Crystal inclusions in Portland cement clinker
The nature of inclusions in clinker crystals provides clues about the burning environment and are therefore important indicators of clinker quality.
Figure 1: blue belite inclusions in alite crystals
Of the four main crystalline compounds that are found in Portland cement clinker, two are produced as solids and two are present in the burning zone of the cement kiln as a liquid, which then crystallises on cooling.
Microscopic examination of clinker has been employed as a quality control tool for many years and the shapes of the crystals, as seen in polished sections of clinker, are familiar in many parts of the industry. Alite is characterised as angular crystals generally 30-40µm across, belite crystals are rounded and little smaller than alite at 15-20µm and the C3A and C4AF, which crystallise from the liquid, can range from <1µm to >20µm, depending on the rate at which the clinker liquid cools.
