It removes drilling fluids.

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Note added at 2 hrs (2015-04-20 22:31:34 GMT)
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Compare:
Klocek cementowy
a) zapobiega mieszaniu się płuczki z cementem .
b) ułatwia przetłaczanie cementu w rurociągach tłoczących.
c) służy do dobrego wymieszania cementu.
d) służy do rozdziału fazy stałej z cementu.

Elsewhere in the document we ONLY have

Rys. 18. Klocki cementacyjne do cementowania jednostopniowego, a) klocek górny, b) klocek dolny [2, s. 291]

http://naftowka.pl/zsp4/media/pomoce/wiertnik/konstrukcja_ce...

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klocek cementacyjny - wiert. klocek wykonany z gumy lub innego materiału, oddzielający zaczyn cementowy od płuczki wiertniczej podczas cementowania.
http://old.teberia.pl/encyklopedia.php?a=ArtShow&ArtId=5832

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cementing plug
A rubber plug used to separate the cement slurry from other fluids, reducing contamination and maintaining predictable slurry performance. Two types of cementing plug are typically used on a cementing operation. The bottom plug is launched ahead of the cement slurry to minimize contamination by fluids inside the casing prior to cementing. A diaphragm in the plug body ruptures to allow the cement slurry to pass through after the plug reaches the landing collar. The top plug has a solid body that provides positive indication of contact with the landing collar and bottom plug through an increase in pump pressure.
Synonyms: wiper plug
http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=te...

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Note added at 3 hrs (2015-04-20 23:48:46 GMT)
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Cementing plugs

Cementing plugs are highly recommended to separate drilling fluid, cement, and displacing fluid. Unless a well is drilled with air or gas, the casing and hole are usually filled with drilling fluid before cementing. To minimize contamination of the interface between the mud and the cement in the casing, a bottom plug is pumped ahead of the cement slurry. This plug wipes the mud from the casing inner diameter (ID) as it moves down the pipe. When it reaches the float collar, differential pressure ruptures a diaphragm on top of the plug, allowing the cement slurry to flow through the plug and the floating equipment and up the annular space between the pipe and the hole (Fig. 5). The top cementing plug, pumped behind the cement slurry, is pumped to a shutoff on the float collar, causing a pressure increase at the surface. This is a signal that the cement has been displaced. Top and bottom plugs are similar in outward appearance, but are always different colors. The top plug (black) has a solid insert with rubber wipers molded to the insert. The bottom plug (red, orange, and yellow) has a cylinder-type insert with molded wipers and a plastic or molded rubber diaphragm designed to rupture at 200 to 400 psi. Inserts are manufactured of plastic or aluminum. Aluminum inserts increase the strength and temperature ratings of the cementing plug, and should be used when the Bottom Hole Circulating Temperature (BHCT) exceeds 300°F, and should be drilled out with conventional tricone rock bits. The recommended landing pressures for aluminum-insert plugs vary, depending on casing size, but are normally higher than the recommended landing pressures for wiper plugs with plastic inserts. Plastic-insert plugs can be used in wells with a BHCT below 300°F and can be drilled out with tri-cone rock or polycrystalline-diamond compact (PDC) bits.


Fig. 5—Five-wiper (left) and nonrotating (right) cementing plugs (courtesy of Halliburton).

http://petrowiki.org/Cementing_hardware#Cementing_plugs