THIS CEMENT AGE

Waipa Post, Volume 45, Issue 3210, 30 July 1932, Page 2 (Supplement)

THIS CEMENT AGE

THE ORIGIN AND GROWTH OF NEW STONE WORKING PROCESSES.

This present age, far removed as it is from the so-called " stone age," makes increasing use of all sorts of scone-like materials. Foremost among these is cement, or more correctly, that mixture of sand, stone and cement which we call concrete.

One striking feature about cement is that it enables us to build structures of almost any shape and size in one block. We can build small or moderately sized articles of clay or plaster in one piece, but cement enables us to carry out our designs on a scale vastly greater and more intricate. One could apply the term plasto-lithic to the present age. Ordinary cement, the so-called Portland cement, has a curious origin. It might almost be called a marine produce, and its discovery was almost accidental. Smeaton who was entrusted with the buJding of the Eddystone lighthouse about 1756, was a fine examplar of thoroughness. He recognised that he required for his structure the very best class of mortar then available, and also data on which to rely for the selection of the best material for the making of such a niortar. In those days lime mortars only were used, so Smeaton studied the subject, and made himself proficient in analytical chemistry. He was thus able to analyse all the best lines then known,, and the limestorf; from which they were made, and so rely confidently on his own data. / VALUABLE " REFUSE." The limestone which he finally selected was one containing a decided proportion of clay, and it happened occasionally when this limestone was " burnt," i.e., in the process of lime making, that semi-fused nodules were found in the resulting lime. Before the lime was actually used these nodules were sieved out, and thrown aside as useless. One inquisitive person, however, took it into his head to grind up some of this "refuse," and to test its capability for making mortar. It was found to be superior to good lime, setting more quickly and harder. Hence arose our modern cement industry. When set, the greycoloured, close-grained material remained Aspidin, one of the first man-

ufacturers of Portland building ston P ; hence the name Portland cement. The widespread uses of cement to make concrete for buildings, bridges, dams, and so on, emanates from the reliance which can be placed on the proper use of good material. The proper use of the material is mainly the business of the builder. The goodness of the material,, assuming the competency of the cement manufacturer, is the outcome of many yeais of intense study and research, and although cement is to be classed as a " crude " chemical yet its manufacture is rigorously controlled by chemical and physical tests. Its extensive use is a testimony to the combined effort of engineer and chemist. SCIENTIFIC STUDY. The scientific study of cement presents a type of problem which is met with also in the study of such substances as glass, ceramic ware, rocks and alloys. It is the problem of the constitution of matter in the solid state —not the ultimate analysis, but the grouping of the elements present, the relative number and size -of the groups, and their special relationships. The value of so many of the solid materials we use depends not so much on what they are as on how they behave; a knowledge of their chemical composition is of less actual importance than that of their minute structure, since this latter determines largely their behaviour. It might be asked, "Why worry about all these scientific data when it is possible without them to make a satisfactory cement? " Fortunately for the public, the business man and the scientist have too much pride and inquisitiveness to accede to such a laissez fair doctrine of the " that'll do " type. Both need to know not only what should be done, but also what should not, or need not, be done, and, further, what might be done to give a better and cheaper product. Cement}, and, of course, c oncrete has one shortcoming common to all stone-like materials. It is inelastic and does not withstand well a pulling strain. Its tensile strength is poor. Sparklets and selzogenes for example, have to be bound on the outside with a metal webbing. Fortunately, by imbedding bars of steel in concrete we can combine the crushing strength j of the steel, and the two components j show no antipathetic behaviom*. j Hence, nowadays, most large concrete structures are made with "reinforced" j concrete,