Do you think because they look identical to you that they formed in the same way? That's a heck of an assumption. You are comparing Apple's and oranges. A diamond and synthetic diamond look the same, but they were formed by a very different process.
Limestone caves dissolve at a very slow, consistent rate that can be measured. You simply cannot compare stalagtites that form naturally in a cave with mineral deposits from a farmer's field, or a cement basement. In cement the rock has already been crushed and dissolved, greatly speeding up the process. In a limestone cave the process happens naturally and that process takes time. Barometric dating puts the stalagtites in limestone caves at 190,000 years
From Konceptuals link above:
The fast-growing stalactites form via processes very different from calcium carbonate stalactites found in limestone caves. Limestone is not soluble in water. When carbon dioxide (from decaying plants in the soil above the cave) mixes with water, it forms a very weak carbonic acid. This turns the calcium carbonate into calcium bicarbonate, which dissolves. When drips are exposed to air in the cave, a little carbon dioxide escapes from them into the atmosphere, which reverses the process and precipitates a small amount of calcium carbonate. The upper average rate for limestone stalactite growth is ten centimeters per thousand years, with lower growth rates outside of tropical areas.
