Recycled PET Geopolymer Bricks and Strength Testing

ISEF Category: Chemistry

Subcategory: Materials Chemistry  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Plastic waste does not just sit in landfills. You can turn some of it into building materials. That means one science fair project can ask both a chemistry question and a climate question. Your data can show whether recycled PET helps bricks or weakens them.

What Is It?

This project studies geopolymer bricks made with sand, recycled PET, and a sodium hydroxide, or NaOH, activator. A geopolymer is a cement-like material that forms when an aluminosilicate source reacts in a basic solution. Think of it like a chemical glue that hardens rock grains together.

The main question is simple. As you add more PET, does the brick get stronger or weaker? PET is the plastic used in drink bottles. It can change the way particles pack, the way the paste bonds, and the tiny holes left in the final brick. Those changes affect compressive strength, which is how much squeezing force a material can handle before it fails.

You can add a second layer by comparing cost and embodied CO₂, which means the greenhouse gas emissions tied to making the material. Open databases like ICE can help you estimate the impact of the raw ingredients. That lets you ask not just, 'Does it work?' but also, 'Is it worth it?'

Why This Is a Good Topic

This is a strong science fair topic because you can test one clear variable, PET fraction, and measure a real material property, compressive strength. You also get a built-in engineering tradeoff. A mix can be lighter, cheaper, or lower-carbon, but still too weak for use. That makes the project useful, not just decorative. You can learn experimental design, materials testing, and basic life-cycle analysis without needing a university lab.

Research Questions

• How does PET fraction change the compressive strength of sand-geopolymer bricks? ?
• What is the effect of PET fraction on brick density and visible porosity? ?
• Does adding recycled PET change the cost per brick compared with a PET-free control? ?
• To what extent does PET fraction change the estimated embodied CO₂ of the brick mix? ?
• Which PET fraction gives the best balance of strength, cost, and embodied CO₂? ?
• How does curing condition affect strength differences between PET-rich and PET-poor bricks? ?

Basic Materials

• Fine sand or builder's sand.
• Recycled PET flakes or pellets from cleaned plastic bottles.
• Sodium hydroxide solution or other school-approved alkali activator.
• Aluminosilicate source for geopolymer mix, such as fly ash or metakaolin, depending on school access.
• Digital kitchen scale with 0.1 g accuracy.
• Mold set for small brick or cube samples.
• Safety goggles, nitrile gloves, and a lab apron.
• Ruler or digital caliper.
• Texture-safe release agent for molds.
• Permanent marker and labels for sample tracking.

Advanced Materials

• Metakaolin or fly ash with known composition.
• Recycled PET particles with measured size distribution.
• Sodium hydroxide pellets and deionized water for controlled activator preparation.
• Optional sodium silicate solution, if your protocol includes it.
• Compression testing machine with calibrated load cell.
• Vacuum desiccator or drying oven for controlled sample conditioning.
• Analytical balance with 0.01 g accuracy.
• Vernier calipers or micrometer.
• SEM access for fracture surface imaging.
• XRF or XRD access for confirming precursor chemistry.

Software & Tools

• Google Sheets: Organizes mix design data, strength results, and life-cycle estimates in one place.
• Excel: Helps you plot compressive strength, density, and cost trends across PET fractions.
• Python: Lets you run cleaner statistics, fit curves, and compare groups with reproducible code.
• ImageJ: Measures pore area, crack width, or fracture features from sample photos.
• PubChem: Helps you confirm chemical information for sodium hydroxide and related materials.

Experiment Steps

1. Define the exact brick formulation and choose the one variable you will change first, such as PET fraction.
2. Set up a control mix with no PET so you have a baseline for strength and cost.
3. Plan your sample groups so each PET level has enough repeats for fair comparison.
4. Decide how you will measure compressive strength, density, and visible pore structure with the same method for every sample.
5. Build a life-cycle comparison plan using open embodied-carbon data for each ingredient.
6. Predefine how you will judge the best mix, using strength, cost, and CO₂ together instead of one number alone.

Common Pitfalls

• Using PET pieces of different sizes, which changes packing density and makes the strength data hard to compare.
• Mixing the bricks by hand with no repeatable order, which creates uneven PET distribution from sample to sample.
• Comparing samples that cured under different moisture or temperature conditions, which can change geopolymer formation.
• Testing bricks before they reach the same drying state, which can make one sample look weak just because it still holds water.
• Estimating embodied CO₂ with mismatched database entries, which can produce fake precision in the final life-cycle analysis.

What Makes This Competitive

A stronger project goes past a simple strength graph. You can compare multiple PET sizes, add density or porosity measurements, and use a real statistical test instead of eyeballing trends. You can also combine mechanical data with cost and embodied CO₂, which makes the work feel like a design study, not just a materials demo. If you can explain why one mix wins on multiple criteria, your project becomes much more convincing.

Project Variations

• Test shredded bottle PET instead of pellets to see whether particle shape changes brick strength.
• Compare recycled PET with other plastic fillers, such as HDPE or PP, to see whether polymer type matters.
• Add a porosity analysis from sample photos to link internal structure with compressive strength.

Learn More

• ICE Database: A free embodied-carbon database from University of Bath, useful for comparing raw material impacts, search for the ICE database online.
• PubMed: Search review articles on geopolymers, recycled plastics, and construction materials to find background science.
• Construction and Building Materials: Search this journal for papers on PET in cement and geopolymer composites.
• Cement and Concrete Composites: Search for studies on recycled aggregate, porosity, and compressive strength.
• MIT OpenCourseWare: Look for materials science and civil engineering lecture notes on ceramics, composites, and structure-property links.