MECHANICAL PROPERTIES OF SULFUR CONCRETE

Fossil fuel consumption is rapidly increasing around the world, as is the amount of sulfur generated as a byproduct of the industrial refining process. However, sulfur concrete made with unmodified sulfur has limitations for practical use because it has inferior properties such as poor resistance to water and is significantly more brittle than traditional concrete. Sulfur concrete with modified sulfur binders exhibits excellent durability at high acid or salt concentrations.

This study investigated the mechanical and durability properties of sulfur concrete made with a modified sulfur binder instead of Portland cement. Preliminary tests were conducted to evaluate the effects of maximum coarse aggregate size, proportion of binder, aggregate and modified sulfur binder, degree of replacement of SPB with fly ash on workability, strength and modulus of elasticity. Compressive and tensile tests of split strength were carried out and the modulus of elasticity of sulfur concrete was measured.

For the samples made with the maximum size of 19 mm, 13 mm and 25 mm coarse aggregate, the average compressive strength was 76, 53 and 50 MPa, respectively. When the proportion of fly ash was increased to 5, 12, and 15% to replace SPB, the compressive strength of sulfur concrete showed 76, 83, and 72 MPa, respectively. Therefore, the case with 19 mm coarse aggregate and 15% fly ash (by weight) showed the best mechanical properties.

Based on the results of the preliminary tests, three types of samples were tested to investigate the properties of sulfur concrete in harsh environment, such as frost and thaw resistance, coefficient of thermal expansion and chemical resistance.

INTRODUCTION AND BACKGROUND

LITERATURE REVIEW

TEST METHODS

Manufacturing Process of Specimens
Strength and Elastic Modulus Tests
Chemical Resistance Tests
Coefficient of Thermal Expansion Tests
Resistance of Freezing and Thawing Tests

To measure the stress-strain relationship of sulfur concrete, three compressometers with a 100 mm gage length were used. The 1500kN SATECTM Series 1500HDX hydraulic universal testing machine shown in Figure 3 was used to perform strength tests and measure the modulus of elasticity. The modulus of elasticity of concrete under uniaxial compression is taken as the slope of the stress-strain curve.

First, the tangent modulus of elasticity at a given point is given by the slope of a line drawn tangent to the stress-stain curve at any point. The secant elastic modulus is given by the slope of a line drawn from the origin to the point on the curve corresponding to 40% of the maximum load. Finally, the modulus of elasticity of the tendon is given by the slope of the line running from the point representing a strain of 50 × 10-6 mm/mm to the point corresponding to 40% of the maximum load.

11] In this study, the elastic secant modulus was calculated and used to compare the elastic modulus of sulfur concrete. To evaluate the chemical resistance of sulfur concrete, cylindrical samples were immersed in three different aggressive chemical environments: 10% HCl solution, 20% H2SO4. Before determining the mass change and compressive strength, the specimens were removed from the chemical solution, washed and dried in an oven at 105°C.

The mass change and compressive strength of three samples were also measured after immersion for each case. The expansion and contraction of sulfur concrete due to temperature changes can affect its durability. In this study, the AASHTO designation: TP was followed to evaluate the thermal expansion coefficient of sulfur concrete.

First, the sample was soaked in water for 2 days and the length of the sample was measured. Tests for the freezing and thawing resistance of sulfur concrete were carried out according to ASTM C 666 procedure B on mm prismatic samples. The test was terminated when the number of cycles reached 300 cycles or the dynamic modulus of elasticity decreased below 60% of the initial value before 300 cycles.

PRELIMINARY TESTS FOR OPTIMUM MIX PROPORTION

Test Variables and Mixing Proportions

Materials

The recycled coarse aggregates used have a lower density and a higher water absorption ratio than natural coarse aggregates. Natural coarse aggregates are crushed aggregates (Figure 6) whose maximum size is 25, 19 and 13 mm. The maximum size of recycled coarse aggregate and natural fine aggregate is 25 and 10 mm, respectively.

Result of optimum mixing proportions

1, 2 and 4 were compared to each other to confirm the effect of the size of coarse aggregate on strength. 4 mixed with coarse aggregate of maximum size 19 mm showed the best performance of the strength among three samples (sample no. 1, 2 and 4). The possible reason why sample 2 had a low strength could be improper particle size distribution of coarse aggregate.

The small difference of 7 MPa in compressive strength between two samples may be due to deviation of samples. This means that sulfur concrete reaches its final strength at an early age and does not need a long curing time like Portland cement concrete. To determine the correct proportion of fly ash and SPB, the strengths of sample no.

5, which is mixed with 15% SPB and 12% fly ash, achieved the highest compressive and splitting tensile strength among all samples. The test result shows that an increase in the proportion of fly ash, while reducing the amount of SPB, is useful in improving the strength of the sulfur concrete. However, the strength was reduced when the amount of SPB was reduced to 12%, because the amount of SPB as a plastic state determines the machinability.

6 was produced using recycled coarse aggregate of maximum size 25 mm to investigate an applicability of recycled aggregate to sulfur concrete. The elastic modulus of sulfur concrete was compared with calculated elastic modulus of Portland cement concrete. The average elastic modulus of sulfur concrete is 89 % of that of ordinary Portland cement concrete.

TESTS FOR MACHANICAL AND DURABILITY PROPERTIES

Test Variables and Mix Proportions

The effect of using fly ash on mechanical and durability properties was investigated by comparing type S and type F specimens. In these tests, the same type of modified sulfur as used in the preliminary tests was used as binder. The properties of fly ash used as mineral filler in these tests are shown in Table 9.

The recycled coarse aggregates used have the same properties as those used in the primary tests as shown in Table 5. The natural coarse aggregate is crushed granite and the maximum size of the natural fine aggregate is 10 mm.

Test results

The stress-strain curves for sulfur concrete with different mixing ratios are shown in Figure 10. The results show that the modulus of elasticity of sulfur concrete is generally lower than that of ordinary Portland cement concrete, which has the same compressive strength as sulfur concrete. This means that sulfur concrete has a greater load than ordinary Portland cement concrete under the same load.

The result of the compressive strength of sulfur concrete after immersion in acid and salt solution is shown in Table 13 and Figure 12. In this test, the average compressive strength of sulfur concrete decreased by 10% after immersion for 60 days. The result shows that the compressive strength of these specimens decreased up to 2~3% by HCl and H2SO4 solutions and NaCl solution did not reduce the strength of sulfur concrete after 360 days immersion.

The reaction mechanism with sulfur concrete in acidic environment mentioned by Vlahovic et al. Aggregates and fillers used in the production of sulfur concrete consist of mineral oxides. Sulfur concrete's resistance to frost is important for structures exposed to such climatic conditions.

9] showed that when sulfur concrete's moisture absorption exceeds the limit (0.05%), its resistance to freeze-thaw damage drops dramatically. He expected that the possible reason for high resistance in freezing and thawing cycles is low water absorption of sulfur concrete. To evaluate the applicability of sulfur concrete for harsh environment, concrete durability evaluation for freezing and thawing resistance proposed by KCI [22] was calculated.

In this study, 7 cases of sulfur concrete were prepared and tested with different types (natural and recycled) and sizes mm) of aggregate, and different ratios of fly ash and SPB to find different optimal mixing ratios as a preliminary test. The amount of fly ash and SPB was adjusted to maintain the correct price of the sulfur concrete by minimizing the amount of SPB while maintaining workability. To ensure workability, the unconsolidated sulfur concrete was recorded with a video camera before casting.

Fly ash is expected to increase the density of the sulfur concrete by improving aggregate size distribution and filling the pores. The elastic modulus of sulfur concrete is generally lower than that of ordinary Portland cement concrete, which has the same compressive strength as sulfur concrete.