Page 443 - Carrahers_Polymer_Chemistry,_Eighth_Edition

406 Carraher’s Polymer Chemistry

TABLE 12.1
Important Inorganic Polymers
Agate Chabazite Glasses (many kinds) Spodumene
Alumina Chett Graphite Stilbite
Aluminum oxide Chrysotile Imogolite Stishorite
Amphiboles Concrete Kaolinite Sulfur nitride
Anthophylite Cristobalite Mesolite Talc
Arsenic selenide Crocidolite Mica Thomsonite
Arsenic sulfi de Diamond Montmorillonite Tremolite
Asbestos Dickite Muscovite Tridymite
Berlinite Epistilbite Phosphorus oxynitride Valentinite
Beryllium oxide Feldspars Polyphosphates (many) Vermiculite
Boron nitride Flint Quartz Wollastonite
Boron oxides Fuller’s earth Rhodonite Xonotlite
Boron phosphate Garnet Serpentine Ziolites
Calcite Germanium selenide Silicon dioxides (many) Zirconia
Carbon black Gibbsite Silicon carbide

detail, yet its structure and the process whereby it is formed are not completely known. This is due
to at least two factors. First, its three-dimensional arrangement of various atoms has a somewhat
ordered array when a small (molecular level) portion is studied, but as larger portions are viewed,
less order is observed giving only an average overall structure. This arrangement is referred to as
short-range order and long-range disorder and is a good description of many three-dimensional,
somewhat amorphous inorganic and organic polymers. Thus, there exists only an average structure
for the cement that varies with amount of water and other components added, time after applica-
tion (i.e., age of the cement), and source of concrete mix and location (surface or internal). Second,
three-dimensional materials are insoluble in all liquids; therefore, tools of characterization and
identifi cation that require materials to be in solution cannot be employed to assist in the structural

identification of cement.
When anhydrous cement mix is added to water, the silicates react, forming hydrates and calcium
hydroxide. Hardened Portland cement contains about 70% cross-linked calcium silicate hydrate and
20% crystalline calcium hydroxide.

2Ca SiO + 6H O → Ca Si O 3H O + 3Ca(OH) 2 (12.1)
.
3
5
2
7
2
2
3
.
2Ca SiO + 4H O → Ca Si O 3H O + Ca(OH) 2 (12.2)
2
2
4
7
2
3
2
A typical cement paste contains about 60%–75% water by volume and only about 40%–25% sol-
ids. The hardening occurs through at least two major steps (Figure 12.1). First a gelatinous layer is
formed on the surface of the calcium silicate particles. The layer consists mainly of water with some
calcium hydroxide. After about 2 h, the gel layer sprouts fibrillar outgrowths that radiate from each

calcium silicate particle. The fibrillar tentacles increase in number and length, becoming enmeshed

and integrated. The lengthwise growth slows, with the fibrils now joining up sideways, forming stri-
ated sheets that contain tunnels and holes. During this time, calcium ions are washed away from the
solid silicate polymeric structures by water molecules and react further, forming additional calcium
hydroxide. As particular local sites become saturated with calcium hydroxide, calcium hydroxide
itself begins to crystallize, occupying once vacant sites and carrying on the process of interconnect-
ing about and with the silicate “jungle.”
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