BP is currently pumping great quantities of mud into the rupture in the Gulf and plan to dump cement in when the flow permits. Cement has to dry to be effective? So how does it dry under water and the pressures it will encounter at that depth.
Simply put: Cement doesn't 'dry', it hydrates. It's a chemical reaction. the cement combines with water to form crystals that bind it, (and the sand normally mixed in), together. The water becomes part of the 'new rock' formed.
Depends on how picky you are. Some of the water WILL evaporate, so there will be SOME weight loss; just not a lot. Also, because your talking about weight and you didn't specify location, if the dry concrete was on the moon, and the wet cement was on the earth; then the dry concrete WOULD weigh less. If you had specified mass that would be a different story. (I know it's dumb, but it's a good way to screw with your coworker)
it doesn't dry; it sets - it's a chemical reaction; not simply evaporating its water content. Setting time depends on the exact chemical mix of the cement - unsurprisingly, fast-drying cement sets very quickly but is usually comparatively brittle, while slower formulations take longer but tend to be stronger. Considerable heat is given off during the setting process and the cement tends to expand, particularly if too much is added at once so that the heat in the middle can't dissipate quickly enough - this is another reason why slow-setting is preferred in many situations. Setting is also speeded up by spraying with water which not only aids the chemical reaction but also helps in cooling. Under the sea the water would actually assist in the setting process. The main problem is that it will be subjected to the pressure of the escaping oil while setting which will reduce its strength - this is probably why they're also pumping mud into the hole - this will partially clog up the hole and reduce the pressure and so give the cement a better chance of working.
