Mineralogy and Chemistry of Cements

Abstract

In the hierarchical materials triangles (see Figure 1.6 ), cements are shown to belong to the group of hydraulic ceramics , also termed chemically bonded ceramic s ( CBC s), to indicate that they obtain their properties not by thermal processes (as do ceramics, sensu strictu ), but rather by their reaction with water. Moreover, the product of this reaction is stable against further reaction with water. This type of hydraulic reaction was fi rst utilized many centuries ago by the Romans, who mixed hydraulic volcanic ash (pozzolan) with lime, sand and crushed rock to yield " opus caementicium , " from which expression the term " cement " was derived. Artifi cial hydraulic " Roman cement " was fi rst developed in Britain by James Parker in 1796. For this, the septarian concretions of organically derived calcite nodules, embedded in marine mudstones, were burned; subsequently, when the burned product was ground to a fi ne powder and mixed with sand, the mixture set hard in about 15 minutes. Earlier observations by John Smeaton, in relation to the construction of the famous Eddystone Lighthouse in the English Channel between 1755 and 1759, revealed that the " hydraulicity " of the then -available hydraulic lime mortars was directly proportional to the clay content of the precursor limestones. Armed with this knowledge, Smeaton selected those mortars for the lighthouse construction that would set and develop a reasonable strength during the 12 -h period between successive high tides.In 1824, Joseph Aspdin patented a hydraulic material, obtained by burning of an intimate mix of chalk and clay, that he called " Portland cement, " on the basis that the set product was similar in appearance to the prestigious Portland limestone. The strength of the mortar made from early Portland cement was rather low, however, owing to the fact that Aspdin had fi red the mixture at a temperature well below 1250 ° C. Such a temperature was too low to obtain alite (Ca 3 SiO 5 ) which, in modern Portland cements, is responsible for early strength (see Figure 5.2 ); instead, the reaction yielded essentially belite ( β -Ca 2 SiO 4 ), which develops its strength more slowly owing to a delayed hydrolysis. The situation was remedied in 1844, simultaneously by Isaac Johnson, Louis Vicat, and Joseph Aspdin ' s son, William, when the high -temperature Portland cement that they produced 119 5 Classic and Advanced Ceramics: From Fundamentals to Applications. Robert B. Heimann