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ANCIENTLY, THE ROMANS USED a mixture of hydrated lime and fine-grained pumice to form strong and incredibly durable concrete, mortar and plaster—and the evidence of that cementitious wisdom still stands some 2000 years later.



bigThe Romans didn’t have a quick setting cement like we do today, rather, they used hydrated lime—a cementitious product made from limestone that was heated to drive off the carbon dioxide and transform the Calcium Carbonate into Calcium Oxide (lime)—and water. Lime does not act like a hydraulic cement on its own, and will only form Calcium Silicate Hydrate (CSH)—the all-important cementitious binder that makes concrete work—in the presence of water and pumice (pozzolan).

When lime and water are mixed with a finely graded amorphous silica (known to the Romans as pulvis puteolanus; referred to today as volcanic ash or pumice pozzolan), nearly 100% of the lime is converted to the cementitious binder chemical CSH. That's the key: in Portland-cement-based formulations, the hydraulic reaction that forms the CSH binder happens quickly and without the need for a pozzolan, but the reaction forms a trouble-making by-product—Calcium Hydroxide (CH)—at the expense of CSH.

This CH by-product not only contributes nothing to concrete strength and durability, it instigates a handful of problems that actively work against and drastically affect the integrity of the concrete (and plaster). When the CH migrates out of the concrete’s interior via capillary action, it leaves behind a network of density-compromising wormholes that both weaken the concrete and allow for the future ingress of water. This infiltrating water can contain sulfates, chlorides, and other damaging chemicals.

In the case of LimeStrong plasters, our simple lime + pozzolan + water formulation triggers none of the deleterious by-products that make modern plaster, mortar and concrete—made using Portland cement—so problematic, and short-lived. Thus, Roman methods produce densified, flexible, self-healing mortars and plasters that are not nearly as susceptible to brittleness, chemical attacks, and other problems that plague modern cement products.

As you might expect, the addition of a high-performance natural pozzolan, like finely-ground pumice, will improve modern concrete performance as well. An extensive body of research details how pozzolan-charged concrete mix designs ignite a secondary (pozzolanic) reaction that captures deleterious CH and turns it into additional CSH. And that newly-created CSH does what you’d expect: it further densifies and strengthens the concrete, welding the aggregates into a dense, durable, almost impermeable matrix. This pozzolanic reaction—a molecular reclamation process, if you will—continues until the pozzolan is used up.


Under the guidance of Stan Petersen, the renowned expert in old-world masonry and lime plastering, Hess Pumice formulated LimeStrong: a natural pozzolan and lime plaster product that puts the strength, durability and beauty of old-world genius in the capable hands of today’s plasterers.

LimeStrong is safe to use and simple to mix, handle and apply. It is ideal for both outside and inside finishes, and creativity with color and texture is unlimited: a LimeStrong finish can be worked down to a glass-smooth finish or left rustically rough.

Naturally white in color, LimeStrong is easily tinted and colored using natural oxide color pigments.

Like Roman plaster finishes, LimeStrong finishes are impressively-enduring, are flexible and autogenously heal, give off no volatile organic compounds, provide ample working times, can be mixed and applied in a wide range of weather conditions, and result in unique and beautiful finishes—a medley limited only by the creativity and skill of the hand that applies it.

“Withstanding the test of time” is a common cliché, but is fully applicable when considering the physical body of evidence on and in countless structures world-wide—lime pozzolan plasters last and last and last.


» Ancient Roman Concrete Is About to Revolutionize Modern Architecture (BusinessWeek)
» The Secrets of Ancient Roman Concrete (History Channel)
» Riddle of Ancient Roman Concrete
» Roman Concrete (Wikipedia)

» Hess Pozz: High Performance Concrete (Website)
» How Pumice Pozzolans Super-Charge Concrete Performance (PDF)
» Pumice Pozzolan vs. Fly Ash (PDF)
» The Pozzolanic Reaction (InfoGraphic)