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Materials of Construction - Concrete
May 9, 2003
Introduction
Concrete is found throughout an acid plant both above and
below ground. It is used extensively for foundations and buildings but may also be
found as an integral part of some equipment such as pits, holding tanks, cooling towers,
etc.
Sulphur Concrete
Sulphur concrete is a thermoplastic material that is produced
by mixing modified molten sulphur with mineral fibers and aggregates at 132 to 141°C (270
to 285°F). The sulphur replaces portland cement and water as the binder in
conventional concrete. The result is a very-high-yield material that has superior
mechanical properties and is resistant to many chemicals including sulphuric acid.
| |
Sulphur Concrete |
Portland Cement Concrete |
| Compression Strength (Average) |
8000 psi |
3000 to 4000 psi |
| Flexural Strength |
750 psi |
500 to 550 psi |
Sulphur concrete's high compressive strength is achieved
rapdily upon cooling and solidification. Portland cement requires a long time to
cure and achieve its final strength but sulphur concrete reches 80 to 100% of its ultimate
strength in 24 hours.
Sulphur concrete is suitable for cast-in-place and precast
construction of floors, walls, sumps, trenches, pump bases, tank foundations, etc.
The material will be cost competitive for applications where acid resistance, high
strength and curing within 24 hours is required.
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Composition
Modified Sulphur - Modification is required to the sulphur to
control the expansion and contraction of the cement during thermal cycling. Sulphur
makes up 95.0% +/- 1.0% of the mix with 4.5% +/- 0.1% carbon and 0.5% +/- 0.05%
hydrocarbons. The modified sulphur will have a specific gravity of approximately
1.90 (25°C) and a viscosity of 25 to 100 cP.
Aggregates - The aggregate should be clean, hard, tough,
organic free, clay free and resistant to chemicals. Dense graded aggregates should
be used to minimize the amount of modified sulphur used in the mix and to minimize thermal
expansion of the concrete. Three size fractions are generally used; coarse
aggregate, sand and silica fluor (-200 mesh).
Mix Proportions - The proportion of the various
components that make up sulphur concrete should give the concrete the desired mechanical
properties. As well, the mix should be easy to work and fluid enough to be poured.
A typical mix is as follows:
Modified
Sulphur |
14 wt% |
Mineral Filler |
10 wt% |
Coarse
Aggregate |
38 wt% |
Sand |
38 wt% |
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Surface Preparation
The sub-base must be compacted and dry. Wherever
moisture is present, a 6 mil polyethylene vapour-barrier can be used to prevetn
vapourization of the water when the hot sulphur concrete is applied.
Sulphur concrete can be installed over existing portland
cement concrete. The surface of the existing concrete should be prepared
hydroblasting to expose aggregate and remove all loose material. A continuous
corrosion-resistant flexible membrane should be applied which will keep moisture out and
protect the substrate against chemical attack. The membrane will also reduce stress
cracking of the sulphur concrete and provide and adhesive bond between it and the existing
concrete. Stress crackng is reduce because the membrane provides a slip plane
allowing the sulphur concrete to move upon cooling. The membrane also prevents any
chemical reaction between the sulphur concrete and chlorides in the portland cement
concrete.
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Mixing,
Transporting and Placing
Sulphur concrete is made by mixing the molten modified
sulphur with aggregate that has been heated to 204°C (400°F). The temperature of
the mix is sufficient to allow the concrete to be transported to site and place down.
The mix is transported in special heated concrete-mixer trucks.
Continuous production and placement of sulphur concrete can
lead to finishing problems, due to premature solidification of previously placed material.
To avoid these problems a large enough batch of mix is required to enable placement
as one mass. To achieve this the mixer truck needs to be located as close as
possible to the worksite. Pouring of the concrete should be done as rapidly as
possible.
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Finishing
Finishing of sulphur concrete can be done using conventional
concrete finishing tools. Once the sulphur concrete is poured it must be struck off
(leveled) immediately with a simple screed. After the concrete is levelled there are
only minutes remaining to finish the surface before the sulphur concrete begins to crust.
For a 50 mm (2") thick layer there is only 2 to 3 minutes available to finish
the surface. For a slab 150 to 200 mm (6" to 8") thick there is about 10
to 15 minutes available.
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