Chap. V.
MORTAR AND CEMENTS.
717
CHAP. V.
MORTAR AND CEMENTS.
Calcareous mortars and cements are said to acquire their hardness by the slow absorptionof carbonic acid from the atmosphere, though few mortars that have been submitted to theprocess of analysis have exhibited the quantity of acid necessary for the full saturation ofthe lime, every 28 parts by weight requiring 22 of carbonic acid. This idea was probablysuggested by noticing that the outer surface of a lump of mortar first exhibited the greatesthardness, and then the portion nearest to it, and at last the centre. According to onetheory a chemical affinity exists among the ingredients that compose the mortar, so that thelime acting upon the alumina and sand or silica enters into combination with them ; whileanother supposes that the particles are only affected by mechanical agency, and that thelime adds to their cohesive properties. The minute state of the lime, and its extremedivision, allows it to spread over the entire surface of the particles of sand, bringing themmore closely into contact, or it fills up their interstitial spaces, forming a matrix to holdthem together.
Lime is superior in its adhesive power to that of its cohesive, and therefore attachesitself to hard bodies in preference to its own softer particles. The hydrates of lime andalumina, when powdered and mixed with water, possess great adhesive powers, which is notthe case with anhydrous substances, as carbon and silica: those bodies which hardenquickest have the strongest affinity for water, though some substances which will not hardenin this liquid will in others. Lime-paste slowly dried acquires considerable cohesive pro-perties : rich limes, when mixed with sand or grains of silex to form ordinary mortar,being very soluble in water, remain in a soft state when excluded from the air for a lengthof time; but when a small portion of puzzolana is added in a finely comminuted state,the lime loses its solubility, and in a short time hardens under water, probably occasionedby a chemical combination taking place between the lime and puzzolana.
Hydraulic Mortars are composed of silica and caustic lime in general, and their peculiarproperty may be attributed to their forming a hydrated silicate of lime ; when clay andmagnesia are added, double silicates of greater consistency and strength are produced : thesilica should always be in such a state that it is easily converted into a gelatinous paste bythe addition of an acid, and should be prepared by calcining it w'ith an alkaline earth ata bright red heat, after which it will dissolve in acids. Sand of the quartzose kind, whenmixed with lime in the ordinary way, will not form hydraulic mortar; but if after, whenreduced to fine grains, it be burnt with the lime, it becomes suitable for the purpose ofbuilding in water: those limestones which contain 10 per cent."of clay, when stronglyburnt, form good hydraulic mortars; but if this proportion be increased to double or more,it requires to be well ground before it will set. Marls which contain 30 per cent, of claymake an excellent mortar without adding any other ingredient; when the proportion ofclay is greater, it must not be subjected to any great or prolonged heat; if strongly cal-cined it becomes vitrefied, and requires pounding, or grinding very fine, as well as anaddition of some strong lime to make an hydraulic mortar. When 1 part of silica and4 of caustic potassa are fused together and slowly cooled, a part of the compound may bepoured out of the crucible before the whole has solidified, and pearly crystals are formed inthe residuary portion, composed of 1 atom of silica and 1 of potassa : when 1 part of silicaand of carbonate of potassa are fused together, the carbonic acid is expelled, and abisilicate of potassa is formed ; these silicates are soluble in water ; this solution may bealso obtained by digesting gelatinous hydrate of silica, or very finely divided silica, insolution of potassa.
In Holland a substance called terras or trass has been from time immemorial used as awater cement; it consists ot a substance called Wakke % a species of basalt, and has beenemployed in forming mounds or barriers against tne irruption ot the sea: according toMorveau compact basalt, after burning, made a similar cement to the Dutch terras; and amaterial very nearly resembling it is found in great abundance near the port of Leith, andin the vicinity of Edinburgh .
The Mortar made use of by the Egyptians was formed of sand and lime, nearly in the pro-portions we now adopt: 100 grains taken from the pyramid of Cheops were carefullyanalysed ; after being reduced to a fine powder, dried thoroughly, and immersed in 6 ouncesof pure water for some time, they were heated to remove the soluble salts : when filteredafter this operation the mortar was found to have lost 18£ grains, which consisted of 15’3sulphate of lime, and 3*2 of soda. The residue of 81^ grains had 4 cubic inches of dilute