CONCRETE
CANCER
EXPLAINED.
Alkali-Silica Reaction is a chemical war happening inside your concrete right now, and it started the day the concrete was poured. Here is what it is, how it works, and why the industry failed to stop it.
Alkali silica reaction is an internal expansive reaction between reactive silica in the aggregate and alkalis in the cement paste. It is the most serious mechanism of concrete failure, and it is sometimes called concrete cancer.
ASR is a mechanism, not a symptom. Concrete fails. Map cracking, spalling and dimensional movement are what an owner sees on the surface. The reaction is what is producing them. Every term on this page is defined once, as a standalone sentence, in the PSIP definitions, and the wider failure picture is at The Problem.
WHAT IS ACTUALLY
HAPPENING
INSIDE YOUR CONCRETE
Concrete cancer, formally called Alkali-Silica Reaction (ASR), is not a manufacturing defect, a cheap contractor, or bad luck. It is chemistry. Specifically, it is a reaction between two materials that are both inherent to concrete: the alkali hydroxides (sodium and potassium) in Portland cement paste, and the reactive amorphous silica present in many common aggregates.
When water is present, which it always is in pools, driveways, and any structure exposed to weather, these materials react. The result is a reaction product: sodium silicate hydrate. As that product precipitates inside the confined space of a concrete aggregate particle, it generates solidification pressure. The reaction occurs in aqueous pore solution throughout, but the pressure does not come from the product absorbing water.
Research using the extended Surface Force Apparatus measured a solidification pressure of 6 to 13 MPa as ASR products precipitate in confined conditions. The authors identify that solidification pressure, and not water absorption by the reaction product, as the mechanism that cracks aggregate particles and expands ASR affected concrete. This is a laboratory model of an aggregate's internal surfaces, not a field measurement.
ASR NEEDS THREE
THINGS. ALL THREE
ARE IN YOUR CONCRETE.
Per ACI 221.1R and the American Concrete Institute's documented research, ASR requires exactly three conditions. The reason this matters is that preventing any one of the three stops the reaction entirely. The reason it is so widespread is that all three exist by default in most concrete mixes, and the industry removed the one additive that was managing this problem.
Amorphous (non-crystalline) silica is present in most aggregate sources, quartzite, chert, opaline materials, volcanic glasses, and certain types of sand. In Texas, the limestone-based aggregates common in Central Texas contain reactive silica phases. In Tennessee, river sand aggregates can contain reactive quartz. In Florida, silica-rich sands are the dominant aggregate source. The ACI confirms that "almost all concretes will eventually be impacted", because reactive silica is in virtually every aggregate source in America.
Portland cement produces a pore solution with a pH of 12.5 to 13.5. The alkali content, primarily sodium oxide (Na₂O) and potassium oxide (K₂O), is inherent to the cement manufacturing process. ASTM C150 designates cements with more than 0.6% Na₂O equivalent as high-alkali cements. Most standard Portland cement and Type 1L Portland Limestone Cement fall into this category. The alkalis that drive ASR are not contamination, they are a fundamental product of cement chemistry.
Water is the catalyst. Per peer-reviewed research published in PMC (National Institutes of Health, 2024), ASR stops when internal relative humidity drops below 80%. For pools, constantly filled with water, that threshold is always exceeded. For driveways, patios, and pool decks in Tennessee, Florida, Texas, and Georgia, all high-humidity environments, the moisture threshold is routinely exceeded for months every year. The reaction only needs enough time and enough water to do permanent damage.
THE IRREVERSIBILITY PROBLEM: Once ASR begins, the alkali hydroxides (NaOH and KOH) are continuously regenerated by the reaction itself, they react with calcium hydroxide (portlandite) in the cement paste, which releases them back into solution to attack more silica. Per Wikipedia's documented chemistry: "It is impossible to interrupt the ASR reaction." The only protection is prevention before the pour.
FIVE FRAMES.
CHEMISTRY IN PROGRESS.
Continuing from the interfacial transition zone shown above, the active alkali-silica reaction proceeds through a documented chemical sequence. The reaction proceeds through a documented chemical sequence.
The five frames below visualize each step, from the alkali pore solution filling the open interfacial space, to silica attack, to gel formation, to pressure expansion, to the bond failure that initiates surface-visible cracking. This chemistry proceeds in this exact sequence inside every untreated reactive-aggregate pour from the moment of placement.
WHAT ASR
LOOKS LIKE
ASR produces a distinctive pattern that distinguishes it from other types of concrete cracking. Normal shrinkage cracks follow straight lines along control joints or slab edges. Freeze-thaw damage produces surface scaling. ASR looks different, and once you know what to look for, it is unmistakable.
Random, multi-directional cracking that resembles a road map or alligator skin. Unlike shrinkage cracks that follow straight lines, ASR cracks radiate in all directions from aggregate particles. This pattern is caused by expansion occurring simultaneously throughout the concrete matrix rather than at a specific stress concentration point.
A white, gel-like substance that appears wet or glistening at crack faces. This is the sodium silicate hydrate gel that has expanded beyond the aggregate boundary and pushed through cracks to the surface. In pools, this substance may be washed away by water, but its presence in a core sample confirms ASR definitively. Lab confirmation requires petrographic analysis of 30-micron thin sections under polarized light.
Visible lifting or displacement at crack edges, one side of a crack is higher than the other. This occurs because the aggregate expansion is creating net outward pressure that causes sections of concrete to shift relative to adjacent sections. In pool shells, this manifests as tile loss, coping displacement, and structural failure at wall-to-floor transitions.
Studies published in the Concrete Society's Magazine (2024) confirm that ASR causes up to 35% loss in compressive strength and up to 24% loss in tensile strength once gel reaches the concrete surface. The critical problem: by the time surface symptoms appear, significant internal damage has already occurred. The concrete that looked structurally sound last year may already have lost a third of its load-bearing capacity.
ASR-cracked concrete absorbs water far more readily than intact concrete. In Tennessee and the mountain South, this means that winter freeze-thaw cycles operate inside a concrete matrix that ASR has already compromised. The two failure mechanisms accelerate each other exponentially. What might have been surface scaling becomes structural failure.
ASR is definitively confirmed by petrographic examination of thin concrete sections (30 microns) under plane-polarised and ultraviolet light, identifying ASR gel in aggregate pores. X-ray diffraction (XRD) can quantify crystalline ASR products. ASTM C1293 (2-year concrete prism test) and ASTM C1260 (16-day mortar bar test) are the standard methods for new aggregate evaluation. Core drilling from an affected structure followed by lab analysis is the only definitive confirmation method.
THE FLY ASH
COLLAPSE MADE
ASR INEVITABLE.
ASR has been documented since Thomas Stanton's foundational research in 1940. For eight decades, the industry managed it, primarily through the use of fly ash as a supplementary cementitious material in concrete mixes. Fly ash worked by reducing pore solution alkalinity through pozzolanic reaction, binding the alkalis that drive ASR before they could attack reactive silica. It was not a perfect solution, but it was a working solution.
In 2018, three major Texas coal plants closed simultaneously, removing a critical regional source of Class F fly ash. Concrete suppliers quietly removed fly ash from mix designs. Pool builders, flatwork contractors, and homeowners were never notified. The concrete that arrived on job sites in 2018, 2019, 2020, and 2021 contained the same reactive aggregates and the same alkali-rich cement, but no longer contained the pozzolan that had been managing the reaction for decades.
PUBLISHED IN ASTM: "It is well chronicled that coal fly ash can mitigate the risk of alkali-silica reaction (ASR) in concrete structures." The mechanism: fly ash dilutes pore solution alkalis, consumes calcium hydroxide (portlandite) through pozzolanic reaction, and reduces the availability of alkalis for ASR. Without it, or without a verified alternative, ASR in reactive aggregate environments is not a question of if but when, wherever there is reactive aggregate, moisture, and no mitigation in the mix.
The result was not visible immediately. ASR incubates. The internal cracking begins within months of placement, but surface symptoms typically appear two to four years later in pool shell construction, longer for residential flatwork. The pools built in 2019 started cracking visibly in 2021. The driveways poured in 2020 are cracking now.
The 2018 Texas event was the first regional shock. The pattern has now generalized. 23 coal-fired units at 15 plants stopped burning coal in 2025 alone (EIA). ACAA's 2025 Production and Use Survey documents 14.6 million tons of fly ash consumed in 2024 concrete production, up 2.7 million tons year over year, against a contracting supply base. Multiple major utilities (TVA, Georgia Power, Duke Energy Carolinas) have walked back previously committed coal retirements in 2025 to 2026 under EPA emissions rule rollbacks, preserving regional supply on borrowed regulatory time. The chemistry crisis is not closed. It is rolling, region by region, on timelines that vary by state and by federal regulatory cycle.
SCM REPLACEMENT
STRATEGIES THAT WORK.
With traditional fly ash supply declining nationally as coal plants close through 2030, verified alternative SCMs are no longer optional, they are the only documented path to ASR protection. PSIP documents one of the following on certified pours, confirmed on the batch ticket before any material is placed. The hierarchy reflects the locked PSIP doctrine: two parallel Tier 1 categories anchor the protocol, each with a distinct evidence base.
Calcined clay, shale, or volcanic pozzolan at 25 to 30 percent replacement minimum. The chemistry the Roman builders specified for the Pantheon and the Pozzuoli harbor structures, both standing two thousand years later. The chemistry the Bureau of Reclamation specified for Hoover Dam, documented in continuous strength gain over ninety years. Supply-chain-independent. Regionally sourced. PSIP recognizes the work of Joseph E. Thomas in advancing the natural pozzolan standard.
Verified colloidal nano silica admixture, batched at plant. Mechanism: nano silica particles fill micro-voids cement cannot reach, binding alkalis and reducing pore solution pH below ASR activation threshold. Supply-chain-stable modern admixture.
Minimum 25 to 30 percent replacement by mass of cement. Must be Class F per ASTM C618 (low calcium, high silica), Class C is not equivalent and is less reliable for ASR mitigation. Must be confirmed on the batch ticket with source documentation. Verbal assurance is not accepted under PSIP. Class F has worked historically, but the 2018 Texas supply collapse demonstrated that fly ash availability is not reliable across all regions; natural pozzolan (Tier 1A) is the structurally preferred substitute where regional supply supports it.
Fifty percent or higher replacement rate required for reliable ASR mitigation. Acceptable under PSIP with batch ticket verification and documented replacement rate. Requires longer cure times and careful cold-weather management.
ASTM C1778 designates low-alkali cement alone as insufficient for reactive aggregate combinations. No SCM at all is the Texas 2018 scenario, zero ASR mitigation. Not accepted under PSIP under any circumstances.
When a doctor adds something to a vaccine to make it more effective, an adjuvant, it's not the main ingredient, but it changes what the main ingredient can do. SCMs like natural pozzolan, nano silica, and fly ash work the same way in concrete. They're not the cement. But when they're in the mix, they change how the cement behaves, reducing the chemical activity that causes failure, filling the microscopic pores that let in water, and making the whole matrix denser and more resistant. Take them out and the concrete is exposed. That's exactly what happened to Texas pools after 2018.
When you buy a bottle of supplements, the label tells you exactly what's in it, every ingredient, every amount. A concrete batch ticket is the nutrition label for your concrete. It lists the cement type, the PSI strength, the water-cement ratio, and every admixture in the mix. If your contractor can't show you the batch ticket, or if the batch ticket shows fly ash at zero and no alternative SCM, you have no idea what's actually in the structure you just paid for. PSIP-certified contractors verify the batch ticket before a single yard is placed.
WHAT YOU DO WITH
THIS INFORMATION
DEFINES EVERYTHING.
You just read the peer-reviewed science of concrete cancer. Most contractors doing work in your market today cannot explain what you just learned. That gap is either your problem, or your protection.
QUESTIONS TO ASK
BEFORE YOU SIGN
WHAT YOU'RE CARRYING
RIGHT NOW
ASR CANNOT BE
REVERSED.
IT CAN BE PREVENTED.
The science is settled. The chemistry cannot be stopped once it starts. The only defense is verifying that a Tier 1 SCM (Tier 1A natural pozzolan or Tier 1B colloidal nano silica admixture) was in the mix before placement, and documenting it with a signed certificate. PSIP is that documentation system.