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Sulphuric Acid on the WebTM Technical Manual DKL Engineering, Inc.

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Materials of Construction - Concrete
May 9, 2003

Introduction
Sulphur Concrete
        Composition
        Surface Preparation
        Mixing, Transporting and Placing
        Finishing
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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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