Journal . addition ofpouzzolana to the cement, or the use of calcium silicates(slags, for example), which are inert towards water, butare attacked by carbonic anhydride, are favourable con-ditions. A good way to induce carbonation is to matter, ;/., saw-dust, with the cement, as this isslowly decomposed by the lime with evolution of carbonicanhydride. The Romans used straw in preparing theirfresco surfaces. The part played by magnesium salts requires furtherinvestigation. No hydrated carbonate of calcium is formedabove 5. With regard to testing cements, many of the aboveremark
Journal . addition ofpouzzolana to the cement, or the use of calcium silicates(slags, for example), which are inert towards water, butare attacked by carbonic anhydride, are favourable con-ditions. A good way to induce carbonation is to matter, ;/., saw-dust, with the cement, as this isslowly decomposed by the lime with evolution of carbonicanhydride. The Romans used straw in preparing theirfresco surfaces. The part played by magnesium salts requires furtherinvestigation. No hydrated carbonate of calcium is formedabove 5. With regard to testing cements, many of the aboveremarks are applicable. The cement should not containmore base than is sufficient to saturate all the acids. Ferricoxide must not be regarded as an acid, because ealciuuiferrates slake, and are therefore undesirable ; as examplesof a good cement, setting well, and a bad one which wouldnot set, the author gives the following analytical data,expressed in equivalents, taking the quantity of silicapresent as All the following methods have been found deficieut forascertaining the quality of cements :—Treatment with wateror with ammonium salts ; noting heat evolved on dissolvino-in acid ; examining sections under the microscope ; and theusual mechanical test, even after the cement has beenexposed to hydration three months. Somewhat more satis-factory results have been obtained by placing tue briquettes24 hours after moistening into water at about 100, but notin ebullition as the agitation woidd disintegrate the cements,more especially the slow-setting ones, 48 hours in the hotwater being sufficient to develop most of the effects of ex-pansion due to slaking, and after 7 days the resistance of thebriquette is at its highest ; it is then, to make the operationmore complete, dessicated at 100^ in dry air, inferiorquick-drying cements have been found to lose half theirresistance during this latter operation. The following numbers are from tests made with cylindricalbriquettes, 2 cm.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectchemist, bookyear1882
