Ballast Sand vs. Alternative Materials: A Comparative Analysis

6th Feb 2024 -

Ballast sand is a finely crushed and versatile sand. It provides stability, support and load distribution across diverse applications, from railway tracks to pathways. Despite its popularity, alternative materials are used as replacements. These alternatives aim to provide equal or superior performance while addressing ballast limitations. 

This feature weighs up the pros and cons of ballast sand vs alternative materials. We provide a comparison between ballast sand and promising alternative materials. Key factors examined include load-bearing capacity, drainage, cost and lifecycle.

What is ballast made from?

The composition of ballast can vary slightly depending on its application. However, it typically consists of the following mix of ingredients. 

Silica sand: Silica sand forms the bulk of ballast sand composition. It is primarily composed of silicon dioxide (SiO2) and contains high purity and uniform particle size. Silica sand offers excellent drainage properties and helps in distributing loads evenly. As a result, it enhances the stability of structures.

Gravel or crushed stone: Ballast may contain varying proportions of gravel or crushed stone such as granite and basalt. These coarse aggregates add strength and stability to the ballast. They improve its load-bearing capacity and prevent deformation under heavy-duty loads.

Fines: Small particles of sand and stone dust to improve cohesion. They help in filling voids between larger particles, enhancing the overall ballast stability and structural integrity. 

Clay or silt: In some cases, ballast sand may contain small amounts of clay or silt particles. An excessive amount of both can reduce drainage and increase erosion. However, a controlled amount improves cohesion and prevents excessive settlement.

Water: Water is an important ingredient in the manufacturing and installation of ballast sand. It dampens the sand during compaction, ensuring a good level of density and stability.

Chemical additives: In some applications, you can add chemical additives to the ballast sand mixture. This is to boost specific properties such as water resistance and durability. 

You can also use ballast for concrete mixtures by mixing it with Portland cement, gravel and water. Learn more about the uses and benefits of ballast sand with our guide.

7 challenges with ballast sand

Although it is an important component of structures such as railroad track beds, it also has some challenges. Below are 7 potential issues to think about.

1. Drainage issues

Ballast sand can accumulate silts and clays from the surface, reducing drainage capacity. For example, this can lead to waterlogging under heavy rain or snow melt, causing unstable rail tracks. 

2. Lack of cohesion

Loose, unbound sand particles have little structural strength or stability. This provides inadequate support and allows erosion from wind and water more easily than well-graded angular aggregates.

3. Dust

Dry, exposed ballast sand is prone to particle migration from wind gusts across trackbeds. Wind-blown sand creates clouds of silica dust exposure. This can cause harm to track maintenance workers and nearby residents.

4. Difficult to compact

The movement of small uniform particles can affect how you compact ballast sand to create enough stiffness. Good compaction requires specific methods.

5. Prone to movement

Unbound dry sand can easily shift under loads from passing trains. Freeze-thaw cycling also readily unsettles sand particles, requiring frequent re-tamping.

6. Low-weight

Light sands may lack adequate strength for sufficient resistant force against track movement compared to heavier rock aggregates.

7. Variable performance

Natural sands have less uniform properties than precisely engineered aggregates. This leads to uncertain long-term trackbed stability.

What are the alternatives to ballast?

While crushed granite, basalt and other rocks have been the standard aggregate used for ballast sand, some alternative sand types can also be used.

Steel slag

Steel slag is a byproduct of steel production that can be crushed and sized to create an aggregate material.

Pros

• Angular particles provide shear strength for stability.
• Its hardness adds durability. 
• Using slag reduces the need for landfill disposal. 
• Often available near steel mills. 
• Lower material cost compared to quarried sand.

Cons

• Slag composition variability creates inconsistent properties.
• Slower drainage compared to natural sand because of more fine particles.
• Higher specific gravity adds weight to the track.

Overview: Its angular particles, hardness and reuse of waste make it a viable ballast sand substitute. Particularly if leaching and composition concerns are addressed through treatment methods. However, slower drainage may require the use of additives or filters.

Iron ore

Iron ore tailings and minerals which are leftover from mining operations and processed into a sand material.

Pros

• Provides heavy, strong particles comparable to natural sand.
• Availability near iron mining and processing facilities. 
• The uniform composition creates consistent performance.

Cons

• Durability decreases over time as iron oxidises.
• Production limitations on optimising gradation size.
• Fluctuations in steel production impact supply demand.

Overview: Iron ore sand is a potential replacement where locally available. However, there are longevity concerns due to oxidation. Limitations around optimising size gradation also need to be addressed.

Recycled concrete

Recycled concrete is made from crushed concrete and screened old demolished concrete into properly sized aggregate particles.

Pros

• Reuses waste concrete that would otherwise fill landfills. 
• Contains known original materials from source concrete. 
• Often located near project sites. 
• Lower material cost than procuring newer aggregates.

Cons

• Original concrete's variability results in inconsistent properties. 
• Limited ability to selectively grade parent concrete to optimal sizes. 
• Less shape and texture compared to natural sand deposits. 
• The inclusion of unwanted debris and contaminants. 

Overview: Recycled concrete can be a good option for reducing disposal and material costs. Nonetheless, variability and contamination must be managed through rigorous processing methods.

Asphalt

This alternative type consists of milled scrap asphalt pieces that are refined into sand-sized gradation.

Pros

• Binder between particles creates stability.
• Graded scraper fines achieve dense gradation
• Potential local availability from asphalt roof or road projects.
• Smoother graded particles improve compaction.

Cons 

• Excess asphalt fines limit drainage capacity.
• Temperature sensitivity causes performance fluctuations.
• Long-term oxidation can degrade asphalt over time.

Overview: Asphalt provides stability from particle binding action. However, there are performance concerns within extreme climates and drainage limitations, which could require additive use. 

Recycled plastic

Recycled plastic is produced from shredded plastic waste that is melted, hardened and crushed into a sand aggregate material.

Pros

• Provides sustainable use for waste plastic instead of landfilling.
• Polymer coating improves durability and cohesion.
• Particles resist weathering or degradation over time.

Cons

• Concerns about production energy use and emissions. 
• Higher cost compared to natural types.
• Undetermined long-term impact on the environment.
• Limited large-scale railroad implementation track record.

Overview: Recycled plastic offers some sustainable benefits. However, it has unproven longevity, high costs and an uncertain lifecycle which requires further trial testing. 

Is building sand the same as ballast?

While some ballast and building sand characteristics overlap, there are some key differences between the 2 materials.

1. Composition

Building sand is usually natural sand composed of sub-angular to sub-rounded grains. Ballast contains more angular crushed rock particles mixed with natural sand.

2. Grain shape

The rounded grains in building sand offer ease of use for concreting, plastering and mortars. The crushed angular particles in the ballast improve strength and drainage.

3. Grading

Building sand has a wide range of sizes suited for filling gaps in concrete. Ballast is narrowly graded to fill voids between larger aggregates.

4. Structural strength

The sharp interlocking particles of ballast provide greater strength and load-bearing capacity compared to building sand.

5. Permeability

Ballast's crushed content creates higher permeability and drainage capacity important for railroad track beds. Building sand focuses more on binding properties.

6. Reactivity

Building sand's reactivity assists in bonding cement and mortar mixes. Ballast's mildness is desirable to avoid reactions with railroad ties.

How much do ballast and alternatives cost?

When evaluating alternatives to ballast, a key consideration is whether potential cost savings justify tradeoffs in performance. Below is a comparison of the average costs of ballast and alternative materials, including an overview of their performance.

Material	Average cost (per tonne)	Performance
Ballast	£7 - £12	Good drainage, stability and longevity.
Steel slag	£3 - £6	Durability, hardness and drainage may be inferior to natural sand.
Iron ore	£5 - £10	Prone to break down over time.
Recycled concrete	£2 - £5	The performance level is highly variable.
Asphalt	£5 - £8	Reduced drainage but improved stability.
Recycled plastic	£20 - £30	Unproven longevity.