The silicates were introduced in Chapter 2. There we learned that the mineral quartz is composed of the chemical silica, SiO2. In Chapter 5 we learned that the aluminosilicate minerals contain silica and alumina, Al2O2. Chapter 9 introduced a variety of metal oxides and sulfides. In this chapter we discuss chemicals derived from calcium sulfate and calcium carbonate.

Three calcium sulfate minerals are pertinent to our discussion. The mineral anhydrite is, as its name implies, the anhydrous form of calcium sulfate, CaSO4. Bassanite is a hydrous form, CaSO4 H2O, or calcium sulfate hemihydrate. As with cellulose or kaolinite, the "H2O" in the empirical formula doesn't mean that bassanite is wet. No, the water is part of the formula, but the material itself is bone-dry. We might just as well write the formula "CaH2SO5" but "CaSO4 H2O" is a convenient way to write it for comparison with the other calcium sulfates. The third mineral form of calcium sulfate, gypsum, is another hydrate, CaSO42 H2O, or calcium sulfate dihydrate. It may seem like picking nits to have three different calcium sulfates but they have different crystal structures and different properties. It is easy, however, to convert from one to the other, as shown in Equation 10-1. Heating gypsum to 128C (262F) converts it to bassanite, more commonly known as plaster of Paris, or plaster, for short. Heating plaster to 163C (325F) converts it to anhydrite. You can add water to anhydrite to convert it back to plaster, but this conversion is slow. By contrast plaster absorbs liquid water to become gypsum quite rapidly. The growth of gypsum crystals is what causes plaster to "set" when water is added. Gypsum is the "porous stone" described by Vitruvius for making stucco.

Equation 10-1. From Gypsum to Plaster and Back Again

The "hard" stone mentioned by Vitruvius is limestone, a sedimentary rock composed of calcium carbonate. Chalk and marble are other rocks derived from calcium carbonate. Sea-shells are also composed primarily of calcium carbonate. When calcium carbonate crystallizes, it does so as the minerals calcite and aragonite, both with formula CaCO3. Dolomitic limestone is derived from the mineral dolomite, CaCO3MgCO3, with chemistry similar to that of limestone itself.

Calcium carbonate is not soluble in water; if you add water to limestone, you just get wet limestone. But if you burn limestone, if you heat the bejeezus out of it, it turns into quicklime, calcium oxide. When you add water, that is, when you slake it, it gets hot and turns into calcium hydroxide, or slaked lime. Figure 10-1 shows the whole lime-making process as a schematic. The first reactor, the furnace, should be familiar from Figure 1-3. It's just a container where gas can come off of a solid that's having the bejeezus heated out of it. The second reactor, the slaker, is a container where water is added to a solid. As usual, reactants enter from the left, waste products exit to the top and bottom, and the main product exits to the right of the figure. Calcium hydroxide is sparingly soluble in water and when mixed with sand forms a dandy mortar. You might think that rain would wash the slaked lime right out of the mortar. That's where carbon dioxide comes in.

Figure 10-1. Lime-Making as a Process

Carbon dioxide is a weak acid. When it dissolves in water it forms hydrogen carbonate, H2CO3, also known as carbonic acid. Hydrogen carbonate reacts with calcium hydroxide in a classic metathesis reaction to produce calcium carbonate and hydrogen hydroxide, as shown in Equation 10-2. Calcium carbonate is just limestone and hydrogen hydroxide is just water. In other words, when carbon dioxide reacts with slaked lime it turns back into limestone. Thus the lime in mortar gradually turns to limestone, cementing the silica in the sand together to form a material which is quite impervious to the elements. That's just what happens when a calcium-rich sea dries up; it absorbs carbon dioxide from the air and deposits a layer of limestone or chalk. So the limestone in our mortar has come full-circle, when you think about it.

Equation 10-2. From Lime Back to Limestone

WarningMaterial Safety

Locate MSDS's for limestone (CAS 471-34-1), lime (CAS 1305-78-8), slaked lime (CAS 1305-62-0), gypsum (CAS 10101-41-4), and silica (CAS 14808-60-7). Summarize the hazardous properties of these materials in your notebook, including the identity of the company which produced each MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion. Also include the LD50 (oral, rat) for each of these materials.[1]

Your most likely exposure is dust inhalation. If a persistent cough develops, see a doctor. Lime is caustic, which means it eats skin; in case of skin contact, wash the affected area with plenty of cold water. Be aware that lime gets hot when it gets wet.

You should wear safety glasses and a dust mask while working on this project. Leftover materials may be disposed of in the trash. Lime should be slaked before disposal.

NoteResearch and Development

Well, I guess if you are in a class or something, you might want to know what will be on the quiz.

  • You better know all the words that are important enough to be indexified and glossarated.

  • You better know the Research and Development stuff from Chapter 5 and Chapter 6.

  • You ought to be able to recognize calcite, limestone, and gypsum, either from photographs or from samples. You should also know that sea-shells are made of calcium carbonate.

  • Know the formulas for limestone, lime, gypsum, and plaster and all the equations in this chapter.

  • Know all the hazards of working with limestone, lime, gypsum, plaster, and silica and what to do if things get out of hand.

  • Know that lime is an alkali, what it tastes like, whether it's pH is high or low and what color it turns pH test paper.

  • Know what water of hydration means, and that just because a formula has H2O in it doesn't mean that it literally has liquid water in it.



The LD50 was introduced in Section 7.2.