Can you float in concrete?

• Cement is the binding glue that holds concrete together • Concrete is cement plus aggregate (gravel and sand) • These terms are often confused but have distinct meanings

• 500 kilograms of cement are created yearly per person on Earth • This amount can make two cubic meters of concrete • Cement is the most important man-made substance on the planet

We produce more cementitious material than all other solid materials combined, including copper, aluminum, glass, asphalt, iron, ceramic, and wood.

• Liquid rock that can be poured into any shape • Strong, durable, and inexpensive • Easy to produce, with versions used for thousands of years

• Heat limestone (calcium carbonate) to 1,000°C to drive off CO2 • Results in quicklime (calcium oxide) • Mix with water to create calcium hydroxide • Material hardens as CO2 from air turns it back to calcium carbonate

• Cannot make large molds because CO2 cannot penetrate deeply • Does not work underwater where CO2 is unavailable

Romans added volcanic ash called pozzolana to crushed limestone before heating, creating cement that was much stronger, more durable, and could harden underwater.

• Built the Pantheon, the largest unreinforced concrete dome in the world, still standing after 2,000 years • Constructed concrete piers in the sea that hardened underwater and remain standing today

Concrete is approximately three times as dense as water.

Because of its density, a human can float in concrete with most of the body above the surface, up to the waist.

Despite being able to lift legs initially without suction issues, the dense weight creates concern about breathing pressure as concrete rises higher.

Oxygen is kept on hand as a precaution, and the participant practiced escape techniques beforehand.

For centuries, the Roman cement recipe was lost until rediscovered in a Swiss monastery book in the 1400s.

Roman cement's strength came from pozzolana (volcanic ash) containing silica, which changes cement chemistry fundamentally.

• Silica allows cement to harden without drying • Water becomes an integral part of hardened concrete • Maximum strength is achieved when setting underwater

Nearly 2,000 years later, scientists discovered that adding clay or shale to limestone before heating produces the same effect as pozzolana.

• Every time concrete is poured on a job site, sample cylinders are cast from the material • These samples are tested to ensure required strength

Concrete samples must be kept in 100% humidity, often submerged in a lime bath in the curing room.

• Samples are tested at 7, 14, and 28 days • At 28 days, concrete is considered to have reached full strength • Strength continues increasing after that milestone

Samples are placed in a hydraulic press with pressure increased at 30 psi per second until failure, measured in pounds per square inch.

Portland cement was discovered in the 1840s and named after desirable rocks quarried near Portland, England, though the name is purely marketing.

• Limestone is crushed and mixed with shale or clay for silicates • Ground into fine powder and heated to high temperatures in a kiln • Results in hard nodules called clinker

Clinker was likely discovered by accident when lime mixtures were overcooked, but grinding it produces superior cement far stronger than other known chemistry.

The most common compound in Portland cement is tricalcium silicate.

Three cylinders were tested: pure cement, cement with sand, and cement with sand and gravel (typical concrete mix).

• Pure cement failed at 8,000 psi with significant flaking and chipping • Cement plus sand failed at 9,163 psi • Cement plus sand and gravel failed at 8,300 psi

All three mixtures broke under similar pressure despite pure cement having the most binding glue, contradicting expectations.

• Cement is the most expensive part of concrete • Reducing cement to 30% of the mix still achieves the same strength characteristics • Aggregate helps the sample cohere and stay together under load

• Aggregates are blasted out of quarries • Ground up to particular sizes • Strict requirements exist for size and shape to affect concrete strength

• Well-graded concrete sand helps with finishability • Rounded particles work better than jagged crushed particles • River sand, resembling river stone, is preferred

• Normal weight concrete weighs 150 pounds per cubic foot • Lightweight aggregate can reduce weight to 110 pounds per cubic foot • Used for elevated construction to lighten deck loads

Aggregates are hauled by truck, dumped at the plant, transported on conveyor belts into storage piles, loaded into hoppers, weighed, and poured into mixer trucks.

Each concrete load has a specific recipe with precise weights for components like 3/4 rock (13,000 pounds target), 3/8 rock, sand, water, and chemical additives.

Batch operators manage the plant using a computer system that displays the recipe and automatically measures materials with tolerance controls to prevent overshooting targets.

• Water can be added to mixtures but should not be • Adding extra water can affect concrete strength • Once added, water cannot be removed from the batch

Materials are dropped from hoppers above into mixer trucks where the drum rotates continuously to blend the concrete.

• Concrete must not be too dry or too runny • Water adjustment affects strength, so alternative methods are used • The concrete for buoyancy testing had a 27-inch spread requirement

Modern superplasticizers are dispensed from units at the plant to make concrete easier to work and spread without changing water content significantly.

• A cone is filled with concrete to the top • The cone is lifted and the distance the concrete spreads is measured • Both sides must meet the target spread distance

Correct consistency allows concrete to flow and fill containers properly without being too dry, which would prevent complete filling.

Without agitation, concrete typically sets in about four hours.

The drum on concrete trucks must keep turning during transport to maintain agitation and prevent the concrete from hardening prematurely.

• If a truck breaks down, hits traffic, or encounters other delays, concrete can harden inside the drum • This is considered a terrible outcome requiring intervention

• Sugars in carbonated soft drinks like Coke slow the concrete setting process • Truck drivers carry two-liter bottles to add if needed • This can buy several additional hours before setting occurs

• Water dissolves cement powder grains releasing ions into solution • Calcium hydroxide ions make concrete a very basic solution • pH can reach 12 or 13, as corrosive as bleach on skin

• Tricalcium silicate reacts with water to form calcium silicate hydrates and other hydrate minerals • Crystals grow and become interlocking • This interlocking crystal structure causes hardening

• Water does not evaporate or dry out • Water becomes part of the solid concrete material itself • This process is called cement hydration, not drying

• Freshly poured concrete should be kept in as damp an environment as possible • Las Vegas frequently uses misters to spray new concrete and maintain high humidity • Proper humidity ensures correct hydration and strength development

• Less well-mixed with undissolved calcium oxide (quicklime) particles remaining • When cracked, water dissolves quicklime forming calcium hydroxide and new calcium carbonate growth • This self-healing property is a fascinating advantage over modern concrete

The short answer is no - Roman concrete was not overall superior to modern concrete.

We only see Roman structures that survived to the present day, creating a bias that overestimates their quality and durability.

• Modern concrete can be made very strong to last incredibly long • We choose not to prioritize longevity for cost reasons • Buildings are not expected to last as long as ancient Roman structures

The presenter discovers they are floating in concrete with most of the body above the surface, which is totally unexpected and ridiculous.

• Concrete is three times as dense as water • This density allows human bodies to float • Flotation occurs up to the waist level

• Initially worried about suction preventing exit from concrete • Able to lift legs without problem • Main concern becomes breathing pressure as concrete rises to chest

• Cannot get buried in concrete due to buoyancy • Pushing down is extremely difficult, like fighting against the material • Concrete continuously pushes the body upward out of the material

• Concrete production creates an estimated 8% of global CO2 emissions • This exceeds the entire aviation sector's emissions • A significant environmental concern requiring action

• Limestone core component comes from ancient sea life • Formed from skeletons and shells of organisms that died millions of years ago • Compressed over time into the limestone we use today

• Skyscrapers and large infrastructure are made of concrete • Modern skylines are essentially made from ancient marine life • This creates an interesting perspective on our built environment

• Individual carbon reduction actions are helpful but not sufficient alone • Systemic change requires policy advocacy through organizations like Wren and the Clean Air Task Force • Focus on new technologies and policies for zero-emissions economy