The Alaska daily empire., 1919-08-26
USES OF PORTLAND CEMENT
 By G. FAeERsoN, Concrete waterp roof Engineer.

Owing to it's great range of adapt
 ability, there is practically no class
 of construction into which cement
 may not enter, and this fact is fully
 verified by thousands of buildings
 bridges, dams, roads, equeducts. sew
 CTS. breakwaters and numerous other
 classes of structures.
 During the earlier days of develop
 ment of the use of Portland cement
 the composition of aggregates was not as we'll understood as it is al present, and even with the definite
 knowledge available on the subject,
 this feature does not receive the care full attention it should in great
 many instances.
 During the last decade, refinements have been made in the manufacture
 of steel and it has been possible to
 establish new calculations for rein
 forced concrete work Refnements
 have also been made in the chemical
 composition of cement, and today
 the manufacture of Portland cement
 may be said to be standardized
 For a considerable number of years after the use of Portland cement had
 become general, definite knowledge
 If the action of the cement in con crete was limited, and the majority
 If the users of cement did not diff
 'erentiate between setting and hard
 oning. with decidedly disastrous re- suits in many cases.
 | Many users of cement demand a quick setting cement when they re ally want a quick hardening cement.
 or a cement that will show the greatest strength values in the short, est length of time. If it were gen
 erally known that cement which shows a quick final set develops far less strength in short periods than q slow setting or normal setting ce- ment, architects, engineers and con tractors would refrain from demand ing quick setting cements. The ac- tion of setting. in itself, does not result in strength to any appreciabie degree, as the strength of the con crete results entirely from hardening. | Relative tn the setting and harden ing of Portland cements, the calsined Portland cements are formed essen tially Of a calsium, silicate, which is he active element of hardening. and
 hat the compound is produced by chemical participation in the midst | )f molten silicate and lime to al
 ow them to combine, but which re-
 mains sensibly neutral during their | hardening.
 The fundamental reaction which
 rings about the hardening is the
 'plitting up of basic calcium sili- cate into mano- calsium slicate and ;alsium hydrate. as
 | While it is difficult to differentiate | between setting and hardening chem | cally, the matter is decidedly impor-
 ant The setting is perhaps only the | 'eginning of hardening; it is charac erized by a progressive diminution n the fiuidity of the mortars; it
 'egins to manifest itself by the per | ;istency of breaks of continuity; the | oids produced artificially in the | ,nder the influence of gravity or 1 'apillarity
 The chemical reactions which ac- company the setting may be different rom those which continue to occur turing hardening, but it is not all
 ways so. The hardening, properly called, is occasioned almost ex- | lusively by the slow and progressive | 1ydration of calsium silicate, which, ,fter having begun during the set | ing, continues entirely alone. While the hardness of hydraulic
 nortars depends on the cohesion of he crystalized compounds and their adherence to each other or to a | forcing inert substance, adhesion de bends mainly on:
 (a) Nature of bodies in contact md relative direction of faces in contact, in case of crystalized bodies (b) The extent of surface in con tact and upon the chemical nature md conditions of surfaces of bodies present
 While the aggregate in concrete l may have all the properties for com bining with the cement to produce strong concrete; yet the presence Of lay or other forcing substances on the surfaces of the stone or grave will prevent the crystals of the ce- ment from coming in intimate con tack with the surface proper and the strength that would result from adhesion will be losta large per ,entage of clay or other substance