Your low cement papercrete should hold a screw well. The key is to use a screw that will hold the weight you intend to put on the shelf.
Some nice long lag screws should hold into the papercrete beautifully and support substantial weight.
You may want to predrill a hold through your plaster layer to prevent it from cracking or chipping chunks away. That may not be needed depending upon the characteristics of your plaster. I'm not sure.
If in doubt... you probably have some scrap papercrete blocks laying around. Try test screwing/bolting something to those blocks to make sure your system works before trying it on your finished wall.
You can always paint the bolt/screw heads to get them to match the shelf brackets.
--- In papercreters@yahoogroups.com, JUDITH WILLIAMS <williams_judith@...> wrote:
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>
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> I did really well in getting the cement in my mix down to 1/2 bag per 200 gallon mix. Now that I am ready to hang a shelf I am worried it might be too heavy for the wall. The screws may pull out. I have been thinking of all sorts of different ways to make sure the shelf doesn't fall down. I even looked at some of those fancy fasteners but don't know what to buy. There are some that have a plastic part the goes into the hole first and some that have little wing thingies that stick out if you put them into drywall. I also thought I would glue a decorative piece of punched tin to the wall behind where the brackets will go. I just don't know, but I want that shelf up there soon. It's a 2x12 12 feet long to span an entire wall. I have 4 beautiful cast iron brackets to hold it up.
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> Follow progress on the new project at http://www.papercretebyjudith.com/blog
>
> More papercrete info at http://squidoo.com/papercretebyjudith
>
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