Cement Slurries
Water is added to dry cement to cause hydration and to make a pumpable
slurry. To be used correctly, several properties must be known: the yield per unit (cubic feet per sack), the amount of water required (gallons per sack), and its density (pounds per gallon).
Another important parameter is the cements “absolute volume”. This is the actual volume occupied by the material (the bulk volume includes the open spaces between the cement particles). For example, one sack (94 lbs) of cement has a bulk volume of 1 ft3, but if all the open spaces between the particles were removed, the absolute volume would be 0.478 ft3.
With dry materials (cement and additives), the absolute volume is used along with the water requirements to determine the slurry. For example, the absolute volume of one sack of cement (0.478 ft3) plus the water volume (5.18 gal/sk or 0.693 ft3) yields a slurry volume of 1.171 ft3 (0.478 +
0.693).
The absolute volume of the cement's components are normally found in tables, but may be calculated using:
For components that dissolve in water (sodium chloride, etc.), since they do not occupy as much space as the specific gravities would indicate, the absolute volume is determined from experimental data and placed. Slurry density is also determined. Since one sack of cement weighs 94 lbs, and 0.693 ft3 of water weighs 43.2 lbs, when mixed they yield 137.2 lbs of slurry. The slurry's density is then calculated by dividing slurry weight by slurry volume, 137.2 lbs / 1.171 ft3 equals 117.1 lbs/ft3 (15.7 ppg). Yield is converted to cubic feet per sack by using the constant 7.4805 (62.4
lbs/ft3 / 8.34 lbs/gal).
Fly ash, a synthetic pozzolan, is another major constituent of cements. A fly ash/cement mixture is designated as the ratio of fly ash to cement (expressed as 50:50 or 60:40, etc.) with the total always equaling 100. The first number is the percentage of fly ash (74 lbs/sack), the second number is cement (94 lbs/sack). A sack of fly ash and a sack of cement have the same absolute volume.
If other additives are included (gel, accelerators, retarders, etc.), the mixture is expressed as a percentage of weight of both cement and fly ash. The slurry is then expressed: 50:50:2% gel
