HOW TO
PREVENT
ASR.
Once Alkali-Silica Reaction begins, it cannot be stopped. The chemistry is self-perpetuating. The only defensible position is verified prevention before concrete is placed, documented with a signed certificate.
Alkali silica reaction is prevented before placement, not repaired after it. The mitigation decisions are made when the mix is specified, and the record of those decisions is made on the day of the pour.
Concrete fails. ASR is the mechanism, cracking and spalling are symptoms of it. Every term on this page is defined once, as a standalone sentence, in the PSIP definitions, and the record that captures what was actually specified and delivered is the documented pour.
THREE WAYS TO
STOP ASR.
ONE MUST BE VERIFIED.
Per ASTM C1778, the standard practice for determining aggregate reactivity and selecting preventive measures, ASR can be reliably prevented by eliminating any one of its three required conditions: reactive silica, sufficient alkalis, or moisture above 80% RH. In practice, the moisture approach is not viable for pools and is unreliable for outdoor flatwork. That leaves two real strategies: eliminate reactive aggregate (expensive and often impractical), or reduce pore solution alkalinity through supplementary cementitious materials (SCMs). The SCM approach is the industry standard, and it is the approach PSIP documents on every certified pour.
Blended cement now accounts for the majority of U.S. cement shipments, and Type 1L Portland-Limestone Cement accounts for the majority of that blended share. Type 1L contains interground limestone (typically 5 to 15 percent) and behaves differently with reactive aggregates than the high-alkali ordinary Portland cement that SCM prevention strategies were originally developed for. The cement supply shift to Type 1L makes verified pozzolanic SCM more important, not less, on every pour.
The fly ash supply story is rolling, not closed. The 2018 Texas event was the first regional shock. 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) walked back previously committed coal retirements in 2025 to 2026 under EPA emissions rule rollbacks, preserving regional supply on borrowed regulatory time. Step NP (Tier 1A natural pozzolan) becomes more critical under this framing, not less. Pozzolan-side mitigation is supply-chain-independent. Class F fly ash is not.
SUPPLEMENTARY
CEMENTITIOUS MATERIALS
Natural pozzolan is a reactive aluminosilicate material that reacts with the calcium hydroxide (portlandite) in cement paste to produce additional calcium silicate hydrate (C-S-H) gel. The C-S-H reaction product binds the alkalis that drive ASR before they can attack reactive silica, densifies the matrix over time, and continues to gain strength with age. Calcined clay, calcined shale, and volcanic pozzolan all qualify as Class N pozzolans per ASTM C618. Replacement rate: 25 to 30 percent of cement by mass.
The Roman builders specified pozzolana from Pozzuoli, Italy, for the Pantheon (built 126 AD, still standing) and for the harbor structures at Pozzuoli (still functional after two thousand years). Modern petrographic analysis of marine Roman concrete (Jackson et al., American Mineralogist 2017) documented crystal formation in the pozzolan-bound matrix that continues to strengthen the concrete over time. The Bureau of Reclamation specified natural pozzolan for the Hoover Dam (built 1936); field cores extracted in 1995 and subsequent decades show continued compressive strength gain consistent with ongoing pozzolanic reaction. No other ASR mitigation strategy has comparable field-data evidence.
Supply-chain status: Natural pozzolan is supply-chain-independent. Volcanic pozzolan from the Western U.S., calcined clay and calcined shale from the Southeast and Mid-Atlantic, and volcanic ash from multiple regional sources are all increasingly available at commercial scale. The chemistry that worked for two millennia is not subject to coal plant closures or industrial supply disruption. PSIP recognizes the work of Joseph E. Thomas in advancing the natural pozzolan standard for modern residential and pool construction application.
Class F fly ash, the low-calcium, high-silica byproduct of burning bituminous or anthracite coal, has been documented as an effective ASR mitigation strategy since the 1970s. The mechanism is dual: fly ash dilutes pore solution alkali concentration (alkali dilution), and the pozzolanic reaction consumes calcium hydroxide (portlandite), the compound that sustains ASR chemistry by regenerating alkali hydroxides.
A multi-analytical study confirmed that addition of Class F fly ash to reactive mortar systems resulted in generation of significantly fewer ASR products compared to control systems. Raman spectroscopic analysis revealed aluminate monomer formation at aggregate crack interfaces, confirming that fly ash introduces an additional mitigation mechanism beyond simple alkali dilution, aluminate-bearing reaction products compete with ASR gel formation sites.
Critical limitation: The required replacement rate for reliable mitigation is 25 to 30% of cement by mass for Class F fly ash, and higher for Class C (high-calcium) ash. Consistency depends entirely on the chemical composition of the specific ash, which varies by coal source. The supply collapse that caused the Texas crisis is ongoing nationally. Class F fly ash availability will continue declining as coal plants close through 2030. A supply-chain-dependent mitigation strategy is not a documented protection strategy.
Nano silica (colloidal nanoSiO₂) operates through a different and more powerful mechanism than fly ash. At the nanoscale, particles measured in nanometers rather than microns, nano silica fills the sub-microscopic voids in the concrete matrix that Portland cement particles are too large to reach. This produces a measurably denser, less permeable matrix that physically limits moisture ingress, the trigger condition for ASR activation.
The Correct Durability Benchmark: The most disciplined concrete markets in America are in Minnesota, Wisconsin, and the Upper Midwest, states where contractors have spent a century learning what happens when you under-spec concrete in a freeze-thaw climate. In those markets, strength targets run well above code minimum, air entrainment is required, and curing discipline is non-negotiable. That is the level of discipline PSIP documents, not the minimum code that most of the country still accepts.
Calcined clay, calcined shale, or volcanic pozzolan at 25 to 30 percent cement replacement. 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. Governed by ACI 240 / ASTM C618 / AASHTO M295. 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, colloidal 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.
TIER 1B SPECIFICATION, POOL SHELL SHOTCRETE APPLICATION: The Tier 1B category for pool shell construction is a colloidal nano silica admixture formulated for shotcrete and gunite applications, batched at the ready-mix plant, providing three simultaneous functions: pozzolanic ASR mitigation (binds alkalis, reduces pore solution pH), internal curing (retains mix water in the matrix for more complete hydration on vertical surfaces), and viscosity modification (reduces rebound rate, producing denser shell sections with fewer voids). Independent testing per ASTM C39 shows increased compressive strength at 28 days versus control for documented Tier 1B colloidal nano silica admixtures.
ASTM C150 designates cement with less than 0.60% Na₂O equivalent as low-alkali cement. Using low-alkali cement reduces the initial alkali contribution from the cement paste, but does not eliminate the alkali source entirely, and cannot compensate for external alkali contributions from de-icing salts or pool water chemistry. Per ASTM C1778, low-alkali cement alone is considered an insufficient mitigation strategy for highly reactive aggregate combinations. It is most effective as a supporting measure combined with SCM use, not as a standalone approach.
WHICH APPROACH
ACTUALLY WORKS?
| Mitigation Method | ASR Reduction | Supply Reliability | Pool Application | Documentable? | PSIP Standard? |
|---|---|---|---|---|---|
| Tier 1A · Natural Pozzolan (Class N, 25 to 30%) | ✓ Effective · Two millennia of field data | ✓ Supply-independent · Regionally sourced | ✓ Works for flatwork + pools | ✓ Via batch ticket + source documentation | ✓ PSIP Tier 1A Required |
| Class F Fly Ash (25 to 30%) | ✓ Effective | ✗ Supply-dependent · Declining nationally | ✓ Works, if available and verified | ✓ Via batch ticket | Accepted, with verification |
| Class C Fly Ash | ⚠ Variable, less reliable | ✗ Supply-dependent | ⚠ Higher replacement rate needed | ✓ Via batch ticket | Not preferred, Class F required |
| Tier 1B · Colloidal Nano Silica Admixture | ✓ Expansion reduction confirmed (ASTM C1260) | ✓ Supply-independent · Modern manufactured product | ✓ Works for flatwork + pool shells | ✓ Via batch ticket + admixture lot number | ✓ PSIP Tier 1B Required |
| Slag Cement (GGBFS, 50%+) | ✓ Effective at high replacement rates | ⚠ Regional availability varies | ✓ Works for flatwork + pools | ✓ Via batch ticket | Accepted, with PSIP verification |
| Low-Alkali Cement Only | ✗ Insufficient for reactive aggregates (ASTM C1778) | ✓ Widely available | ✗ Not adequate standalone | Partial, batch ticket shows cement type | ✗ Not accepted as sole mitigation |
| No SCM (post-2018 Texas practice) | ✗ Zero mitigation | N/A | ✗ Produced the Texas crisis | ✗ Cannot document what isn't there | ✗ PSIP certification refused |
KNOWING ISN'T ENOUGH.
PROVING IT IS.
The Texas ASR lawsuits established one thing with absolute clarity: verbal assurances about concrete mix design are worth nothing in court. Pool builders who told homeowners "we always use good material" have spent years in litigation because they had no batch ticket to show, no pre-shoot photograph to produce, no signed certificate to present. The mitigation that was never documented is the mitigation that never legally happened.
PSIP's prevention protocol adds one critical layer to the chemistry: documentation. The verified Tier 1 SCM (Tier 1A natural pozzolan or Tier 1B colloidal nano silica admixture) must appear on the batch ticket. The batch ticket must be photographed before any material is placed. The photograph is GPS-timestamped. The shell thickness is measured. The curing method is documented. The certificate is signed by both the contractor and the homeowner. And the record is submitted to PSIP HQ for review.
This is not bureaucracy. This is the defensible record that separates a contractor who did everything right from a contractor who says they did everything right. In a courtroom, only one of those has value.
THE PSIP AQUATIC CRITICAL GATE: If a verified Tier 1 SCM (Tier 1A natural pozzolan or Tier 1B colloidal nano silica admixture) does not appear on the batch ticket at the time of delivery, the shoot does not begin. Not "we'll add it later." Not "the supplier said it's in there." Not "we've worked with them for years." The batch ticket shows it or the concrete goes back. This is the single requirement that would have prevented the entire Texas pool crisis.
THE CHEMISTRY
AND THE PAPER.
BOTH REQUIRED.
You now know what works, why it works, and exactly what the science says about ASR prevention. The contractors and homeowners who act on this knowledge are the ones who will be protected when the next wave of claims arrives. Everyone else will be reliving Texas.
THE THREE THINGS
TO DEMAND
WHAT PSIP
GIVES YOU
EVERY POUR
IS A CHOICE.
MAKE THE RIGHT ONE.
The science of ASR prevention is settled. The mitigation methods are documented in ASTM standards. The gap is not knowledge, it is implementation and documentation. PSIP closes both gaps, with a verified protocol and a signed certificate on every project. The institutional architecture is published on the PSIP Independence Standard; the protocol's technical governance is held by the Technical Advisory Council.