This technical information might not be everyone’s cup of tea but I found it extremely helpful since I had never worked with concrete before. It helped me become familiar with the actual characteristics of concrete and the necessary precautions I should take before making my first sphere.
Concrete ~ cement is the key ingredient in concrete with sand or gravel and water added. Cement is what binds the ingredients and acts as the glue to hold all the components together.
Two most important things to remember when working with cement is the
curing process and the
strength/longevity of your sphere.
Curing: To produce strong concrete that will last 20 years or more, you need to aim for a slow, cool and damp curing process, at least a minimum of a week for the finished sphere. The percentage of water you use for mixing is the main factor affecting the strength and quality of your finished piece. Any cracking you experience will be directly related to water content and shrinkage.
Use only the absolute minimum water/cement ratio necessary - The higher the ratio of the mix, the greater the chance of cracks resulting from the natural reduction in mass that occurs as the water evaporates.
I’ve already given you the most important tip but I’ll stress it again.
Add water slowly to avoid a mix that’s too liquid and continue to add small amounts until your mix is workable. When your mix is the consistency of dry cookie batter I find it’s easier to use a spray bottle to add water to the mix. I then continue to work it in with my hands, much like kneading bread, until it looks like peanut butter.
Strength/Longevity: There are many different types of cement you can use to make your sphere and even hypertufa (cement/peat blend) can be used. However, it's recommended to add alkali resistant fibers and/or a liquid bonding agent for added strength. Alkali resistant fibers do not
increase strength as most people think. They were originally engineered to minimize early cracking but research has proven that higher end strength is indeed a by-product and these fibers are engineered to take advantage of this.
Any Portland cement product can be used. However, all of my research led me to believe that
QuikWall Surface Bonding Cement was the perfect choice. This Portland cement based formula includes the alkali resistant glass fibers which were created specifically for
vertical applications. It’s a little more expensive but it’s worth it. It’s the perfect solution because the ingredients provide everything you need so you don’t have to add sand, bonding agents or fibers for a stronger bond. An added bonus, the smaller bags of Quikwall are easier to lift and much lighter than 80-90 lbs bags of Portland cement.
TIP: Take heavy duty trash bags with you and ask the sales personnel to cut the bag in half. They’re usually more than willing to help you lift the bags into your car but when you get home there’s no one there to help with the heavy bags. The lighter weight 'half' bags will be much easier for you to carry. 
FibaTape for Cement Board]/b]
FibaTape is a fiberglass mesh tape which is alkali resistant. The heavy fiberglass yarn used in this [b]specific fiberglass mesh increases strength and structural integrity and the alkali resistance enhances long-term performance in any cement application. There are other products available but unless they include
“AR” on the label they are
not alkali resistant and will eventually deteriorate. I had difficulty locating this specific product locally. FYI, Lowes does not carry FibaTape products. Home Depot does sell these products and FibaTape for Cement Board is the
best mesh tape to use for any cement applications.
You may find other information online for making hollow concrete spheres with instructions to use wire mesh versus fiberglass mesh tape. Poultry mesh or hardware cloth which is used to add strength in concrete stepping stones is often recommended. Take it from me, the wire mesh is almost impossible to form around the ball and the mesh tape is just as strong and so much easier!
A few more TIPS:
You'll find it much easier if you start applying the cement patties around the middle of ball versus the top. Working "up" the ball helps to prevent the cement from slumping (sliding) down the ball. Once you've finished the top half you can work from the middle down to the edge and the cement you're applying will have something to grab on to.
You'll find that a spray bottle of water will become your best friend. You can use it to spray the cement in the bowl to keep it moist if it begins to stiffen. You can also spray the ball itself to prevent the cement from drying out too fast as you continue to apply cement patties.
In addition to the spray bottle, I've also found it a tremendous help to keep a small bowl of water handy. Sometimes it's much easier to just dip your gloved fingers in the bowl to wet them instead of grabbing a spray bottle.
I think that's everything! If you're interested, I did post photos of how I created the pedestal and shelf for my sphere.
http://s777.photobucket.com/al... This was my first attempt at making a pedestal and I think I might take a different approach on any future attempts. The pedestal is not exactly
light, I think it weighs about 60 lbs.
Let me know if you have any questions!
