Portland cement clinkers from the rotary kiln vary with their grain size in both chemical compositions and microscopic textures, according to the wide variation in the burning and nodulization environment in the kiln. The dense interior of large clinker nodules, first formed at high heating rates on and near the surface of the moving raw mix mass, is enriched with K2O and SO3 and made up from coarse-grained components of the raw mix. The alite crystals consist mostly of the M3 phase with inclusions in the core. By contrast, the porous exterior, formed inside the mass at lower heating rates and firing temperatures, is less in K2O and SO3 content and made up from fine-grained components of the raw mix. The alite crystals are generally zoned with M1 occurring in the core. Clinker nodules of medium size, similar in both the chemical composition and the fine textures of alite, are formed concurrently with the exterior of large nodules. Fine clinker nodules come from the core of the mass where the radial motion is stagnant and are formed, due to the large temperature gradient in the mass, at low heating rates and firing temperatures. Dust components comprise, besides small fragments of clinkers, separate alite and belite grains in quantity, indicating that they are separated mostly in the quenching cooler from the porous surface layers of the large clinker nodules. K2O and SO3, as well as the fine textures of alite, are useful as an indicator of the progress of firing and nodulization in the kiln.