Concrete Making

Part of Lime & Cement

Combining lime binder, sand, gravel, and water into concrete — the most versatile structural material available without industrial infrastructure.

Why This Matters

Concrete is arguably the single most important construction material in human history. It can be molded into any shape, sets into a stone-like mass, resists fire, weather, and insects, and can be made from materials found almost everywhere on Earth. The Romans built roads, aqueducts, harbors, and the Pantheon’s unreinforced concrete dome — still the largest in the world — using nothing more than lime, volcanic ash, aggregate, and water.

In a rebuilding scenario, concrete gives you capabilities that no other material provides. You can cast foundations, build water tanks, construct bridges and retaining walls, and create flat roofs and floors — all without the precision joinery that timber framing demands or the skilled cutting that dressed stone requires. Anyone can mix concrete. The challenge is mixing it correctly so the result is strong, durable, and fit for purpose.

This article covers the complete concrete-making process using lime-based binders — the only type achievable without industrial kilns capable of Portland cement temperatures (1,450 degrees Celsius). Lime concrete is slower to cure than modern Portland cement concrete, but it is more flexible, self-healing, and has proven its durability over millennia.

Understanding Lime Concrete

Components

Every concrete mix has four components:

Component	Function	Proportion (by volume)
Binder (lime)	Glues everything together	1 part
Fine aggregate (sand)	Fills gaps between coarse aggregate	2 parts
Coarse aggregate (gravel/stone)	Provides bulk and strength	3-4 parts
Water	Activates the binder; makes the mix workable	As needed

Types of Lime Binder

Your binder choice determines what you can build:

Non-hydraulic lime (air lime):

• Made from pure limestone
• Sets only by absorbing CO2 from air (carbonation)
• Will NOT set underwater or in thick sections
• Suitable for: thin renders, plaster, mortar in above-water masonry
• Setting time: weeks to months

Hydraulic lime:

• Made from limestone containing clay impurities (naturally) or by adding pozzolans to air lime
• Sets by chemical reaction with water (hydration) AND by carbonation
• Will set underwater and in thick sections
• Suitable for: foundations, cisterns, bridge piers, any structural work
• Setting time: days to weeks

For Structural Concrete, You Need Hydraulic Lime

Non-hydraulic lime concrete is too weak and too slow for foundations, load-bearing walls, or any significant structure. Always use hydraulic lime or add pozzolanic material to your lime. See the pozzolan section below.

Making Pozzolanic Lime Concrete

If you do not have natural hydraulic lime (from clay-bearing limestone), you can create hydraulic concrete by adding pozzolanic material to ordinary lime.

What Are Pozzolans?

Pozzolans are materials containing reactive silica and alumina that react chemically with lime in the presence of water to form calcium silicate hydrate — the same compound that gives Portland cement its strength. They transform weak, air-setting lime into strong, water-setting cement.

Pozzolan	Availability	Effectiveness
Volcanic ash/tuff	Volcanic regions	Excellent — the original Roman pozzolan
Crushed fired brick	Anywhere with brick-making	Very good — widely available
Calcined clay	Anywhere with clay deposits	Good — heat clay to 700 degrees Celsius, grind fine
Rice husk ash	Rice-growing regions	Good — high silica content
Wood ash (selected)	Everywhere	Poor-moderate — variable quality
Diatomaceous earth	Near lakes/marine deposits	Good — high reactive silica

Pozzolanic Concrete Formula

Component	Parts by Volume
Lime putty or slaked lime	1
Pozzolan (crushed brick dust, volcanic ash, etc.)	1
Sand	2
Gravel (10-40 mm)	3-4
Water	As needed for workability

The pozzolan replaces half the sand in a standard mix. Some formulas use even more pozzolan for greater strength and faster setting.

The Mixing Process

Preparation

1. Screen all aggregates. Remove organic matter, clay lumps, and oversized stones.
2. Prepare lime. If using quicklime, slake it at least 24 hours before mixing (preferably weeks — aged lime putty makes stronger concrete). If using lime putty, ensure it is smooth and lump-free.
3. Measure components. Use consistent volume measures — a bucket, a box, a basket. The exact size does not matter as long as you use the same measure for all components.

Mixing Steps

Step 1: Dry mix the aggregates

Combine the sand, pozzolan, and gravel on a clean, flat mixing surface (a wooden platform, flat rock, or packed earth). Mix by turning the pile with shovels or hoes until the color is uniform — no streaks of pure sand or unmixed pozzolan visible.

Step 2: Add lime

Spread the lime putty over the aggregate pile. If using dry hydrated lime, add it to the dry aggregate mix.

Step 3: Mix thoroughly

Turn the pile repeatedly, folding the material from the edges into the center. The goal is complete, uniform distribution of lime throughout the aggregate. This requires at least 5-10 minutes of vigorous mixing for each batch.

Step 4: Add water

Add water gradually — never dump it all in at once. Mix and add, mix and add, until the concrete reaches the right consistency.

Getting the Water Right

The water-to-binder ratio is the single most important factor controlling concrete strength. Too little water and the mix is unworkable and will not compact properly, leaving voids. Too much water and the excess water creates pores as it evaporates, weakening the concrete dramatically.

The slump test:

1. Fill a bucket or cone-shaped vessel with your wet concrete mix, packing it down.
2. Turn it over and lift the container.
3. The concrete should slump slightly — settling 3-5 cm from its original height.
4. If it holds its shape rigidly — too dry, add water.
5. If it collapses into a puddle — too wet, add more dry aggregate.

The squeeze test:

Grab a handful of the wet mix and squeeze firmly. Open your hand:

• Mix holds its shape and your hand is damp but not wet = correct consistency
• Mix crumbles apart = too dry
• Water squeezes out between your fingers = too wet

The Driest Workable Mix

Always aim for the driest mix you can still place and compact. Wetter mixes are easier to work but produce weaker concrete. If a slightly drier mix means you have to work harder packing it into place, that extra effort is worth it — the result is significantly stronger.

Placing and Compacting

Formwork

For foundations, walls, and other shaped structures, you need formwork — a temporary mold that holds the wet concrete in shape until it sets.

• Timber boards: The most common formwork material. Oil or wet the boards before pouring so concrete does not stick.
• Stone walls: For foundations, the excavation walls themselves can serve as formwork.
• Woven screens: For curved shapes, bend flexible screens (woven branches, bamboo) into shape and pour against them.

Placing

1. Place in layers of 10-15 cm thickness. Do not dump the full depth at once — it is impossible to compact thick layers properly.

2. Compact each layer by tamping with a heavy wooden ram, stomping with feet, or pounding with a mallet. Compaction removes air pockets that become weak points. Continue until water just starts to appear on the surface.

3. Score the top of each layer before placing the next one. This provides a key (rough surface) for the next layer to bond to.

4. Work continuously. Do not leave a partially completed pour for hours — the lower layer begins to set and will not bond properly with the upper layer. Plan to complete each structural element in one continuous session.

Joints

If you must stop and restart (a “cold joint”), follow these steps:

1. Leave the surface rough (do not smooth it).
2. When resuming, dampen the old surface thoroughly.
3. Apply a thin layer of lime-pozzolan paste (no aggregate) as a bonding slurry before pouring the next batch.

Curing

Curing is the period after placement when the concrete gains strength. Lime concrete cures by two mechanisms:

1. Hydraulic set (pozzolanic reaction): Chemical reaction between lime and pozzolan, requiring water. Occurs in the first days to weeks.
2. Carbonation: Lime absorbs CO2 from air and converts back to calcium carbonate. Occurs over weeks to months (and continues for years in thick sections).

Curing Requirements

• Keep the concrete damp for at least 7-14 days. Cover with wet cloths, straw, or soil. Sprinkle with water daily if the covering dries out.
• Protect from freezing. Lime concrete that freezes before it has set will crumble. Do not pour in freezing weather, or insulate heavily.
• Protect from direct sun and wind. Rapid surface drying causes shrinkage cracks. Cover or shade new concrete.
• Do not load the concrete for at least 28 days. Lime concrete gains strength much more slowly than Portland cement concrete. Full strength may take 3-6 months.

Strength Timeline

Age	Approximate Strength (% of final)
7 days	15-25%
28 days	40-60%
90 days	70-85%
1 year	90-100%
10+ years	Can exceed 100% (continued carbonation)

Patience Is Structural Integrity

The single most common mistake with lime concrete is loading it too early. It feels hard and looks solid after a week, but it has less than a quarter of its eventual strength. Wait. Build the next section of wall, work on another project, but do not place heavy loads on fresh concrete. Time is the cheapest reinforcement you have.

Troubleshooting

Problem	Cause	Prevention
Surface cracking (map cracking)	Dried too fast	Cover and keep damp during curing
Crumbling, weak concrete	Too much water in mix	Use drier mix; compact thoroughly
Layers separating (delamination)	Cold joint not bonded	Wet old surface; apply bonding slurry
Surface dusting	Insufficient lime binder	Increase lime proportion; cure longer
Concrete will not set	Using non-hydraulic lime without pozzolan	Add pozzolanic material
Efflorescence (white surface deposits)	Salt in aggregate or water	Use clean aggregate; wash before use