Geopolymer Concrete Strength From Ash and Glass

ISEF Category: Environmental Engineering

Subcategory: Recycling and Waste Management  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Trash can become building material. That sounds simple, but the chemistry behind it is not. You can turn wood ash, baked clay, and crushed glass into a concrete-like material, then see how strong it really is. This kind of project connects waste reuse with real engineering tests.

What Is It?

Geopolymer concrete is a cement-like material made without regular Portland cement. Instead, you mix an aluminosilicate source, which is a material rich in aluminum and silicon, with an alkaline solution that triggers hardening. In this project, wood ash can stand in for part of the fly ash, metakaolin can come from baked kaolin clay, and crushed post-consumer glass can act as a recycled filler or sand replacement.

Think of it like making a new rock from powdered ingredients. The powder particles lock together and form a solid network as the chemistry changes. If the mix is balanced well, the final block can get quite strong. If the mix is off, the block may stay crumbly, crack early, or fail under compression.

Compressive strength means how much squeezing force a sample can take before it breaks. That makes this project a good match for a DIY hydraulic-jack rig, because you can compare different mixes under the same type of load. The result gives you a real engineering number, not just a visual guess.

Why This Is a Good Topic

This is a strong science fair topic because you can change one ingredient at a time and measure a clear outcome, compressive strength. It also connects to a real waste problem, since wood ash and glass are both materials that often end up discarded. You can learn mix design, experimental controls, and basic strength testing without needing a university lab. A careful student can also collect enough data to compare patterns instead of just picking the strongest block by eye.

Research Questions

• How does the percentage of wood ash in the binder affect compressive strength?
• How does the particle size of crushed glass affect compressive strength?
• What is the effect of replacing natural sand with recycled glass on cracking and failure mode?
• To what extent does the ratio of metakaolin to wood ash change the peak load a sample can تحمل?
• Which mix design gives the best strength-to-mass ratio for recycled-content geopolymer blocks?
• Does adding glass as a fine aggregate change the variability between repeated samples?

Basic Materials

• Kaolin clay, dry.
• Oven or kiln access for making metakaolin from kaolin clay.
• Wood ash that has been cooled, sieved, and dried.
• Crushed post-consumer glass, sorted by color if needed.
• Alkaline activator ingredients approved by your school, such as sodium silicate solution and sodium hydroxide solution.
• Digital kitchen scale with at least 0.1 g precision.
• Mixing bowls or plastic beakers.
• Sturdy mold boxes for casting test blocks.
• Ruler or digital caliper.
• Curing containers or sealed plastic bags.
• Safety goggles, gloves, and dust mask.
• Hydraulic jack compression rig or a supervised school-built loading frame.
• Measuring tape or caliper for sample dimensions.
• Notebook or spreadsheet for sample tracking.

Advanced Materials

• X-ray diffraction access for checking phase changes in metakaolin.
• Scanning electron microscopy access for looking at pore structure and glass-binder bonding.
• Universal testing machine for more precise compressive strength data.
• Particle size analyzer or sieve stack for characterizing ash and glass fractions.
• Muffle furnace for controlled calcination of kaolin and burn-off tests.
• FTIR or Raman spectroscopy for comparing binder chemistry.
• Vacuum mixer for reducing trapped air in samples.
• Precision balance with at least 0.01 g precision.
• Data-logging load cell setup for continuous stress-strain measurements.

Software & Tools

• Google Sheets: Organizes sample groups, calculates averages, and graphs strength trends.
• ImageJ: Measures crack patterns, porosity, and sample dimensions from photos.
• Python: Runs statistical tests, fits trend lines, and compares mix groups.
• GeoGebra: Helps model relationships between mix ratios and strength.
• NIH ImageJ macros forum: Helps you find free scripts for batch image measurements.

Experiment Steps

1. Define one clear mix variable, such as ash fraction, glass fraction, or particle size, and keep the others fixed.
2. Design a control mix with no recycled replacement so you have a baseline for comparison.
3. Plan a sample layout that gives you repeated blocks for each mix, so you can test variation, not just one lucky result.
4. Set up a measurement plan for sample size, mass, and failure load before you cast anything.
5. Decide how you will score failure, such as peak load, crack pattern, or strength-to-mass ratio.
6. Build a data table and graph plan so you can compare mixes with statistics, not just one-number claims.

Common Pitfalls

• Using wood ash with mixed debris or soot, which changes the chemistry from batch to batch.
• Crushing glass into uneven pieces, which makes the aggregate behave differently in every sample.
• Changing sample size or shape between trials, which makes compression results hard to compare.
• Curing blocks in different humidity or sealing conditions, which can shift strength before testing.
• Treating one strong sample as proof, which hides the large variation that often appears in recycled composites.

What Makes This Competitive

A competitive version of this project does more than compare a few recipes. You can separate the effects of ash content, glass size, and curing condition, then use statistics to test which change really matters. Strong projects also explain the failure mode, not just the peak strength. If you connect the mix design to waste reduction and show a thoughtful materials-testing method, the project feels much more like engineering research.

Project Variations

• Replace wood ash with rice husk ash to compare two waste-derived binders.
• Test glass powder instead of crushed glass to see whether finer particles improve the binder reaction.
• Compare air-cured samples with sealed-cured samples to study how moisture changes strength development.

Learn More

• USGS Mineral Commodity Summaries: Search for glass, construction aggregates, and industrial minerals data to learn the materials context.
• NIH PubMed: Search for review articles on geopolymers, fly ash alternatives, and recycled aggregate concrete.
• NASA Technical Reports Server: Search for papers on alkali-activated materials and recycled construction composites.
• MIT OpenCourseWare, Materials Science and Engineering: Find free lecture notes on cement, ceramics, and fracture behavior.
• Cement and Concrete Research: Search recent articles on geopolymer mix design, strength testing, and waste-derived binders.
• NOAA Climate.gov: Use background articles to connect lower-cement materials with carbon reduction ideas.