⚡ electrical engineering

Experimental Investigation of the Temperature-Dependent Mechanical Properties of Sulfur Mortars with Different Binder Compositions

This study experimentally demonstrates that while increasing sulfur content enhances the mechanical performance of sulfur-based mortars at elevated temperatures, the addition of bitumen significantly reduces strength despite improving ductility, leading to a recommended safe service temperature limit of approximately 75°C.

Original authors: Sayed Nematullah Sadat, Ömer Faruk Balalan, Ahmet Özbayrak

Published 2026-07-07

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CC BY 4.0

Original authors: Sayed Nematullah Sadat, Ömer Faruk Balalan, Ahmet Özbayrak

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a house out of a very special, unusual material: molten sulfur.

Normally, when we build with concrete, we mix sand with cement and water. But sulfur is different. It's a byproduct of cleaning oil and gas, and instead of water, we melt it down and mix it with sand to create a "waterless" building block. This material is super strong, resists acid like a superhero, and hardens incredibly fast.

However, there's a catch: Sulfur is a bit of a mood ring when it comes to heat. It gets hard and brittle when it's cold, but it starts to get soft and squishy when it gets hot.

This research paper is like a stress test for this sulfur-sand mixture. The scientists wanted to answer two main questions:

How does the temperature change the strength of this material? (Does it get stronger or weaker as it heats up?)
Does adding a little bit of "road tar" (bitumen) help or hurt? (Think of bitumen as a softener or plasticizer, like adding butter to a stiff dough to make it more flexible.)

Here is what they found, broken down simply:

1. The Temperature Rollercoaster

The researchers tested the material at five different temperatures, ranging from a freezing -10°C (like a winter day) to a scorching 100°C (like a hot summer day or a boiling pot).

The Cold Snap (-10°C): The material was very stiff. Interestingly, the mixture with less sulfur (30%) was actually stronger than the one with more sulfur (40%) when it was freezing cold.
The "Sweet Spot" (75°C): This is the most surprising part. Usually, things get weaker as they get hotter. But for the sulfur mixture with more sulfur (40%) and no bitumen, the material actually got stronger as it warmed up to 75°C. It was like a rubber band that suddenly snapped back tighter. The scientists call this an "anomalous strength increase."
The Melting Point (100°C): Once the temperature hit 100°C, the party was over. The sulfur started to soften significantly (remember, sulfur melts around 115°C). At this point, the material lost its stiffness, and its strength crashed. It was like trying to build a wall out of warm butter.

2. The Bitumen Experiment (The "Butter" Test)

The scientists added 1.5% bitumen (a sticky, black substance used on roads) to see if it would make the sulfur less brittle and more flexible.

The Result: It made the material much weaker, but a little more stretchy.
The Trade-off: Imagine a chocolate bar. The plain sulfur bar is hard and snaps cleanly. The bitumen bar is softer and bends a bit before breaking, but it breaks under much less weight.
The Numbers: Adding bitumen caused a massive drop in strength.
- Bending Strength: Dropped by 67% to 82%.
- Squeezing (Compressive) Strength: Dropped by 28% to 67%.
- Essentially, the bitumen acted like a "weak link" in the chain, disrupting the strong crystal structure of the sulfur.

3. The Load vs. The Bend (Ductility)

The researchers also looked at how much the material bent before breaking (deflection).

Without Bitumen: The material was very strong but brittle. When it finally broke, it snapped suddenly, like a dry twig.
With Bitumen: The material was weaker, but it bent more before breaking. It was more "ductile," like a piece of taffy.
The Catch: While being able to bend is usually good, the bitumen made the material so weak that it couldn't hold much weight to begin with. Also, its behavior became unpredictable as the temperature changed.

The Big Takeaway

If you want to use this sulfur-sand material for building something that needs to hold heavy weight:

Don't add the bitumen. It makes the material too weak, even though it makes it slightly more flexible.
Use more sulfur (40%). This creates a stronger material, especially in warm conditions.
Watch the temperature. The material is happiest and strongest between 50°C and 75°C. If it gets too hot (100°C), it loses its strength. If it's freezing, it's okay, but the 30% sulfur mix is better there.

In short: This study tells us that while sulfur mortar is a promising, eco-friendly building material, it is very sensitive to heat. Adding bitumen to make it "tougher" actually makes it too weak to be useful for heavy-duty jobs. The best recipe is a high-sulfur mix without bitumen, used in environments where the temperature stays below 75°C.

Technical Summary: Experimental Investigation of the Temperature-Dependent Mechanical Properties of Sulfur Mortars with Different Binder Compositions

Problem Statement
The construction industry faces significant environmental challenges due to the high energy consumption and carbon emissions associated with Ordinary Portland Cement (OPC). Concurrently, the energy sector generates vast quantities of elemental sulfur as a byproduct of desulfurization, creating a waste management issue. Sulfur-based mortars (SBM) offer a sustainable alternative, utilizing molten sulfur as a binder to create "waterless concrete" with rapid strength development and high chemical resistance. However, a critical limitation of sulfur-based composites is their thermoplastic nature; the binder softens as temperatures rise, leading to reduced stiffness and mechanical strength. While the softening point of sulfur (approximately 88–93°C) is known, the specific mechanical behavior of SBM under varying temperature conditions, particularly when modified with additives like bitumen and varying sulfur contents, remains insufficiently clarified. This study addresses the need to understand the thermal-mechanical performance of SBM to ensure its reliability in engineering applications.

Methodology
The study experimentally investigated the mechanical properties of SBM across a temperature range of -10°C to 100°C. The experimental design involved:

Materials: $\alpha$ -sulfur (binder), river sand (0–4 mm), Class F fly ash (mineral additive), and B50/70 bitumen (modifier).
Mix Designs: Mortars were prepared with two sulfur contents (30% and 40% of sand mass) and a constant fly ash content (15% of sand mass). Bitumen was added at 1.5% of the sand mass for modified mixtures.
Specimens: 60 prismatic specimens (40 × 40 × 160 mm) were produced for flexural testing, with corresponding half-specimens used for compressive testing.
Testing Conditions: Samples were tested at five specific temperatures: -10°C, 25°C, 50°C, 75°C, and 100°C.
Procedures: Flexural strength was determined via three-point bending (TS EN 196-1), and compressive strength was tested on the resulting half-specimens. Load-deflection behavior was also analyzed to assess ductility and brittleness.

Key Results
The experimental results revealed complex interactions between temperature, sulfur content, and bitumen modification:

Effect of Bitumen: The addition of 1.5% bitumen consistently resulted in significant performance losses across all temperatures. Flexural strength decreased by 67–82%, and compressive strength decreased by 28–67% compared to unmodified samples. While bitumen increased post-peak ductility (allowing deformation after peak load), it acted as a "weak phase" that disrupted the crystalline structure of the sulfur matrix, severely compromising load-bearing capacity.
Effect of Sulfur Content: Increasing sulfur content from 30% to 40% generally improved mechanical performance at medium and high temperatures (25–75°C). The 40% sulfur mixtures (SM40) achieved the highest flexural and compressive strengths, particularly at 75°C. Conversely, at low temperatures (-10°C), the 30% sulfur mixtures (SM30) exhibited superior performance.
Temperature Sensitivity and Anomalous Strength Increase:
- Low Temperatures (-10°C): Materials exhibited higher strength but brittle behavior.
- Medium to High Temperatures (50–75°C): An "anomalous strength increase" was observed, particularly in the SM40 samples. At 75°C, the unmodified SM40 sample reached a peak flexural strength of 8.44 MPa and a compressive strength of 38.91 MPa. This aligns with literature suggesting sulfur-based composites can exhibit increased strength up to 74–80°C.
- Critical Temperature (100°C): At 100°C, all samples experienced a sharp decline in strength. This is attributed to exceeding the safe service temperature limit (88–93°C), where the sulfur polymer softens and approaches its melting point, leading to a transition to viscous flow and loss of rigidity.
Flexural vs. Compressive Strength Relationship: The study found no direct linear correlation between flexural and compressive strength. In many cases, the trends were opposite; for instance, while compressive strength often peaked at 50–75°C, flexural strength showed different sensitivity patterns. This indicates that evaluating SBM based on a single mechanical parameter is insufficient.
Load-Deflection Behavior: Rising temperatures and bitumen modification increased material ductility (higher deflection values) but significantly reduced load-bearing capacity. Unmodified samples failed in a brittle manner after peak load, whereas bitumen-modified samples continued to deform under lower loads.

Significance and Claims
The authors conclude that the safe service temperature limit for sulfur-based composites in engineering applications should be optimized to approximately 75°C. Beyond this point, specifically at 100°C, the material's structural integrity is compromised due to the softening of the sulfur binder.

The study emphasizes that while increasing sulfur content to 40% offers a mechanical advantage in the 50–75°C range, the use of plasticizing additives like bitumen must be carefully balanced. Although bitumen improves ductility, the substantial reduction in flexural and compressive strength (up to 82% and 67%, respectively) presents a distinct disadvantage for structural applications requiring high load-bearing capacity.

Ultimately, this research provides a scientific foundation for designing sulfur-based binders, highlighting that the mechanical performance of SBM is highly sensitive to temperature fluctuations and binder composition. The findings suggest that for optimal structural performance, unmodified mixtures with 40% sulfur content are preferable within the 50–75°C operating window, provided that the brittle failure mode is accounted for in design.

1. The Temperature Rollercoaster

2. The Bitumen Experiment (The "Butter" Test)

3. The Load vs. The Bend (Ductility)

The Big Takeaway

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