Building Materials, Third Edition

ƒ™—2g



2—2g

2g™

 RTU

Soluble glass is melted from quartz sand, ground and thoroughly mixed with soda ash,
sodium sulphate or potassium carbonate in glass tanks at a temperature between 1300 and
1400°C. Melting takes from 7 to 10 hours. The resultant glass mass flows from the furnace into
cars where it cools rapidly and breaks up into pieces called “silicate lumps”. This glass is
soluble in water under normal conditions, but when exposed to the action of high-pressure
steam (5–6 atm at about 150°C), it fairly readily becomes liquid.
Soluble glass hardens in the air because atmospheric carbon dioxide causes amorphous
silica SiO 2 .2H 2 O[Si(OH) 4 ] to settle out and dry according to the reaction

Na 2 SiO 3 + CO 2 +2H 2 O
Si(OH) 4 + Na 2 CO 3
But this process is very slow in the air. Hardening of soluble glass is accelerated by adding
a catalyzer sodium fluosilicate (Na 2 SiF 6 ) which interacts rapidly with soluble glass to produce
a silicate gel.

Na 2 SiF 6 + 2Na 2 SiO 3 +6H 2 O
6NaF + 3 Si (OH) 4
The introduction of sodium fluosilicate not only accelerates hardening, but also enhances
the water and acid-resistant properties of cement.
The acid resistant cement should not be used in constructions subjected to the action of
water, alkalis and phosphoric, hydrofluoric or fluosilicic acids for long periods of time. To
enhance the water resisting property of cement, 0.5% of linseed oil or 2% of ceresit are added
to it. The hydrophobic cement thus obtained is known as the acid and water-resistant cement.

… Acid-resistant cements are used for lining chemical apparatus and for building towers,
tanks and other installations for chemical industry. Soluble glass is also used for preparing
acid-resistant and heat-resistant coatings.

PHQ iˆ€exhsxq2giwix„ƒ

This group of binding material encompasses cements that show a slight increase in volume in
the course of hardening under moist conditions, or that do not shrink when hardened in the air.
Two kinds of expanding cements are available the water-impermeable and gypsum-aluminous
varieties.

‡—2s —˜2i
—2gis a quick-setting and quick hardening hydraulic
cementing material obtained by grinding or mixing finely ground aluminous cement, gypsum
and high basicity calcium aluminate in a ball mill. Calcium aluminate of high basicity
(4CaO.Al 2 O 3 .12H 2 O) is obtained by hydrothermal curing over a period of 5 to 6 hours at a
temperature between 120 to 150°C of a 1:1 mixture of aluminous cement and lime, mixed with
30% of water. The product is dried and ground. Setting begins not earlier than in 4 min and
ends not later than in 10 min after water is added. Setting may be retarded by the addition of
such ingredients as acetic acid and borax. Compressive strength of specimens from cement
paste should not be lower than: in 6 hours, 7.5N/mm^2 in 3 days, 30N/mm^2 ; in 28 days,
50N/mm^2. After one day of hardening, the specimens should be fully impermeable to water
applied under pressure of 6 atm. Linear expansion after one day of hardening should not be
less than 0.2 and not more than 1%.