Clinker petrography has long been used in the examination of clinker materials. ASTM 1356M [2] provides guidance in phase abundance analysis using a microscopical point-count procedure. The original certificate phase abundance estimates were performed using a procedure similar to that of ASTM 1356M while the round robin data were collected following that ASTM standard test method. The advantages of optical microscopy (OM) over chemical analyses are that it provides direct examination of the clinker, characterization of both phase composition and texture, and a rapid, relatively simple method for estimating the phase volume fraction via point count analysis. Procedures for preparation and examination of clinker may be found in Campbell [3].