Also have a look into silica;
"Silica is more prevalent than calcium in bone. This is because it also makes up the collagen matrix of bone which is actually 80% of total bone mass. A recent study of the effectiveness of calcium supplementation in preventing bone fracture revealed that calcium alone was not effective. The study involved 35,000 middle aged and older women and is the largest study to date examining the effects of calcium and vitamin D on bone health. While previous studies investigating calcium supplementation have produced mixed results, this study quite convincingly concluded that calcium did not produce the expected results. Calcium supplementation provided no protection from fractures or colorectal cancer, but did increase the risk of kidney stones. Had the researchers looked at silica or a combination of silica and calcium, the results would have been dramatically different. Bones, like cement, require a compliment of minerals, the most important being silica followed by calcium, magnesium, and phosphorus.
Concrete is the blend of three basic components: cement, sand or gravel, and water. When these components are mixed together in the right proportions, a substance as hard as rock is created that can be shaped into any form imaginable. Any one or even a combination of two of these components is useless because it takes all three to perfect the process.
The first ingredient, cement is made up of a number of minerals including calcium oxide (limestone)-65%, silicon oxide-25%, aluminum oxide-5%, ferro oxide (iron)-5%, and a touch of sulfate. Does this sound familiar? These elements are also found in bone. The
second ingredient is sand and or gravel which is generally composed of silicates. Lastly, there is water. Mix together two parts cement, with one part sand and one part gravel, add water and you have concrete.
What is very interesting here is the fact that while cement is made of primarily calcium, it will not harden into strong concrete without silica. It will harden, but it is very brittle, just like osteoporotic bone. Silica plays a small role in the ingredients of cement, and a larger role as part of the second ingredient of concrete, sand and gravel. In fact, the amount of silica in concrete is about equal to the amount of calcium.