Text 6. Portland Cement

Portland cement is made by heating an intimate mixture of chalk and clay to a white heat (temperature of incipient fusion) and, after the resultant clinker has cooled, grinding it to extremely fine powder.

Portland cement is usually made by the wet process. The chalk and clay mixed with water are reduced to a creamy consistency in washmills, circular tanks in which a central vertical spindle carries a rotating steel framework to which are suspended heavy harrows with projecting teeth. The circumference is fitted with gratings, and the process conti­nues until the creamy liquid called slurry is able to pass the screen. It passes then to a second and third washmill, fitted with screens of ever smaller mesh, until after passing the last washmill only 5 per cent is retained on a sieve of 32,400 me­shes to the square inch. During this time the chemists are making periodic tests to ensure the correct proportion of lime to chalk and adjusting when necessary. These proportions are vital.

The slurry is then passed to much larger tanks known as mixers, where it is kept stirred by rotating arms with vertical paddles until the kilns are ready to receive it.

The calcining or burning is generally done in rotary kilns. They are lined with firebrick and set at an angle of about 8 degrees to the horizontal.

The slurry is introduced at the top end by a rotating spoon feed, and gradually works its way down the kiln owing to the combined action of the slope and rotation. In so doing it meets the hot flames (the fuel, generally pow­dered coal, is introduced into the lower end) which pass up through the kiln and then to the chimney. In this way the slurry gets hotter as it descends and reaches the zone of maxi­mum temperature (about 2,800 °F) some distance from the lower end, when chemical combination of the constituents takes place; all the water having, of course, long since been driven off.

At this stage the cement has formed itself into extremely hard nodules about the size of walnuts known as clinker, which now drops into a lower but parallel rotating and in­clined tube where it is cooled from a white heat by meeting a current of air. This air is thus heated to about 600 °F and used for blowing with the powdered coal into the kiln, so economising in fuel.

The next process is the grinding of the clinker in tube mills (horizontally rotating cylinders) divided into three or four compartments. Each compartment contains exceptional­ly hard steel balls, which, when the mill rotates and clinker is introduced, are lifted and fall on to the clinker and so crush it. The cement passes from one compartment to an­other, and grinding continues. During grinding about 2 per cent to 3 per cent of gypsum is ground in to make the cement slow setting. The cement then goes to large circular silos, or into sheds where it is stored. The cement then has to be tested.

Vocabulary notes:

an intimate mixture of chalk and clay	однорідна суміш крейди й глини
temperature of incipient fusion	температура початкового плавлення
the resultant clinker	одержаний в результаті клінкер
grind to extremely fine powder	перемелювати на міні­мально тонкий порошок
are reduced to a creamy consistency	доводяться до кремо-подібної консистенції
heavy harrows with projecting teeth	важкі борони із зубцями, що виступають
adjust when necessary	регулювати, коли необхідно
stir by rotating arms with vertical paddles	перемішувати обертовими важелями з вертикальними лопатками
line with firebrick	обкладати вогнетривкою цеглою
a rotating spoon feed	лопать, що обертається й падає
has formed itself into extremely hard nodules	перетворив­ся на виключно важкий конгломерат
to make the cement slow setting	щоб уповільнити його тужавлення