352 SECTIONAL TRANSACTIONS.— B. 



real existence. For various reasons hemihydrate is probably 3CaS04 . 2H2O, 

 which it is proposed to call sub-hydrate. From a study of the solubility 

 curves, sub-hydrate belov^r 98° C. and anhydrite below 38° C. can take up 

 water to form gypsum, and both should be capable of functioning as 

 plasters at normal temperatures. Setting is a simple recrystallisation 

 process, and it is not necessary to assume colloidal phenomena. The setting 

 of sub-hydrate plasters in mixes of plastering consistency follows the law 



-J- = Kx{a — x). In dilute slurries, accelerated anhydrite plasters follow 

 at 



the law TT = K{a — x). In mixes of plastering consistency, anhydrite 



plasters follow no simple law. 



Retarders fall into two classes. One appears to work through adsorption 

 on crystal faces. The other modifies the solubility relationships. In 

 retarders, the anion is apparently the deciding factor. 



Accelerators actually speed up the crystal growth of gypsum, although 

 the mechanism of this action is obscure. There is a very rough analogy 

 with the coagulation of sols in colloidal phenomena. It is possible that 

 accelerators function through the removal or discharge of a protective layer 

 on a growing crystal. 



Mr. F. H. Clews, Mr. H. H. Macey and Dr. G. R. Rig^y .—Some 

 important properties of clay (11.30). 



Problems arise in the manufacture of clay products from the nature and 

 properties of the clay itself. Some of these problems are reviewed in the 

 order of stages of treatment of the clay. 



The natural deposits fall into different types and are used for different 

 industrial purposes. Many of the essential minerals have been identified ; 

 investigations have been made into their atomic structure, and some have 

 been synthesised. 



The plasticity of clay is important. Reference is made to problems of 

 measurement of plasticity and of the de-airing of plastic clay masses. 



Clay shrinks on drying and strains are then inevitably produced. Problems 

 therefore arise associated with the maximum safe rate of drying of masses 

 of different shapes and dimensions. The mechanism of drying is discussed 

 with special reference to recent work on the permeability of plastic clay. 



The action of heat on clay during the firing of products brings about 

 thermal decomposition of the clay minerals and other chemical changes, as 

 for example, changes in soluble salt content, of interest in building products 

 manufacture. Physical changes, such as changes in volume, specific 

 gravity, colour, amount and character of pores, also occur and are technically 

 important. 



Dr. D. G. R. BoNNELL. — Some problems connected with porous building 

 materials (12.10). 



The fact that many building materials are porous bodies gives rise to 

 a number of problems which have to be studied if the materials are to be 

 used to the best advantage by the building industry. 



A brief summary is given of the behaviour of water in capillary spaces, 

 and the relation between such behaviour and the properties of building 

 materials is illustrated by : 



{a) The influence of the pore structure on the frost resistance. In this 

 connection mention is made of the important factors which must be 



