Sulphtes

DETERMINATION OF SULPHATE CONTENTS IN

WATER SOIL(2:1) EXTRACT

CONTENTS

● Sulphates● Water soil extract● Procedure of the experiment

WHAT ARE SULPHATES

● The sulphate ion is a polyatomic anion with the empirical formula so

42-

● Acid sulphate soils are naturally occurring soils, sediments or organic substrates (e.g. peat) that are formed under waterlogged conditions.

SULPHATES.......

● Soils containing large amount of sulphates, when exposed to air or water form sulphuric acid.

● This acid furthur lead to release of heavy metals and both these acid and heavy metals lead to following effects

● Killing vegetation, degrading groundwater, degrading the quality of soil.

● Sulphates may also enter the food chain and may have adverse effect on bodiversity.

EFFECTS OF SULPHATES FROM ENGINEER VIEW POINT

Presence of sulphates poses problem for a civil engineer in two ways

● If the soil used for making building material contains sulphates

● If the soil on which structure is standing contains sulphates

Sulphate sources:

1. Internal Sources:

This is more rare but, originates from such concrete- making materials as hydraulic cements, fly ash, aggregate, and admixtures.

● portland cement might be over-sulphated.● presence of natural gypsum in the aggregate.● Admixtures also can contain small amounts of

sulphates.

2. External Sources:

External sources of sulphate are more common and usually are a result of high-sulphate soils and ground waters, or can be the result of atmospheric or industrial water pollution.

●Soil may contain excessive amounts of gypsum or other sulphate.

●Ground water be transported to the concrete foundations, retaining walls, and other underground structures.

●Industrial waste waters.

SULPHATE ATTACK● Sulphate attack is a chemical breakdown mechanism

where sulphate ions attack components of the cement paste.

● The compounds responsible for sulphate attack are water-soluble sulphate-containing salts, such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) sulphates that are capable of chemically reacting with components of concrete.

What happens when sulphates get into concrete?

It combines with the concrete paste, and begins destroying the paste that holds the concrete together. As sulphate dries, new compounds are formed, often called ETTRINGITE .These new crystals occupy empty space, and as they continue to form, they cause the paste to crack, further damaging the concrete.

C3A.Cs.H

18 + 2CH +2S+12H = C

3A.3CS.H

32

C3A.CH.H

18 + 2CH +3S + 11H = C

3A.3CS.H

32

PREVENTIVE MEASURES

● Main factors affecting sulfate attack:1. Cement type and content:

The most important mineralogical phases of cement that affect the intensity of sulphate attack is: C

3A

2. Fly ash addition:

The addition of a pozzolonic admixture such as fly ash reduces the C3A

content of cement.

3. Sulphate type and concentration:

The sulphate attack tends to increase with an increase in the concentration of the sulphate solution up to a certain level.

Control of sulphate attack:

The quality of concrete, specifically a low permeability, is the best protection against sulfate attack.

● Adequate concrete thickness

● High cement content

● Low w/c ratio

● Proper compaction and curing

WATER SOIL EXTRACT

● Water soil extract can be said solution of soil in water.

● Test is performed for soils where water table is high.

EXPERIMENT● APPARATUS :➔ Drying oven.

➔ Balance, readable to .001g.

➔ Desiccator

➔ Test sieves

➔ 500 ml glass beakers

➔ 500ml conical flasks

➔ Electric hot plate

➔ Spatula

➔ Dropper

➔ Flocculator

➔ Whatman filter papers

➔ Funnels

➔ Measuring cylinders

➔ silica crucibles

PROCEDURE● PREPRATION OF SOIL AND ITS ACID

EXTRACT :➔ Soil is prepared according to the BS 1377-3:1990

➔ Minimum mases of soils required are as follows:

fine grained soils : 100g

medium grained soils : 500g

coarse grained soils : 3Kg

➔ DRYING OF SOIL :

a) oven drying

b) air drying

PREPRATION OF TEST SPECIMEN

● Divide the material passing the 2mm sieve, to produce a sample weighing approximately 100 g.

● Pulverize the sample so that it passes the 425micron test sieve. Take about 60 g of sample from the soil that is retained in pan.

● Again dry the sample at a temprature of 75º to 80º C.

● Allow the specimen to cool to room temperature in the dessicator.

PREPARATION OF WATER SOIL EXTRACT

● 2:1 water soil extract is to be made to find the sulphate contents from the prepared specimen.

● Weigh a sample of 50g. And transfer it to a clean extraction bottle or beaker.

● Add exactly 100ml of distilled water to the extraction bottle.

● If extraction bottle is to be used we must use shaker to agitate for 16hours.

● If beaker is used then flocculator is to be used to shake the sample.

● Filter the soil suspension into a clean and dry flask through a suitable filter paper e. g. Whatman no. 50.

● Transfer 50ml of filtereed extract to a 500ml conical beaker.

● Add distilled water about 300 ml.● Prepare a barium chloride 5%(m/v) solution by

dissolving 50g of barium chloride in 1L of water.

TEST PROCEDURE● Test the solution for acidity with litmus paper and if

necessary make slightly acidic by the addition of 20 drops of hydrochloric acid.

● Bring the solution to the boil and add 10mL barium chloride solution drop by drop with constant stirring. Continue boiling gently until the precipitate is properly formed.

● Allow the solution to stand at just below the boiling point for at least 30 minutes, then leave to cool to room temperature.

● Transfer the precipitate with extreme care to a suitable filter paper e. g. Whatman no. 42 in glass filter funnel and filter.

● Now transfer the filter paper and precipitates to a previously ignited and weighed crucibles.

● Place the crucible and its contents in a muffle furnace at the room temperature and gradually raise the temperature to red heat. Ignition for about 15 minutes at red heat is usually sufficient.

● Cool the crucibles to the room temperatures using desiccator and weigh it to the nearest 0.001g.

● Calculate the mass of the precipitate from the increase in the recorded mass of the crucibles.

RESULT

● Sulphate content can be expressed either in terms of the concentration of sulphate in the extract as so

3 of in g/L, or in terms of percentage

as so3 of water soluble sulphate in the soil

sample

METHOD 1:

Calculate sulphate in g/L of the 2:1 water-soil extract from the equation :

Concentration of sulphates so3

= 6.86 * m

4 g/L

Where m4 is the mass of ignited precipitate in g.

METHOD 2

Calculate the sulphate content as a percentage of the soil from the equation:

water soluble sulphate content as SO3

= 1.372 * m

4 (%)

Where m4 is the mass of ignited precipitate in

g.