It is important to know how long a cement takes to harden and set. This is generally roughly ascertained by the impression of the finger nail upon the cakes of cement, as described on page 172, but as a rule no means are used for ascertaining this in a more accurate manner.
It is extremely difficult to define the time required for the setting of different classes of cements, samples from the same lot may take five minutes or five hours to set, according to its age, temperature, the quantity of water used, etc. As a rough guide, however, the following times for setting may be taken under normal circumstances : -
Quick Setting Cements ....
Slow „ „ .
Very Slow „ „ .
1 Grant M.P.I.C.E., vol. lxii.
Fig. 87 shows an apparatus, invented by M. Vicat, for ascertaining the time of hardening and setting of cements. It is taken from Messrs. Currie's circular, together with the following directions for its use.
"To use this apparatus, gauge 14 oz. of neat cement with the requisite quantity of water, mix quickly into a stiff paste, and with this fill the circular brass mould resting on the glass plate, and which has a height of 4 centimetres and a diameter of 8 centimetres. The moment at which the needle having 1 sq. millimetre section and 300 grammes' weight is not able to penetrate completely to the bottom of the paste, marks the commencement of hardening. The interval from the time of gauging till the beginning of the hardening process is the time the cement should be worked and used, if the strength of the work is to correspond to the quality of the cement. As soon as the paste has become so hard that the needle does not leave an observable impression, it is set, and is the time that should be noted as setting time.
"The same apparatus may be used for ascertaining the correct consistency of cement; only the point of 1 sq. millimetre is replaced by a cylinder of 1 centimetre in diameter. The circular mould is filled as quickly as possible, and the piston immediately let down gently into the paste. The consistency of the paste may be considered correct when the piston sticks at a height of about 6 millimetres from the bottom of the mould. In this manner the exact quantity of water required may be ascertained."