By Wiljee Blom, N-Dip (Civ. Eng.), Stage 3 Concrete Technologist, Technical Manager, Penetron Africa

Chemical resistance in concrete is often misunderstood as an inherent property. In my experience, this is where the misunderstanding begins. Chemical resistance is not an attribute per se, but the outcome of several interacting mechanisms acting simultaneously on a material that is inherently permeable, chemically reactive and, although seemingly homogeneous, structurally heterogeneous.

When concrete deteriorates in aggressive environments, it is seldom because it has encountered unknown or exotic chemicals. More often, the exposure conditions are entirely predictable. The internal chemistry of concrete is equally predictable. Deterioration occurs where these two predictable systems interact without sufficient control.

Understanding chemical resistance, therefore, requires understanding permeability, internal reactivity and, most importantly, the depth to which aggressive agents are permitted to penetrate.

Concrete Is Not Inert

Concrete, once hardened, is not chemically inactive. It is an alkaline composite material with a complex and highly interconnected internal structure composed of capillary tacts, pores and microcracks. These features form a continuous network through which fluids and dissolved ions are able to move.

Transport mechanisms include permeability under pressure, diffusion driven by concentration gradients, capillary absorption, osmosis and Brownian movement at the microscale. Together, these mechanisms mean that concrete naturally draws fluids into itself. Wherever those fluids penetrate, chemical reactions will occur.

​Fresh cement paste maintains a pore solution pH of approximately twelve, largely due to the presence of calcium hydroxide, commonly referred to as free lime. This high alkalinity stabilises cement hydrates and protects embedded reinforcement by maintaining passivation of the steel. When exposure conditions reduce alkalinity or introduce aggressive reagents, the cementitious matrix begins to change.

The issue is not whether chemical reactions occur. They will. The question is how much of the concrete mass becomes involved.

Dissolutive and Non-Dissolutive Attack

In practice, chemical attack on concrete can be broadly categorised into two forms: dissolutive and non-dissolutive.

Dissolutive attack occurs where reaction products are soluble and removed from the concrete matrix. This leads to loss of mass and progressive opening of the pore structure. Acids such as hydrochloric and nitric acid fall into this category, as does exposure to soft water. The visible consequences include surface softening, erosion and gradual loss of section. As material is removed, permeability increases, penetration deepens, and the rate of deterioration accelerates.

Non-dissolutive attack is characterised by expansion. Reaction products occupy a greater volume than the original phases, generating internal stresses that result in cracking and powdering. Sulfate attack, alkali–silica reaction and certain forms of hard-water scaling fall into this category. Although the surface may initially appear intact, damage accumulates internally. Through repeated cycles, deterioration compounds.

While the outward symptoms differ, both forms of attack and the rate of deterioration are governed by the same fundamental variable: how deeply aggressive agents are able to penetrate.

Water as a Chemical Driver

Water is often underestimated as a source of chemical attack because it is assumed to be neutral. In reality, its small molecular size makes it the ideal transport mechanism for deterioration.

​Soft water is chemically aggressive because it seeks equilibrium by dissolving calcium compounds from the concrete matrix. Hard water, conversely, may result in precipitation and scaling. Regardless of the mechanism, permeability governs how much reactive material becomes exposed as water moves through the concrete matrix.

Sulfuric Acid and Industrial Environments

Sulfuric acid exposure is frequently misunderstood, particularly in wastewater and industrial environments. In my experience, deterioration in these conditions is driven more by aggressive reactions with calcium, aluminium and iron-bearing phases within the cement matrix.

These reactions form expansive products, resulting in cracking, softening and surface loss. The mechanism becomes both chemical and mechanical. Once initiated, the process accelerates. Newly exposed surfaces increase permeability, allowing deeper penetration and further reaction. The deterioration becomes self-reinforcing.

The damage is not confined to the surface unless permeability is controlled.

Alkali–Silica Reaction in Context

Alkali–silica reaction fits squarely into this broader framework. It requires reactive silica, alkalis and moisture. The reaction produces an expansive gel that absorbs water and swells, generating internal stress.

Although ASR is often treated as a distinct phenomenon, it is fundamentally another example of penetration-driven chemical attack. Moisture ingress enables internal reactivity. Expansion leads to cracking. Cracking increases permeability. The cycle continues.

Again, permeability is the controlling factor.

​Permeability as the Dominant Variable

Across all mechanisms of chemical attack, I find permeability to be the dominant variable.

If aggressive agents penetrate 20, 30, 40 or even 70 millimetres into the concrete, the total reactive volume is substantial. Reactions occur throughout the depth of penetration. Damage becomes difficult to observe and even more difficult to quantify accurately until deterioration is advanced.

If penetration is limited to the first millimetre, the reaction zone is constrained. The reactive volume is dramatically reduced. Deterioration progresses slowly and remains visible, measurable and therefore manageable.

This distinction is critical.

Reducing permeability does not eliminate chemical reactions. It does not make concrete immune to aggressive environments. What it does is reduce the depth of reaction and limit the size of the deterioration zone. Instead of an uncontrolled volumetric reaction occurring throughout the mass of the concrete, deterioration becomes surface-controlled.

Controlled Deterioration Through Crystalline Technology

Crystalline technology operates directly on this principle. Reducing permeability, it limits the ingress of fluids and dissolved chemicals. At the same time, crystallisation consumes and binds reactive constituents within the concrete matrix.

Free lime is most commonly referenced, but aluminium- and iron-bearing phases are also incorporated into stable crystalline structures. Once bound within these structures, these elements are no longer freely available to react with external chemicals.

In highly aggressive environments, external chemicals may still break down these crystalline structures. However, because permeability is significantly reduced, this degradation can only occur incrementally and at shallow depth. Deterioration is effectively forced to occur one millimetre at a time rather than throughout the entire cross-section.

In practical terms, the concrete transitions from behaving like a sponge to behaving more like a dense, glassy material.

Durability as a Managed Outcome

For asset owners and engineers, the implications are significant. When deterioration is surface-controlled, it becomes predictable. Surface wear can be observed and measured. Maintenance interventions can be scheduled rationally. Service life can be extended in a controlled and engineered manner.

Ultimately, I do not view chemical resistance as an attempt to prevent chemistry altogether. Concrete will always react when exposed to aggressive environments. The objective is to control the depth, rate and extent of that reaction.

Permeability determines the extent of exposure.

Durability is not about eliminating change. It is about ensuring that change remains manageable over the life of the structure.

About the Author

Wiljee Blom holds a National Diploma in Civil Engineering, is a Stage 3 Concrete Technologist, and has a postgraduate qualification in Business Management.

He serves as Technical Manager at Penetron Africa, where he specialises in crystalline waterproofing technology and concrete durability solutions.