554 



NATURE 



[April 12, 



far his statement of the existence of these luxurious pro- 

 fessorships is borne out by fact. Still, we do not wish to 

 quarrel with Lord Randolph's economic mood. We are 

 not concerned to defend every expenditure on buildings 

 or art collections in South Kensington or elsewhere ;.and 

 it is quite possible that, if these matters are looked into, 

 an extravagance in this direction may be proved. In 

 that case, in Lord Randolph's words, " the hon. member 

 for South Manchester may have more to spend than he has 

 at present," or, to express this in non- Parliamentary 

 phraseology, a larger proportion of the present grant may 

 be devoted to the pressing and important requirements of 

 science. 



EXPERIMENTAL RESEARCHES ON 

 HYDRAULIC CEMENTS. 

 Recherches Experimentales sur la Constitution des 

 Moriiers Ilydrauliques. Par M. H. Le Chatelier, 

 Ingdnieur des Mines. (Paris ; Vve. Chas. Dunod, 

 1887.) 



THE large employment of concrete for the construc- 

 tion of harbour-works, for building houses, paving 

 streets, and other purposes, has created a considerable 

 demand of late years for hydraulic cements. Cement 

 manufacture is one of the comparatively new industries 

 which have taken root in Ireland. This treatise of M. 

 Le Chatelier is so valuable an addition to our knowledge 

 of the chemistry of a great and important manufacture, 

 that a short abstract of its contents will be welcome to 

 many of the readers of Nature, especially as the work 

 is of unusual scientific interest. 



The chemical reactions which result in the baking and 

 hardening of plaster of Paris, mortars, and hydraulic 

 cements are treated under the following heads : (i) plaster 

 of Paris ; (2) barium silicates ; (3) hydraulic mortars and 

 cements. 



The first scientific investigation of the baking and 

 setting of plaster of Paris was made by Lavoisier, and 

 the process is thus explained by him. There are tv.io 

 stages at which the water is remov^ed from gypsum ; three- 

 fourths of the water of hydration are much more easily 

 expelled by heat than the last fourth. When gypsum is 

 dehydrated by heat it absorbs water again with avidity, 

 and suddenly becomes a confused and hardened mass of 

 crystals. 



Berthier's observation that plaster of Paris ordinarily 

 contains from 4 to 8 per cent, of water has been con- 

 firmed by Landrin. The baking and dehydrating of 

 gypsum was investigated by M. Le Chatelier by observing 

 the periods measured by a chronograph, during which 

 a thermometer marked successive increments of 5° of 

 temperature when plunged into powdered gypsum heated 

 progressively and regularly in a test-tube standing 

 in a bath of paraffin. The longest periods corre- 

 spond with the greatest heat absorption and dehydrating 

 effect. From 130° to 140° C. the period was 20 minutes 

 40 seconds, between 165° and 180° C. it was 5 minutes. 

 Dehydration is partial at 155°, but complete at 194° C. 

 There are two distinct phases of dehydration of the com- 

 pound CaS04 • 2H2O : the first corresponds to the forma- 

 tion of (CaS04)2- 3H2O, the second with (CaSOJj * HgO ; 

 this is plaster of Paris. The hydration which causes the 



quick setting of plaster can be represented by the equa- 

 tion : (CaSOja ' HgO + 3H2O = 2(CaS04 • 2H2O). 



Cause of the Hardening. — It has been shown that a 

 solution of hydrated calcium sulphate dissolves dehydrated 

 plaster, and this after a short interval separates out as 

 crystals of gypsum. This action explains the setting of 

 plaster ; water hydrates the compound partially, and 

 dissolves the hydrate to saturation ; this dissolves anhydrous 

 sulphate to supersaturation, and deposits it as a hydrate, 

 after which more of the anhydrous salt becomes dissolved. 

 These two opposite actions take place simultaneously at 

 contiguous points. The more rapid the hydration, the 

 greater is the degree of supersaturation, and the quicker 

 the setting of the plaster. Many anhydrous salts harden 

 when in contact with water, as for instance sodium sul- 

 phate, but in every case there is a previous formation of 

 a supersaturated solution. 



It is established that crystallization which accompanies 

 the setting of plaster of various kinds results from the 

 difference in solubility of the compounds which set, and 

 those which are formed during the setting : the first occur 

 in a state of unstable equilibrium in presence of water, 

 and can have only a transitory existence. 



The crystals which form during setting are frequently, 

 if not always, extremely delicate prisms united by one of 

 their ends round central nuclei so as to form little 

 spherical groups. 



The mechanics of setting and hardening can be 

 referred to crystallization. Starting with the idea that 

 the hardening of mortars is not an isolated phenomenon 

 without analogy, and that it is certainly similar to, if not 

 identical with, one or other of the known methods, M. 

 Le Chatelier describes these as follows : — Hardening by 

 cotnpression, of powders ; by desiccation, as with clay or 

 gelatine ; by fusion and solidijication, metals ; by crystal- 

 lization, soluble salts. 



These can be referred to two simpler and more general 

 phenomena ; — 



Mutual adherence of solid particles, produced at a 

 minute distance from each other. 



Mobility of the particles, which admits of their 

 coming together. The momentary solution of a salt 

 which sets affords the necessary mobility of the particles. 

 The setting of mortar evidently enters into the category 

 of phenomena of hardening by solution and crystalliza- 

 tion. When the solid particles have once come together 

 the specific hardness will depend upon the internal 

 cohesion of the crystals and their mutual adhesio?t. 



The cohesion of substances varies within very wide 

 limits, of which the extreme terms amongst substances 

 which enter into the composition of cements are : plaster, 

 which is soft enough to be scratched by the nail, and 

 quartz, hard enough to scratch steel. All we know 

 about cohesion is that it is a primordial property of 

 matter. 



Adhesion, unlike cohesion, is a very complex and 

 consequently a very variable phenomenon. Its variations 

 can almost exclusively serve to explain the considerable 

 differences in resistance which are often a distinguishing 

 property in analogous cements. It varies with the 

 chemical nature of the bodies in contact. The adhesion 

 of a crystal of calcium sulphate to a glass plate is nil; on 

 the contrary, it is so great with barium silicate that the 



