Make a Cement Bowl

http://www.youtube.com/watch?v=xvB-caf_k4U&feature=uploademail

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NOTE in video: Fill one bowl with pure Portland Cement halfway and press a second bowl down until the cement squirts out. Let it set for 24 hours. Add water after setting and firm to harden the cement.

At 2:01 min / 7:29.

I want to make the concrete bowl in the video. I have no experience with concrete, so have questions. You wrote (at 2:01 min):
"Fill one bowl with pure Portland Cement halfway and press a second bowl down until the cement squirts out. Let it set for 24 hours. Add water after setting and firm to harden the cement."
Are you saying to fill a bowl with wet Portland Cement & then_ add more water after 24 hours setting? How much water afterwards? Fill the entire bowl with water to "firm the cement?"

CommentsSurvey 1 second ago

Response:

Hi,
Pure portland cement vs adding sand creates a stone like structure vs cement deck. Builders mix because of cost and also decks should have rough surface for wet no slip etc. The lower the sand to cement mix the higher the strength rating. Bowls are small volume so one 80lb bag can make many. Cement is endothermic when hardening. once the_ molded material is firm (cure in the shade higher humidity) about 36hrs. spray with water. After 72 hours fill with water or sink in water.

GREENPOWERSCIENCE in reply to CommentsSurvey (Show the comment) 6 minutes ago

You wrote: "after 72 hours, fill with water"... how long to keep water in the bowl? When to empty and let concrete bowl dry out?

Response:

Sink in water for 1 hour. Remove and allow to dry for 24 hrs. Then you can place in the sun to dry. The water anneals (sort of annealing usually refers to heat) the cement allowing slower cure time and a better cross linked structure preventing tiny cracks/fissures. Bubble holes can be filled with cement. Wet the bowl first prior to smoothing layer as this provides better adhesion. 24 hours is fine if your mixture was perfect. Most add too much water. Depends on humidity and weather.

GREENPOWERSCIENCE in reply to GREENPOWERSCIENCE (Show the comment) 10 minutes ago