What are some chemical/physical properties that make calcium hazardous or useful? 50 points for this

"Calcium's chemical properties are attributed to its location in group 2 of the periodic table. As an alkaline earth metal, it is a good reducing agent for preparing other metals, such as uranium and thorium. It reacts violently when placed in water, forming calcium hydroxide, more commonly known as lime. This is the reason why water that is high in calcium causes lime buildup on the interior of pipes. Calcium also reacts rapidly when exposed to air. It develops an oxide coating, which prevents the mass of calcium from corroding. If exposed to air at a high temperature, calcium burns to produce nitride. The most common calcium isotope is Calcium-40. It has an ionic radius of 0.099 nm, a standard potential of -2.87 volts and an ionization energy of 589.6 kJ per mole. Calcium compounds are important in a number of industries. Calcium oxide, for example, is used in high intensity lights. Calcium hydroxide is useful for breaking down wood and rocks".
-Reference.com
"The calcium oxide, Ca(OH)2, has many applications in which the hydroxyl ion is necessary. In the process of calcium hydroxide quenching, the volume of blown out lime [Ca(OH)2] expends to double the initial quantity of quick lime (CaO), fact that makes it useful to break down rocks or wood".

"The quick lime is an excellent absorbent for the carbon dioxide, because it produces carbonate, which is very insoluble".

"The calcium silicate, CaSi, prepared in an electric oven from lime, silica and reducing carbonated agents, is useful as a steel-deoxidizing agent. Calcium carbide, CaC2, is produces when heating up a mixture of lime and carbon at 3000ºC in an electric oven and it is an acetylate which produces acetylene by hydrolysis. The acetylene is the base material of a great number of important chemicals for the organic industrial chemistry".

"The pure calcium carbonate occurs in two crystalline forms: calcite, hexagonal shaped, which possesses birrefringent properties, and aragonite, rhombohedric. The natural carbonates are the most abundant calcium minerals. The Iceland spar and the calcite are essentially pure carbonate forms, whilst the marble is impure and much more compact, reason why it can be polished. It’s very demanded as construction material. Although the calcium carbonate is very little soluble in water, it is quite soluble if the water contains dissolved carbon dioxide, for in these solutions it forms bicarbonate when dissolving. This fact explains the cave formation, where the lime stone deposits have been in contact with acid waters".
-LENNTECH

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They make up examples of physical properties