalped product is crushed once to transform the block into broken stone. The process is repeated as many times as necessary to obtain the desired fragment size.

The resulting material is then screened to obtain aggregates of the desired grade. The larger pieces that are rejected are returned to the crusher and subsequently re-screened, with the process continuing until the desired size is obtained.

Some categories of aggregates, such as sand and gravel, undergo complementary processing including washing, cycloning and scrubbing, primarily to make them cleaner.