Chemical Analysis

The character of the cement materially affects the time of set. Other things being the same, the finer the cement is ground the quicker it sets.

Sulphate of lime (plaster of paris) is usually added to portland cement by the manufacturer to retard the time of set. The addition of 1 or 2 per cent is sufficient to change the time of setting from a few minutes to several hours. Cement which has been made slow setting by the addition of sulphate of lime, usually becomes quick setting again after exposure to the air; and cement which has not had its time of setting changed by the addition of sulphate of lime, usually becomes slower-setting with age. Cement which has become slow-setting by the addition of sulphate of lime will become quick setting if mixed with a solution of carbonate of soda.

Calcium chloride (chloride of lime) in the water used in mixing will affect the time of set, a weak solution accelerating the set and a strong solution retarding it. A 10-per-cent solution will cause ordinary portland cement to set in about one third of the normal time. Ordinary carpenter's glue dissolved in the water will retard the set. Glue equal to 1 per cent of the dry cement about doubles the time of set—both initial and final,—but weakens the mortar about 20 per cent.

The standard tests for activity are usually made on neat cement on account of the interference of the sand grains with the descent of the needle. The rate of setting of neat mortar gives but little indication of what the action may be with sand. Sand increases the time of setting, but very differently for different cements. With some cements a mortar composed of one part cement to three parts sand will require twice as long to set as a neat mortar, while with other cements the time will be eight or ten times as long.

Time of Set.

A few of the quickest natural cements when tested neat with the minimum of water will begin to set in 5 to 10 minutes, and set hard in 15 to 20 minutes; while the majority when tested with the standard quantity of water will begin to set in 20 to 30 minutes and will set hard in 60 to 100 minutes; and a few of the slowest will not begin to set under 60 minutes.

The quickest of the portlands will begin to set in 20 to 40 minutes; but the majority will not begin to set under 60 to 90 minutes, and will not set hard under 5 or 6 hours. The 1887 standard German specifications reject a portland cement which begins to set in less than 30 minutes or which sets hard in less than 3 hours.