Testing of Concrete in Structures: Fourth Edition

Chemical testing and allied techniques 265obtained. The operations should also be performed as quickly as possibleto minimize exposure to atmospheric carbon dioxide.The portion is first crushed to pass a 5.0 mm sieve and a sub-sample of500–1000 g obtained, which is then crushed to pass a 2.63 mm sieve andquartered to give a sample which is ground to pass a 600 m sieve. Thisis also quartered and further ground to pass the 150 m sieve. Where amechanical means of sample preparation is employed, it is possible to omitsome of these steps and go directly from a 5.0 mm sample to a samplepassing 150 m.9.2.2.2 Determination of calcium oxide (CaO) content aloneA portion of the prepared analytical sample weighing 5 ± 0005 g is treatedwith boiling dilute hydrochloric acid. Triethanolamine, sodium hydroxideand calcein indicator are added to the filtered solution, which is then titratedagainst a standard EDTA solution. The CaO content may be calculatedto the nearest 0.1% in this way. Atomic absorption spectrophotometricmethods are also often used.Reliance upon measurements of CaO content alone may be consideredacceptable if the calcium oxide content of the aggregate is less than0.5%, but additional determination of soluble silica content is recommended.It should be noted that the calcium in the concrete is not inthe form of calcium oxide, CaO, but it is reported as being CaO byconvention.9.2.2.3 Determination of soluble silica, calcium oxideand insoluble residueSoluble silica is extracted from a 5±0005 g portion of the prepared sampleby treatment with hydrochloric acid, followed by sodium carbonate solutionand the insoluble residue collected by filtering. The filtrate is reduced byevaporation and treated with hydrochloric acid and polyethylene oxidebefore again being filtered and diluted to provide a stock solution.The filter paper containing the precipitate produced at this last stage isignited in a weighed platinum crucible at 1200 ± 50 C until constant massis achieved, before cooling and weighing. The soluble silica content can becalculated to the nearest 0.1% from the ratio of the mass of the ignitedresidue to that of the analytical sample. It is also possible to measure thesoluble silica content directly on the stock solution extract, using a nitrousoxide/acetylene flame and atomic absorption spectrophotometric methods.This can be considerably faster.The calcium oxide content is determined from the stock solution usingprocedures similar to those in Section 9.2.2.2. The insoluble residue isdetermined from the material retained during the initial filtration process