The Molecular Weight of Sealants: Penetrating vs. Film-Forming Concrete Sealers

A concrete sealer’s function is determined by where its active chemistry resides in the concrete matrix, not by what it looks like on the surface. A penetrating sealer works inside the pores. A film-forming sealer works on top of the surface. These are not equivalent approaches to the same problem. They address different failure modes, have different service lives, and fail in different ways.

The dividing line between penetrating and film-forming is molecular weight. Molecules small enough to enter concrete’s capillary pore structure, generally below about 1,000 g/mol for monomers and small oligomers, penetrate. Molecules too large to enter the pore structure, typical of polymer coatings above 2,000 g/mol, remain at the surface and form a film.

Our finding: for most exterior concrete applications, penetrating sealers based on silane chemistry provide superior long-term performance and lower lifetime maintenance cost than film-forming alternatives. Film-forming sealers are appropriate for specific applications that require surface-level functional properties that penetrating sealers cannot provide.

The Concrete Pore Structure

Concrete is not a solid. It is a porous matrix of calcium silicate hydrate (C-S-H) gel, calcium hydroxide crystals, and incompletely hydrated cement particles, with pores of varying sizes distributed throughout. The pore structure determines how water and water-soluble salts migrate through the concrete, which determines how the concrete weathers and deteriorates.

Three pore types are relevant to sealer chemistry:

Capillary pores (0.01–10 µm diameter): Formed by the spaces between partially hydrated cement particles. These are the primary transport pathways for water, chloride ions, and sulfate ions. Their size range is accessible to small-molecule sealers. A lower water-to-cement (w/c) ratio during mixing produces fewer and finer capillary pores, creating a denser concrete more resistant to penetration.

Gel pores (0.001–0.01 µm diameter): Within the C-S-H gel structure itself. Too small for most sealer molecules to access. Water in gel pores is considered structurally bound. Gel pores contribute to moisture movement but are not addressable by conventional sealers.

Macro pores and air voids (0.1–1.0 mm diameter): Created by entrapped air and incomplete consolidation. Visible to the naked eye in poorly consolidated concrete. Sealer molecules can enter these freely but the volume is large and coverage may be incomplete.

The effective pore size accessible to a sealer molecule determines the depth of penetration and the tortuosity of the treatment zone. For a silane monomer with a molecular diameter of approximately 0.5 nm, even the narrowest capillary pores are accessible. For a polyurethane coating molecule at 10–50 nm, capillary pore entry is physically impossible.

Silanes: The Chemistry of Penetrating Protection

Silane sealers are the most technically sophisticated category of concrete waterproofing. The active chemistry is an organosilicon compound, most commonly an alkyltrialkoxysilane such as isobutyltriethoxysilane or n-octyltriethoxysilane.

The mechanism has three stages:

Penetration: The silane molecule, small enough to enter concrete capillary pores, migrates by capillary action and vapor phase diffusion into the concrete matrix. Typical penetration depth on a well-prepared, quality concrete substrate is 3–6mm. On porous concrete (high w/c ratio, or lightweight aggregate), penetration can reach 10mm+. On very dense, low w/c ratio concrete, penetration may be limited to 1–2mm.

Hydrolysis: In the presence of the moisture naturally present within concrete, the alkoxy groups on the silane hydrolyze. Methyltriethoxysilane, for example, reacts with water to produce methylsilanetriol, releasing ethanol as a by-product. This hydrolysis reaction occurs spontaneously at ambient temperature and humidity within the concrete pore environment.

Condensation and bonding: The methylsilanetriol condenses with silanol groups on the concrete mineral surfaces (silica and calcium silicate hydrate surfaces both provide reactive silanols) and with adjacent hydrolyzed silane molecules. This produces a Si-O-Si network covalently bonded to the concrete matrix. The organic methyl groups project into the pore space, orienting hydrophobically. The result is a hydrophobic lining within the pore that repels water penetration while allowing vapor transmission.

The covalent bonding to the concrete matrix is the critical difference between silane and all surface-applied coatings. The treatment becomes part of the concrete. It cannot delaminate. It cannot peel. It does not reduce vapor transmission (water vapor molecules are small enough to pass through the treated pore lining). It does not change the appearance of the concrete surface.

Bold Takeaway: Silane protection is not a coating. It is a chemical modification of the concrete pore surface. This is why it cannot delaminate and why it does not require reapplication on the schedule that coatings do.

Siloxanes: The Oligomeric Alternative

Siloxanes are oligomers (short polymer chains) of siloxane units, with molecular weights typically in the 400–800 g/mol range. They are larger than silane monomers but still small enough to penetrate concrete capillary pores, though with less depth than monomers.

Siloxanes react by a similar hydrolysis-condensation mechanism to silanes, but their oligomeric size creates a denser treatment zone near the concrete surface rather than the distributed treatment through the depth provided by monomers.

The practical consequence: siloxane treatments provide excellent surface waterproofing performance but are more affected by surface contamination than silanes, because the larger molecules are more easily blocked from pore entry by surface films, curing compound residue, or prior sealer remnants. Silane treatments, due to their smaller size and greater penetration depth, are more robust to minor surface contamination.

Silane-siloxane blends attempt to combine the deep penetration of silane monomers with the dense surface treatment of siloxanes. These are widely used commercial products (Prosoco’s Consolideck LS, for example, or Chemmaster’s Silasec) and represent a practical engineering compromise for most residential concrete applications.

Film-Forming Sealers: The Surface Protection Category

Film-forming sealers create a continuous polymer film on the concrete surface. The film provides protection by functioning as a physical barrier against liquid water entry and, depending on the polymer, against abrasion, chemical attack, and UV degradation.

Acrylic sealers are the most widely used decorative concrete sealers in the residential market. They are available in solvent-borne and water-borne formulations, with solids content typically 20–30% by weight. Molecular weights vary widely but are generally in the range where film formation occurs rather than penetration. Acrylics are UV-stable (acrylic polymer does not yellow significantly under UV exposure), available in matte to high-gloss finishes, and provide moderate water resistance and reasonable concrete protection.

Acrylic sealers are appropriate for decorative concrete (stamped concrete, colored concrete, concrete overlays) where the surface appearance requires enhancement and where the aesthetic justifies the maintenance cycle. Acrylic films require reapplication every 1–3 years on exterior surfaces, more frequently under heavy traffic or UV exposure, because the film degrades by UV photolysis and abrasion.

Epoxy coatings are two-component systems consisting of an epoxy resin and an amine hardener. When mixed, the amine reacts with epoxide groups in the resin to form a thermoset polymer network with high tensile strength, excellent adhesion (on prepared surfaces), and very good chemical resistance. Molecular weights of the cured film are effectively infinite (crosslinked network). Epoxy coatings are appropriate for garage floors, industrial floors, and laboratory surfaces where abrasion resistance and chemical resistance are primary requirements.

Epoxy coatings are sensitive to moisture vapor transmission from below (vapor drives delamination), UV exposure (aromatic epoxies yellow and chalk; aliphatic hardeners are more stable), and surface preparation adequacy (epoxy bond failure is almost always surface preparation failure, not an epoxy quality problem).

Polyurethane coatings provide better UV resistance than most epoxies (aliphatic polyurethanes are the standard choice for exterior applications) and excellent abrasion resistance. They are often used as topcoats over epoxy base coats in industrial floor system applications. For residential concrete, polyurethane coatings are appropriate for exterior flatwork (driveways, patios) where UV resistance is needed.

Bold Takeaway: Film-forming sealers require surface preparation, maintenance reapplication cycles, and careful management of vapor transmission from below. Penetrating sealers do not. For exterior, below-grade, or high-moisture-exposure concrete, penetrating chemistry is the correct specification baseline.

Surface Preparation: Why Chemistry Is Irrelevant Without It

The correct sealer applied to an inadequately prepared concrete surface will underperform or fail regardless of the chemistry’s quality. Surface preparation is not a pre-treatment step. It is the primary determinant of sealer performance.

For penetrating sealers: The concrete pores must be open. Any surface film, curing compound, existing sealer, or contamination blocks pore access. The standard test for penetrating sealer suitability is the water drop test: place water drops on the surface and observe penetration. If the water beads, a water-repellent material is already present. If it sits flat without penetrating (sitting for more than 2–3 minutes), the surface is blocked and must be cleaned or mechanically prepared before treatment.

Surface cleaning for penetrating sealers requires removal of all chemical residues. Pressure washing alone does not remove curing compounds or silane-based form-release agents, which are hydrophobic and water-insoluble. Chemical cleaning with a curing compound remover or mechanical abrasion (light grinding or shot blasting) is required.

For film-forming sealers: The surface must be clean, dry, and structurally sound. Moisture vapor transmission from the substrate is the primary threat to film adhesion. The ASTM D4263 polyethylene sheet test (tape a 450mm square sheet of plastic to the floor for 16 hours and observe moisture condensation) is the standard field test for moisture vapor transmission. If significant moisture is present, a moisture-tolerant primer or penetrating silane pre-treatment is required before the film-forming sealer.

Surface profile for film-forming sealers is also important. Smooth, trowel-finished concrete surfaces may require grinding or acid etching to create a mechanical anchor profile for coating adhesion. The ICRI surface profile scale (CSP 1–9) provides standardized descriptions of surface roughness; most epoxy and polyurethane coating manufacturers specify minimum CSP 2–3 for adhesion.

Substrate Density and the w/c Ratio: When Penetrating Sealers Are Limited

Silane-based penetrating sealers work through a diffusion mechanism that depends on the concrete pore structure being accessible. Very dense concrete, produced with a low water-to-cement ratio (w/c of 0.30–0.35 versus the typical residential 0.45–0.55), has a finer capillary pore structure. Silane penetration depth is reduced. Treatment effectiveness is still achieved, but the margin between the treated zone depth and the concrete surface is smaller.

For concrete produced with supplementary cementitious materials (fly ash, slag, silica fume), which further refine the pore structure, penetrating sealer effectiveness at standard treatment rates may be limited. In these cases, penetrating densifiers (lithium or sodium silicates that react with calcium hydroxide to form additional C-S-H gel) are often specified alongside or instead of silane treatments to refine the pore structure further and increase density.

For residential applications, the w/c ratio concern is typically secondary. Ready-mix concrete delivered for residential flatwork in the United States is typically produced at w/c 0.45–0.55, which provides adequate porosity for effective silane treatment. Higher w/c ratios (above 0.55) produce more porous concrete that actually benefits more from silane treatment due to greater water and chloride transport risk.

Application Rates and Coverage: The Overlooked Specification

Penetrating silane sealers must be applied at sufficient rate to achieve the required penetration depth. Under-application is a common performance failure: the silane penetrates, but only to a depth of 1–2mm, where concrete carbonation and wear abrade through the treatment zone within 2–3 years.

ASTM C1501 (Test Method for Water Absorption of Hydraulic Cement-Based Materials) and the product manufacturer’s application specifications define the correct coverage rate for the specific concrete porosity. For typical residential concrete flatwork, silane coverage rates of 4–6 m² per liter (approximately 40–60 sq ft per gallon) are common. More porous concrete absorbs more sealer and requires higher application rates to achieve treatment depth.

The “flood coat” method for silane application, maintaining a wet front on the surface for 5–10 minutes before the surface goes dry, ensures maximum penetration relative to single-pass application. For exterior vertical surfaces (retaining walls, foundation walls), multiple applications with time between coats may be needed.

As we cover in our guide to exterior materials maintenance, the relationship between surface preparation, material selection, and application technique determines performance more than product brand selection in most cases. This is as true for concrete sealers as for any other maintenance material.

For those specifying concrete finishes in a new kitchen or bathroom remodel, the considerations in our bathroom tile selection guide address the substrate preparation requirements for tile installation, which overlap with the surface preparation principles relevant to sealer selection.

The selection decision simplifies to this: if your objective is long-term water resistance with minimal maintenance and without changing the appearance of the concrete, specify a silane or silane-siloxane blend penetrating sealer. If your objective is enhanced appearance, increased abrasion resistance, or a functional coating surface, specify the appropriate film-forming sealer and plan for the maintenance cycle it requires.