Aggregate Selection

Part of Lime & Cement

Choosing, grading, and preparing the sand, gravel, and stone that make up 60-80% of concrete and mortar by volume.

Why This Matters

When people think about concrete, they think about cement. But cement is actually the minority component — aggregate (sand, gravel, crushed stone) makes up 60-80% of concrete by volume and 70-85% by weight. The aggregate is the skeleton; the cement paste merely glues it together. Choosing the wrong aggregate — too fine, too uniform, contaminated with clay, or poorly graded — produces concrete that cracks, crumbles, or never reaches useful strength, regardless of how good your lime or cement is.

In a rebuilding scenario, you will not have concrete plants with computerized grading systems. But the Romans built the Pantheon, the Pont du Gard, and harbor structures that have lasted two millennia using hand-selected aggregates and lime-based binders. The principles they followed are simple and learnable: use clean, hard, well-graded material in the right proportions.

Understanding aggregate selection also matters because it determines what you can build. Fine sand mortar works for laying bricks and rendering walls. Coarse gravel concrete works for foundations and massive structures. The wrong aggregate for the application means structural failure — and in a post-collapse world, a collapsing building can mean death.

Types of Aggregate

Fine Aggregate (Sand)

Sand particles range from 0.1 mm to 5 mm. Fine aggregate fills the spaces between larger stones and creates the smooth matrix of mortar and concrete.

Sand Type	Source	Quality	Notes
River sand	Stream and river beds	Best	Rounded, clean, well-graded
Pit sand	Excavated from deposits	Good	Angular particles; may contain clay
Crushed stone dust	Manually crushing rock	Good	Angular; consistent mineral composition
Beach sand	Seashore	Poor	Contains salt (corrodes); too fine and uniform
Desert sand	Dune deposits	Poor	Too fine, too rounded, too uniform

Salt Contamination

Never use beach sand or sand from tidal areas without thorough washing. Salt (sodium chloride) absorbs moisture, causes efflorescence (white surface deposits), and in reinforced concrete (if you get that far), corrodes metal reinforcement. Wash beach sand in at least 3 changes of fresh water and let it drain completely.

Coarse Aggregate (Gravel and Stone)

Coarse aggregate ranges from 5 mm to 40 mm or larger. It provides the bulk structural strength of concrete.

Type	Source	Characteristics
River gravel	Riverbeds, floodplains	Rounded, clean, naturally graded
Crushed stone	Manually broken from rock	Angular, interlocks well, stronger concrete
Broken brick	Demolished structures	Acceptable for non-structural work; porous
Volcanic rock (scoria)	Volcanic regions	Lightweight; good insulation; weaker
Broken pottery	Waste ceramics	Acceptable filler; used historically

Pozzolanic Aggregates (Special)

Some aggregates react chemically with lime to produce hydraulic cement — concrete that hardens underwater and achieves greater strength than ordinary lime concrete.

• Volcanic ash/tuff: The secret of Roman concrete. Contains reactive silica and alumina.
• Crusite brick dust: Ground-up fired clay brick. A widely available pozzolan.
• Calcined clay: Any clay heated to 600-800 degrees Celsius and ground fine.

Grading: Why Size Distribution Matters

“Grading” means the distribution of particle sizes in your aggregate. Well-graded aggregate — with a mix of large, medium, and small particles — is essential because:

1. Smaller particles fill the gaps between larger ones, reducing the void space that cement paste must fill.
2. Less void space means less cement required — critical when lime production is labor-intensive.
3. Dense packing produces stronger concrete — fewer weak points, less shrinkage.

The Jar Test

A simple field test for grading:

1. Fill a clear glass jar one-third full with your aggregate sample.
2. Add water to cover. Shake vigorously for 30 seconds.
3. Let settle for 1 hour.
4. The material separates into layers: gravel on bottom, coarse sand, fine sand, silt, and clay on top.

Reading the results:

• A clearly layered jar with gravel, coarse sand, and fine sand in roughly equal proportions = well-graded.
• A jar showing mostly one particle size = poorly graded (too uniform). Add material of the missing sizes.
• A thick layer of cloudy silt or clay on top = contaminated. Wash the aggregate.

Ideal Proportions

For general-purpose concrete, aim for:

Component	Proportion by Volume
Coarse aggregate (10-40 mm)	40-50%
Fine aggregate (0.5-5 mm)	25-35%
Very fine sand (0.1-0.5 mm)	10-15%
Cement paste (lime + water)	15-20%

Quality Tests

Cleanliness Test

Dirty aggregate — contaminated with clay, silt, organic matter, or soil — produces weak concrete. Clay coats aggregate particles and prevents the cement paste from bonding to them.

Squeeze test: Grab a handful of damp sand and squeeze it. Release. Clean sand falls apart immediately. Sandy material that holds its shape like a ball contains too much clay or silt.

Rub test: Rub damp sand between your palms. Your hands should come away relatively clean. If your palms are stained brown or coated with fine residue, the sand has too much silt or clay.

Smell test: Aggregate should have no smell. A musty or organic smell indicates decaying plant material, which produces gas bubbles in concrete and weakens it.

Hardness Test

Aggregate particles must be harder than the cement paste surrounding them. Soft aggregate crumbles under load, creating weak points.

Scratch test: Try to scratch an aggregate particle with your thumbnail. If you can mark it, it is too soft. Good aggregate (granite, basalt, quartzite, hard limestone) cannot be scratched with a fingernail.

Impact test: Drop a handful of coarse aggregate onto a hard stone from shoulder height. Hard aggregate bounces and rings. Soft aggregate crumbles or thuds.

Shape Assessment

• Angular particles (from crushed rock) interlock and produce stronger concrete.
• Rounded particles (river gravel) flow more easily and are easier to work but produce slightly weaker concrete.
• Flat or elongated particles (shale, slate fragments) are weakest — they create planes of weakness. Reject aggregate with more than 15-20% flat particles.

Washing Aggregate

If your aggregate fails the cleanliness tests, wash it:

1. Place the aggregate in a container (basket, trough, barrel).
2. Add water and agitate vigorously — stir, shake, or stump with a stick.
3. Pour off the muddy water. The clay and silt wash away; the heavier sand and gravel stay.
4. Repeat until the wash water runs clear.
5. Drain and let the aggregate dry before use, or use it damp — damp aggregate is acceptable and actually preferred for mixing.

For large quantities, build a washing trough: a sloped channel with running water. Shovel aggregate into the upper end; clean material collects at the lower end while fines wash away with the current.

Proportioning for Different Applications

Application	Lime	Sand	Gravel	Notes
Brick-laying mortar	1	2-3	0	Fine sand only; no coarse aggregate
Plastering/rendering	1	2.5-3	0	Very fine sand for smooth finish
General concrete	1	2	3-4	Standard foundation mix
Massive works (dams, retaining walls)	1	2	5-6	More aggregate, less paste; use large stones
Waterproof concrete	1	1.5	3	More cement paste; add pozzolan

The Fist Rule

When proportioning by volume, use this rule of thumb: if you grab a handful of your mixed aggregate (before adding lime), you should be able to close your fist around it and feel particles of at least three distinct sizes — fine sand between your fingers, medium grit against your palm, and larger stones pressing into your hand. If it all feels the same size, your grading needs work.

Stockpiling

If you find a good source of clean, well-graded aggregate, stockpile it:

• Store on a clean surface (wooden platform, stone slab) to prevent contamination with ground soil.
• Separate coarse and fine aggregate in different piles — this lets you adjust proportions at mixing time.
• Cover with thatch or cloth if possible to keep rain from washing fines out of the pile.
• Keep aggregate piles away from soil, manure, leaf litter, or anything that could contaminate them.

A reliable aggregate source is as important as a reliable lime source. Map good deposits — riverbeds with clean gravel, rock outcrops of hard stone — and protect them as community resources.