Clinker Micrographs

Portland cement is manufactured by heating a mixture of limestone and clay in a kiln to 1400 to 1600°C. At this temperature the raw ingredients chemically interact to form new phases. The heat treatment is called clinkering, rather than sintering (where no melting occurs) or fusion (where complete melting occurs), because partial melting occurs. The material that comes out of the kiln is called clinker. Clinker emerges as marble to golf ball size lumps, and must be finely ground and mixed with gypsum to be sold as portland cement.

Optical and scanning electron microscopy are useful techniques to identify and quantify the phases present in unground clinker or in the ground cement grains. The phases present in the clinker are dependent upon the original raw ingredients, temperatures in the kiln, and time of heating. The images below illustrate the potential of microscopic techniques.

Clinker structure of NIST RM 8488 with HF etch (approx 150µm full width) [Optical micrograph by Paul Stutzman, NIST]

Backscatter electron image of NIST RM 8488, a coarse-grained portland cement clinder with euhedral to subhedral alite crystals, rounded, occasionally ragged-edged belites, and a coarse-grained interstitial aluminate and ferrite (approx 200 µm full width) [BEI by Paul Stutzman, NIST]