January 24, 19 18] 



NATURE 



413 



THE SETTING OF CEMENTS. 



A GENERAL discussion on the setting of cements 

 ■^*- and plasters was held by the Faraday Society on 

 .Monday, January 14, when several interesting papers 

 were contributed. The subject is one of great tech- 

 nical importance, in view of the large and continually 

 increasing appJication of calcareous cements, especially 

 of the Portland class, in engineering and building work. 

 Although this country leads in the manufacture of 

 Portland cement, very little attention has been given 

 to its chemical stud\' by British chemists, and it is not 

 surprising that the most important papers in the dis- 

 cussion came from France and the United States. 



Whilst the setting of plaster of Paris is now agreed 

 to be brought about by the crystallisation of a super- 

 saturated solution, there still exists a difference of 

 opinion as to the mechanism of the process in the case 

 of Portland cement. The recent work of the U.S. 

 Bureau of Standards, as described in a paper by Mr. 

 A. A. Klein, supports the view, originally due to 

 -Michaelis, that the products of hydrolysis are colloidal 

 in nature, and that the desiccation and induration of 

 gelatinous aluminates and silicates, and even of free 

 alumina and silica, are responsible for the mechanical 

 strength of the cement when set. On the other hand, 

 the veteran cement chemist, Prof. Le Chatelier, to 

 whom the first explanation of setting is due, reiterated 

 his opinion that the process is essentially identical with 

 the setting of plaster, the hardness being caused by 

 crystallisation. Incidentally Prof. Le Chatelier 

 offered some criticisms of the tendency, observable in 

 much of the literature dealing with colloids, to use new 

 technical terms as if they afforded an explanation in 

 themselves, whereas they only express known facts in 

 new language. 



Dr. C. H. Desch, who opened the discussion, and 

 Mr. Hatschek pointed out that the difference between 

 the two schools is in great part one of terminology. 

 It is agreed that the particles of the hydration products 

 are usually too small to be distinguished, so that they 

 fall within the region of ultramicroscopic dimensions, 

 and surface forces become comparable with those 

 which bring about the crystalline arrangement. Under 

 such conditions it is almost immaterial whether the 

 particles be described as crystalline or colloidal, espe- 

 cially in view of the work of von Weimarn, who has 

 done much to show the continuity of the passage from 

 one condition to the other with diminishing size of 

 particles. 



The contributions of Prof. Donnan, Dr. Lowry, and 

 Mr. Hemming dealt with the agglomeration and dis- 

 integration of simple salts, and it was shown that 

 these phenomena have a close connection with those 

 of setting. In both cases the greater solubility of un- 

 stable as compared with stable solid phases plays a 

 part. Dr. Rostmhain carried the discussion a step 

 further by comparing the hardening of plasters and 

 salts with the process of solidification of a metal. The 

 solid formed in each case is a crystalline aggregate, 

 which breaks more readily, under ordinary conditions, 

 across the individual crystals than between their boun- 

 daries. This has been attributed to the formation of 

 an amorphous intercrystalline layer, and it is possible 

 that the strength of hydrated plaster may be due, not 

 merely to friction between the interlocking radiating 

 needles of adjacent spherulite?, or to 'their simple adhe- 

 sion, but to the presence of such amorphous material. 

 Portland cement would presumably contain a much 

 higher proportion of the amorphous products. 



An.other group of papers dealt with questions more 

 closely allied to engineering practice, and the discus- 

 sion rendered evident the fact, well known to those 

 w ho have studied the somewhat complex subject of the 



NO. 2517, VOL. 100] 



chemistry of cement, that there are numerous un- 

 solved problems in connection with the setting and 

 hardening processes, some of which bear in the most 

 direct manner on the utility of cement and concrete 

 as structural materials. Mr. Blount spoke of these 

 difficulties from the point of view of the technical 

 chemist, and Mr. Caroe from that of the architect. 

 For the physical chemist some of the most interesting 

 of these problems concern the spontaneous changes of 

 setting time and their acceleration or inhibition in the 

 presence of catalysts. The chemical constitution of 

 Portland cement clinker is now established, thanks to 

 the splendid work of the Geophysical Laboratory in 

 Washington^ a summary of which was given by Mr. 

 Rankin, who was responsible for the investigation. 

 The exact part played by impurities, such as mag- 

 nesia, iron, and alkalis, still remains .to be deter- 

 mined. 



The addition of puzzolanic materials, containing 

 soluble silica, has been practised since ancient times 

 as a means of improving the qualities of lime mortar, 

 and similar additions to Portland cement have been 

 recommended. The work of the Bureau of Standards 

 indicates that the strength after setting should be im- 

 proved by such additions, and the practical question 

 was directly raised in a paper by Messrs. Lew.is and 

 Deny, who showed a marked improvement in the 

 strength of good brands of Portland cement, due to the 

 addition of finely ground blast-furnace slag of suitable 

 composition. The discussion brought out the fact that 

 a difference of opinion exists on this question, although 

 the evidence for improvement is very strong. Blast- 

 furnace slag as a raw material for Portland cement 

 manufacture has received little attention from chemists 

 in this country, although the industry is now becoming 

 an important one, and the utilisation of such a trouble- 

 some w^aste product deserves much closer study. 



Discussions of this kind do a great service in review- 

 ing the field for investigation in the branch of science 

 or industry discussed, and also in bringing together 

 work undertaken from quite independent viewpoints, 

 the relations between which may have been quite un- 

 suspected by the original investigators. Portland 

 cement was an English invention, and this countrx- 

 has always led in its manufacture; it would be of ad- 

 vantage to the industry and to engineering if it were 

 to receive more attention from British chemists than 

 it has hitherto obtained. C. H. D. 



SECONDARY-SCHOOL EXAMINATIONS 

 AND ADVANCED COURSES. 



THE Consultative Committee of the Board of Edu- 

 cation some years ago prepared a report on exam- 

 inations in secondary schools, and this was published by 

 the Board in 191 1. Following the Committee's recom- 

 mendation, the Board of Education invited the English 

 universities to confer with representatives of the Board 

 on the whole subject. These conferences took place 

 during 1913, and in the same year the Board explained 

 the general nature of the proposals it was about to 

 make to representatives of local education authorities 

 and of associations of secondary-school teachers. In 

 Julv, 1914, the Board issued the now well-known Cir- 

 cular 849, on " Examinations in Secondary Schools,'" 

 and invited criticisms from responsible authorities 

 upon the scheme proposed in it. The scheme provides 

 for the annual examination of grant-earning schools 

 in connection with the Board. Two examinations are 

 proposed, and they are to be conducted by one of the 

 recognised university examining bodies. The first ex- 

 amination is to be suitable for forms in which the 

 average age of the pupils ranges from about sixteen 



