Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  


    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 73



Portland Cement is made from materials containing certain proportions of lime, silica, alumina, and oxide of iron.

In England it is made from a mixture of chalk and river mud—the two being washed, ground, dried, calcined and ground. This is known as the "Goreham," or "semi-wet process, and is the one adopted by us.

The old-fashioned method, known as the "wet" process, viz., washing with an excessive quantity of water, pumping into "backs" (large tanks) to precipitate the solid matter, and then draining off the excess of water, is one never adopted by modern manufacturers.

H. K. Bamber, F.I.C., on Portland Cement manufacture, gives the following analyses as being most suitable for cement-making materials, which, compared with our own, appear analogous:— page 8
English. "Maori Brand
Lime (carbonate) 96.50 96.07
Silica 1.15 2.30
Alumina and oxide of iron (Al2 O3 x F2 O3) 0.78 1.15
Magnesia 0.25 nil
Organic matter 1.32 0.48
100.00 100.00
English. "Maori Brand
Silica 54.84 59.25
Alumina and oxide of iron 25.08 24.32
Lime 0.90 6.27
Magnesia 0.80 1.45
Carbonic acid 0.83 2.15
Sulphuric acid 1.20 Organic matter and alkaline salts 6.56
Organic matter and loss 16.35 Organic matter and alkaline salts 6.56
100.00 100.00
The standard English should be approximately:—
Lime 62.00 "Maori" average is 61.50
Silica 23.00 "Maori" average is 25.00
Alumina and iron 12.00 "Maori" average is 11.50

Magnesia is considered by many experts to be a dangerous compound in cement, and its effects were thoroughly investigated after the failure of the Aberdeen breakwater.

Mr. William Smith, M. Inst. C.E., of Aberdeen (who was employed with Prof. Brazier and others), after making elaborate investigations asserted that—"Magnesia in Portland Cement was only weakening when a higher proportion than two per per cent, was present." The same authority states that—"The presence of two to three per cent, of sulphuric acid in Portland Cement did no appreciable harm."

Dr. Michaelis does not consider that a cement containing up to five per cent, of magnesia should be rejected on that account. Whilst admitting the risk of disturbance of bond due to an excess of magnesium salts, he states that cement in which magnesia was present to the extent of 20 per cent, was under his observation for 10 years, and showed no signs of flaw.

The average of many analyses of our cement gives:—
Magnesia, 1.25 Sulphuric acid, 1.45

Having ascertained by careful analyses that the raw materials are suitable, the next step for a cement manufacturer is to adopt the most suitable plant for treating them, and to institute a proper system of regulating their admixture, that a reliable and equable cement may be produced.

page 9

To illustrate this we may describe our process, which will satisfy the most critical that the utmost care and pains are taken to obtain a first-class cement.

(a)The raw materials are regularly tested to decide the proper proportions.
(b)The mixed compound (technically termed "wash"), after thorough washing and grinding, is Daily analysed—thus making a serious error a matter of impossibility.
(c)The fuel used for burning is frequently analysed for sulphur.
(d)The product of the kilns is all hand-picked, the clinkers Only are sent to the mill for grinding. "Half-burnt" is always worked up and re-burnt. This, we contend, is the secret of our production of the "strongest cement in the market."
(e)The grinding is performed by an Askham Patent Pulveriser, capable of reducing three tons of clinker per hour to an impalpable powder. The sifting is effected by Mumford & Moodies' Patent Separators," which ensure a regular and fine production.
(f)The finished cement is daily tested for tensile hydraulicity and constancy of volume; also frequently analysed and the specific gravity estimated. Occasional experiments are made in the form of concrete blocks immersed in salt water, also subjected to the rise and fall of the tide.
(g)The works are under the constant supervision of a competent English cement expert.