Additives


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Additives

 

(in construction materials), natural or synthetic materials that are mixed into binding agents, concretes, and mortars to impart the required properties to the finished product and reduce its cost. Additives are classified in a number of groups depending on their purpose and characteristics.

Active mineral additives. Active mineral additives are finely ground materials that contain hydrated silica or metakaolinite in a chemically active form. When water is added to a mixture of these materials with lime or portland cement, they react chemically with the latter (at standard temperatures), with the formation of poorly soluble hydrosilicates and other calcium compounds. As a result, the lime acquires the ability to harden in water, and the cement takes on increased strength in fresh and sulfate-containing waters. Active mineral additives may be subdivided into natural materials (diatomites, tripoli, gaizes, and volcanic ash, tuffs, and trasses) and synthetic materials (granulated blast-furnace slags, boiler slags and ash, nepheline slurry, rubble, and argillite). They are used extensively in the production of cement and lime-pozzolana binding agents, as well as for increasing the strength, water resistance, and salt resistance of concretes and mortars.

Filler additives. Filler additives are finely ground limestones, dolomites, sands, low-active slags, and ash. They are introduced to replace large particles of clinker and lime that are not completely hydrated during the hardening process. Such replacement, which does not essentially affect the strength of the binding agents, reduces their shrinkage strains and lowers costs. Filler additives are used in the production of sand, carbonate, and masonry cements, as well as for reducing the expenditure of binding agents and increasing the strength of concretes and mortars.

Acid-resistant, alkali-resistant, and heat-resistant additives. Additives for acid-resistant, alkali-resistant, and heat-resistant concretes and mortars are finely ground inorganic materials that are characterized by high resistance to the action of acids, alkalies, or high temperatures. In playing the role of microscopic fillers, they promote an increase in the resistance of concretes and mortars to aggressive actions. Acid-resistant additives include andesite, basalt, diabase, granite, and quartz, as well as porcelain and stone casting; alkali-resistant additives include compact limestones, dolomite, magnesite, calcareous sandstone, and varieties of these rocks. Finely ground chromite, magnesite, fire clay, crushed material from semiacidic refractory products, and metallurgical magnesite are used as additives for heat-resistant concretes and mortars.

Surface-active additives. Surface-active additives are organic substances that can be adsorbed into particles of cement and can change their surface properties. They are subdivided into plasticizers (concentrates of sulfite-nucleic braggite), which increase the wettability of cement dust by water, and hydrophobic plasticizers (saponite-naphtha, asidol, and organosilicon liquids), which decrease wettability. Both types of these additives increase the convenience of laying concrete and mortar mixes, reduce the expenditure of cement, and increase the frost and corrosion resistance of concretes. Surface-active additives are used to produce plasticized and hydrophobic portland cements, as well as to reduce water requirements of the mixes and to increase the strength of concretes in aggressive mediums.

Foaming and blowing additives. Foaming and blowing additives are materials used in making cellular concretes (foam concretes and gas concretes). Foaming agents are organic surface-active substances whose aqueous solutions form foam when agitated. To increase the staying power of the foam, stabilizers (animal glue or water glass) are usually added to the foaming agent. Blowing additives are substances that as a result of chemical reactions occurring directly in the binding agent paste are capable of giving off gases, which form tiny pores by swelling the mass of paste. Glue-rosin, tarsaponite, and alumosulfonaphthene materials are used to make foam concretes; aluminum powder and industrial Perhydrol are used to make gas concrete.

Additives to accelerate or retard setting and hardening. Additives to accelerate or retard the setting and hardening of binding agents are substances that affect the speed of hydration of the binding agents. Calcium chloride and sodium chloride, as well as sodium sulfate and potash, are used to accelerate the setting and hardening of portland cement and its varieties. Keratin and lime-glue retarders are used to retard the setting of semihydrous gypsum plaster.

Cold-resistant additives. Cold-resistant additives include sodium chloride, calcium chloride, sodium nitrate, and potash. They are added to lower the freezing point of the water in concrete or mortar mix and to ensure their hardening at a temperature below 0°C.

REFERENCES

Volzhenskii, A. V., Iu. S. Burov, and V. S. Kolokol’nikov. Mineral’nye viazhushchie veshchestva. Moscow, 1966.
Stroitel’nye normy i pravila, part 1, sec. C, chap. 2: “Viazhushchie materialy neorganicheskie i dobavki dlia betonov i rastvorov.’ Moscow, 1969.

IU. S. BUROV

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