Determination of Sulfate Content in Expansive Soil

University of Texas at Arlington

Civil Engineering Department

Laboratory Test Report

Sulfate Content of Soil from Austin, Texas

Written by:

Richard Benda

Joseph Muhirwa

Robert Sargent

July 1st, 2012

Table of Contents

Page

Table of Contents ….........................................................................................1
List of Tables and Figures................................................................................2
Abstract….......................................................................................................3
Introduction.......................................................................................................4
Equipment and Materials………………………………….....................…….4

Methods and Procedure......................................................................................6
Data, Results, and Discussion.............................................................................7
Conclusion.........................................................................................................9
References.......................................................................................................10
Appendices

            Appendix A: Sample Calculations……………………………...........11

List of Tables and Figures                                                                                                 Page
Figure 1: Samples on the shaking machine……………………….........3

Figure 2: Filtering of leftover solution from centrifuge tubes………….4
Figure 3: Samples in centrifuge…………………………………….......5

Figure 4: Adding Barium Chloride to boiling solution………………....7

Table 1: Experiment Results……………………………………………8

Abstract

The determination of soluble sulfate concentration is very useful in geotechnical investigations due to its impact on heave-induced problems resulting from chemical soil stabilization. This means that the concentration of sulfate plays a big role in choosing the appropriate method in expansive soil stabilization. To better understand how sulfate content is determined prior to expansive soil stabilization, an experiment was conducted on a soil sample from Austin, Texas. The goal of the experiment was to determine the adequacy of sulfate content in the expansive soil sample from Austin before proceeding with the lime stabilization. This experiment took four days to finish and involved a series of steps. On the first day of the experiment the soil sample was pulverized and subsequently was diluted on a 1:10 soil-water ratio. The next day, the solution was centrifuged and filtered using a filter paper. The filtrate was then mixed with Barium Chloride to get a precipitate which is an indicator of sulfate content. On the third day the resulting solution was filtered using an oven-dried filter paper which retains the precipitate formed. The filter paper was dried again overnight and weighed on the last day to determine the soluble sulfates in parts per million (ppm). After the experiment, it was determined that the soil contained a very low sulfate content ranging from 250 to 300 ppm. Based on the results, it was decided to proceed with lime stabilization since the maximum sulfate content that does not induce any heave is approximately 1000-2000 ppm (Puppala et al. (1999)).

Introduction

Lime has always been a go-to solution for expansive soil stabilization because it lowers the soils plasticity and ability to absorb water. Unfortunately lime treatment, due to high sulfate concentrations in the soil, can lead to an even bigger problem. If lime is used in this situation, the lime and sulfate can form a crystal substance called Ettringite. Because Ettringite can swell even more than the soil, an even bigger heave will be created. It is because of this reaction that finding the sulfate content in the soil is very important. Through the use of the modified UTA method an experiment was conducted to find the sulfate content of the soil in order to see if lime stabilization was the appropriate method. Also to ensure accuracy three samples were prepared for this procedure.

Equipment and Materials

• Pulverizer
• Eberbach shaker
• 300ml flask
• Buchner flask
• 0.1μm filter paper
• Weighing tin
• Filter funnel
• 20 ml Syringe
• 140ml beaker
• 10% Barium Chloride solution
• Distilled water
• Squeeze bottle
• Paper Towels
• Centrifuge machine
• Centrifuge tubes
• Parafilm sealers
• Glass stopper
• Oven
• Glass stir rod with rubber policeman
• 0.001 g sensible mass scale.
• Desiccator

Methods and Procedure

The process of determining the amount of soluble sulfates in the soil sample followed the modified University of Texas at Arlington method. The process in its entirety takes approximately 4 days.

Day 1
1) Add 10 g of the soil sample in question to a flask.
2) Add 100 mL of deionized water to the flask.
3) Cover the top of the flask with wax paper and allow the mixture to sit for approximately 24 hours.

Day 2
1) Using a shaker, shake the sample from the previous day on a low setting for 30 minutes. This is done in order to dissolve the soluble sulfates in the soil.
2) After the shaking process, transfer the solution to centrifuge tubes. If necessary, use a DI spray bottle to remove any leftover solution from the sides of the flask.
3) Place the tubes in the centrifuge on a setting of 10,200 RPM for half an hour. This extracts the sulfates from the soil matrix.
4) Following the centrifuge process, filter the supernatant from the tubes through a filter paper with 0.1 μm pores. If necessary, a 0.45 μm filter can be used prior to this to accelerate the filtering through the 0.1 μm filter. To make sure all of the solution travels through the filter, use a DI spray bottle to rinse the sides of the filter funnel.
5) Transfer the filtered solution into a flask and cover with a glass stopper. Once again, if necessary use a DI spray bottle to remove any leftover solution from the sides of the flask.
6) Dilute this solution to a volume of 200 mL using deionized water.
7) Bring the solution to a light boil.
8) Once boiling, add 40 mL of 10% Barium Chloride to the flask.
9) Allow the precipitate solution to cool for 15 minutes.
10) Put the flask in the oven at 80°C for 12 hours. Also, place a 0.1 μm filter paper (wet with deionized water) in an aluminum weighing tin and put it in the oven to dry overnight.
Day 3
1) Take both the dry filter paper and the solution in the flask out of the oven and allow them to cool.
2) Weigh the filter paper with the aluminum weighing tin and record the value.
3) Filter the precipitate solution with the dry filter paper from the oven. A DI spray bottle and a rubber policeman can be used to ensure all the precipitate from the flask is filtered.
4) Use a DI spray bottle to remove any precipitate solution from the sides of the filter funnel.
5) Put the filter paper with the precipitate on it back into the weighing tin and place in the oven (at 80°C) overnight.
Day 4
1) Remove the filter paper in the weighing tin from the oven and allow it to cool.
2) Weigh the paper and tin using the analytical balance and record the value.
3) Use the excel spreadsheet to calculate the soluble sulfate PPM value from the original mass of the filter paper and the mass of the filter paper

Data, Results and Discussion

Table 1 – Experiment Results

	Sample 1	Sample 2	Sample 3
*Weight 1 (grams)	1.2116	1.2198	1.2156
*Weight 2 (grams)	1.2182	1.2263	1.2216
Sulfate content in parts per million (ppm)	271.63	267.514	246.936

See Appendix A for sample calculations.

*Weight 1= Weight of dry filter paper + weighing tin (before filtering the precipitate).
*Weight 2= Weight of dry filter paper with retained precipitate + weighing tin ( after filtering the precipitate).

Once weight 1 and weight 2 were known the amount of barium sulfate on the filter paper could be calculated by simply finding the difference between the two weights. After that a comparison between a mole of barium of sulfate and a mole of sulfate was used to calculate the parts per million. According to the periodic table barium sulfate has a molecular mass of around 233 grams/mole and sulfate has a molecular mass of around 96 grams/mole. Using the ratio of grams per mole of sulfate to grams per mole of barium sulfate and the amount of barium sulfate caught on the filter paper, a calculation could be performed to determined how much of the barium sulfate left on the filter paper was sulfate. Knowing this value gave the ability to figure out the amount of sulfate in parts per million.

As one can see the results that came out were very accurate. The average ppm came out to be 261 with a standard deviation of 13 ppm which comes out to be 5%. Anything with a standard deviation of less than 7% is considered really good which means that the procedure resulted in accurate and reliable results.

Conclusion

Using the modified University of Texas at Arlington Soluble Sulfate Determination Method, the sulfate content of the soil sample from Austin, Texas, was able to be determined in parts-per-million. Through the use of preparing multiple samples, an average of 261 ppm at a standard deviation of 5% was obtained which is considered quite low in comparison to other soils found in Texas. Puppala et al. (1999), and Viyanat (2000) had found that the baseline level for sulfate-induced heave problems was around 1000-2000 ppm. The sample used in this test was well below this threshold, however; this range may vary depending on the composition of the soil as well as other factors. The result that was obtained was expected because the precipitate solution lacked turbidity, which is a good indicator of how the results will turn out. Though the results of this lab test were excellent as far as accuracy and precision goes, next time all the samples should be started on the same day. This way, all of the steps performed will be more easily reproduced. Even though all the samples in this test were not started on the same day due to lack of experience, the results were accurate as well as precise and they will aid in designing a mix to stabilize the soil and reduce expansive soil heave.

References

Anand J. Puppala et al. (2002). Evaluation of a Modified Soluble Sulfate Determination Method for Fine-Grained Cohesive Soils. ASTM International, West Conshohocken, PA.

Anand J. Puppala et al. (1991). Evaluation of a Sulfate Induced Heave by Mineralogical and Swell Tests. XI Pan-American Conference on Soil Mechanics and Geotechnical Engineering, Foz do Igacu, Brazil.

Miller, D. J., Nelson, J.D. (1992). Expansive Soils: Problems and Practice in Foundation and Pavement Engineering, John Wiley & Sons, Inc., Toronto, Canada.

Viyanant, C., (2000). Laboratory Evaluation of Sulfate Heaving Mechanisms Using Artificial Kaolinite Soil. Masters thesis, The University of Texas at Arlington, TX.

Appendix A

Sample Calculations

From the weight of the Barium Sulfate (BaSO₄) precipitate in grams, the amount of Sulfate must be determined. The percentage of Sulfate in the Barium Sulfate is found using Equation 1:

                                (Eq. 1)

This relationship can then be used to determine the ppm from the weight of the precipitate on the filter paper. To do this, all that has to be done is multiply the percentage relationship by one million (Equation 2).

                                                                  (Eq. 2)

To finalize the calculations, the 1:10 dilution ratio has to be accounted for. This is done by dividing the result from Equation 2 by 10 (Equation 3).

                                                                                                  (Eq. 3)

This results in a conversion factor that can be used to calculate the ppm by multiplying this factor by the weight of the precipitate in grams. Shown in the following equations is a sample calculation for the first sample that was tested. The mass of the weighing tin, filter paper and precipitate was 1.2182 grams. The mass of the weighing tin and filter paper without the precipitate was 1.2116 grams. This leaves the mass of the precipitate at 0.0066 grams. Equation 4 shows the resulting conversion into ppm.

                                                       (Eq. 4)