1. Understand the Goal
You want to use coffee husks or coffee waste (e.g. parchment, husk, grounds) to create a cement additive that:
- Improves compressive strength, durability, or binding properties.
- Reduces cement usage (sustainability).
- Makes use of waste from coffee processing — perfect for Kenya’s coffee ecosystem.
⚗️ 2. Scientific Background
Coffee husks and grounds contain organic carbon, silica (SiO₂), and some alumina (Al₂O₃) — key components in pozzolanic materials like volcanic ash or fly ash used in blended cements.
When properly treated, these materials can react with calcium hydroxide (Ca(OH)₂) released during cement hydration to form additional calcium silicate hydrate (C–S–H) — the main compound responsible for strength in concrete.
3. Preparation Process
Step 1: Collect and Clean
- Use coffee husks (from dry mills) or used coffee grounds.
- Remove contaminants (stones, soil, etc.).
- Wash and dry under the sun or in an oven at ~105°C until moisture <5%.
Step 2: Carbonization or Controlled Burning
You want to create biochar or ash rich in silica and carbon.
Option A – Ash (Pozzolanic Additive):
- Burn the husks at 500–700°C in a muffle furnace or kiln (low oxygen).
- This yields coffee husk ash (CHA) with high silica content.
- Cool, grind, and sieve to <75 µm.
Option B – Biochar (Filler/Strength Enhancer):
- Pyrolyze at 350–500°C in limited oxygen.
- The product is coffee husk biochar, rich in carbon and porous — improving binding and reducing microcracks in cement.
Step 3: Grinding and Blending
- Grind the ash or biochar into a fine powder.
- Mix it with Portland cement at varying replacement ratios:
- 5%, 10%, 15%, or 20% by weight.
- The optimal amount (from studies) is usually 10% replacement, balancing strength and workability.
4. Testing for Strength
To validate results:
- Prepare cement mortar cubes (1:3 cement to sand ratio).
- Cure for 7, 14, and 28 days.
- Test compressive strength (MPa).
In most studies:
- 10% coffee husk ash replacement increased compressive strength by 5–15% after 28 days.
- 20%+ tends to reduce strength due to excessive organic matter.
5. Mechanism of Strength Gain
- Silica in coffee husk ash reacts with lime to form additional C–S–H gel → stronger matrix.
- Fine particles act as micro-fillers, reducing porosity.
- Biochar enhances crack resistance and moisture retention, aiding curing.
♻️ 6. Industrial and Environmental Benefits
- Reduces CO₂ emissions by cutting cement usage.
- Utilizes local coffee waste (cheap and abundant).
- Enhances sustainability image of construction projects (green building material).
⚙️ 7. Possible Product Ideas
- KCS PozzoMix™ – coffee ash blended cement enhancer (packaged additive).
- CoffeeCrete™ – eco-friendly construction blocks or tiles with coffee ash mix.
- BioCem™ – biochar-infused cement for green housing projects.
8. Sample Mix Ratio (for trial)
| Material | Quantity (by weight) |
|---|---|
| Portland Cement | 90% |
| Coffee Husk Ash | 10% |
| Fine Sand | 300% (of cement weight) |
| Water | 0.5 w/c ratio |
Cure specimens at 25°C in moist conditions.
