POLITECHNIKA WARSZAWSKA
WYDZIAŁ INśYNIERII LĄDOWEJ
KATEDRA INśYNIERII
MATERIAŁÓW BUDOWLANYCH
WARSAW UNIVERSITY OF TECHNOLOGY
FACULTY OF CIVIL ENGINEERING
DIVISION OF BUILDING MATERIALS
ENGINEERING
al. Armii Ludowej 16, p. 551, 00-637 Warszawa, POLAND; tel.: (+48 22) 825-76-37, fax: (+48 22) 825-75-47, e-mail:[email protected]
Lech Czarnecki
Paweł Łukowski
Andrzej Garbacz
Bogumiła Chmielewska
Building chemistry – laboratory exercises
collaborative work under the chairmanship of Lech Czarnecki
10. SILICATE BINDERS
Theoretical background
Practical task. Determination of water glass modulus
Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
10. SILICATE BINDERS
THEORETICAL BACKGROUND
Production and chemical constitution of water glass
Water glass comes under a category of air binders. The production process consists in
melting silica (sand) with soda, i.e. sodium carbonate Na2CO3 or potassium carbonate K2CO3
or sodium sulfate Na2SO4 or potassium sulfate K2SO4, with carbon as a reducing agent. The
product of melting – water glass – is a blend of sodium or potassium silicates:
R2O · nSiO2 · mH2O
where R2O stands for Na2O or K2O.
Water glass setting process
Soluble glass occurs in water solution as a mix of silicates and products of their
hydrolysis:
Na2SiO3 + 2H2O → 2NaOH + H2SiO3
↓
SiO2 · H2O
Setting of water-glass consists, first of all, in coagulation (conversion of sol into gel)
of silicate acid. However, sodium hydroxide in the solution, by alkalizing the medium,
maintains silicate acid in a sol form; only reacting with carbon dioxide in the air, it neutralizes
the medium:
2NaOH + C2O → Na2CO3 + H2O,
which makes coagulation possible. Taking into account these partial reactions, the process of
water glass setting may be recorded as follows:
H 2O
Na2O · nSiO2 + CO2 + 2nH2O → Na2CO3 + 2NaOH + nSi(OH)4
The silicate acid gel formed in the reaction undergoes gradual dehydration:
Si(OH)4 → SiO2 + 2H2O
and polycondensation, which results in the formation of silicon oxide chains:


— Si — O — Si —


Polycondensation is a kind of polyreaction, which means formation of
macromolecular compounds; it is characteristic of the polycondensation reaction that polymer
chain formation is accompanied by releasing a by-product of a small-molecule compound,
H2O in this case.
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
Process of setting and hardening of water glass is slow so coagulants are used to
accelerate releasing the silicate acid gel from the solution. The amount of sodium
silicofluoride, Na2SiF6, the most frequent coagulant used, is 15 – 20% of the mass of water
glass. An accelerating activity of sodium silicofluoride consists in neutralizing the sodium
base in the total volume of the binder, while carbon dioxide reacts on the surface. Under its
action, the material gets covered with a hard coat of gel, which hinders gas difusion into the
material and moisture release from internal layers. The presence of sodium silicofluoride
makes the water glass setting process run as follows:
2(Na2O · nSiO2) + Na2SiF6 + 2(2n+1)H2O → 6NaF + (2n +1)Si(OH)4
Comparison of the reactions without and with the accelerator shows that in the latter a higher
amount of silicate acid gel is released from the equivalent amount of silicate:
•
•
without accelerator: Na2O · nSiO2 → nSi(OH)4,
with accelerator: 2(Na2O · nSiO2) → (2n + 1)Si(OH)4 , so
(Na2 O · nSiO2) → (n + ½ )Si(OH)4.
Properties and applications
Water glass solutions have a form of thick, syrop-like liquids (colloids), colourless or
yellow or green. Modulus and density are two quantities used in soluble glass characteristics.
The modulus of soluble glass is expressed as SiO2 to Na2O molar ratio. It is calculated
from the equation below
M =
% SiO2
1.032
% Na 2O
where:
= percentage of silicon dioxide in water glass,
% SiO2
= percentage of sodium oxide in water glass,
% Na2O
1.032
= ratio of molar mass of SiO2 and Na2O (62.00/60.09).
Water glass may have moduli varying from M = 2 to M = 3.5; low-modulus glass has
a substantially slower and longer setting, while using high-modulus glass causes a decrease in
silicate materials strength. For building use, optimal value is 2.7 – 2.9. Determination of water
glass modulus is a subject of a practical task.
The other basic property of water glass is density; usually between 1300 and 1500
kg/m3. Application of lower-density solutions contributes to worse mechanical properties,
while too dense solutions have high viscosity that hinders blending them with fillers. The best
results are achieved when using water glass of 1360 to 1400 kg/m3 density. Detailed
requirements for water glass are given in the standard BN-74/6016-41 “Sodium water glass”.
Water glass is acid-resistant, and therefore is used in silicate acid-proof materials,
mainly in putties and mortars, which are formed by mixing water glass with acid-proof fillers
– mineral flour (granite, silica, andesite, etc.). Sodium water glass is more widely used in
practice than potassium glass; the latter is particularly useful when acting with sulfuric,
phosphoric and acetic acids. The principal defect of silicate materials is low resistance to
moisture, and that is why water-resistant agents are added. For these reasons, they may be
used for building protection in conditions of dry acid environment (e.g. dry chimney gases).
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
PRACTICAL TASK. DETERMINATION OF WATER GLASS MODULUS
The equipment necessary to perform the task:
quartz crucible,
ceramic crucible,
ceramic triangle,
burner,
glass rod,
250 cm3 beaker,
Reagents used:
water solution of water glass,
cresol red,
ceramic plate,
desiccator,
burette.
distilled water,
HCl 0.1 m
Task performance
A tested sample of water glass contains water, sodium oxide and silicon dioxide. The
content of water is determined by measuring a mass loss during heating; the rest is a dry
substance comprising SiO2 and Na2O. Sodium oxide is determined by titration of water glass
solution with hydrochloric acid. During titration, the following reaction takes place:
HCl + NaOH → NaCl + H2O
(sodium hydroxide is a product of sodium silicate hydrolysis). SiO2 makes the remaining part
of dry substance. This determination allows to calculate the water glass modulus.
Determination of water and dry substance contents
Using a glass rod, put about 20 drops of tested glass into a weighed quartz crucible
and weigh the crucible with the sample. Place the crucible on a ceramic plate over a burner
and heat it until characteristic clicks (caused by material stress caused by water evaporation)
stop. Then, move the crucible from the plate to the triangle and keep heating for about 10
minutes and next cool the crucible in the desiccator. Calculate the percentage of water in the
sample (%w) from the formula:
b−c
100
%w =
b−a
where
a – mass of the crucible, g,
b – mass of the crucible with the sample, g,
c – mass of the crucible with the sample after roasting, g.
The percentage of dry substance (%s) is calculated from the difference:
%s = 100 – %w
Determination of sodium oxide and silicon dioxide; calculating the water glass modulus.
Using a glass rod, put about 10 drops of tested water glass into a weighed ceramic
crucible and weigh the crucible with the sample. Place the crucible in a 250 cm3 beaker and
pour 100 cm3 of hot distilled water (about 80ºC) onto it (the water is heated over a burner, in a
flask or beaker placed on a ceramic plate; to save time, it is recommended to prepare the
water simultaneously with roasting a sample for determination of water and dry substance).
When the crucible overturns in the water, move it there with the glass rod to dissolve water
glass. On complete sample dissolution, add 3–4 drops of an indicator – cresol red, and,
leaving the crucible and the rod in place, titrate with 0.1 m solution of HCl until it changes
colour. Titrate according to the instructions given in Chapter 2.3. The percentage of Na2O in
the sample is calculated from the formula:
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
% Na2O =
VHCl ⋅ 0.0031
100
b−a
where:
VHCl – volume of hydrochloric acid used, cm3,
a
– mass of the crucible, g,
b
– mass of the crucible with the sample, g,
0.0031 – analytical factor = amount of Na2O equivalent to 1 cm3 of 0.1 m HCl solution.
The percentage of SiO2 is calculated by subtracting the percentage of Na2O in the
sample from the percentage of dry substance
%SiO2 = %s – %Na2O
Using this data, calculate the water glass modulus and assess the quality of the glass
tested according to this parameter.
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