Concrete is one of the most durable building materials on Earth, but it has a surprising number of enemies. Bacteria, acids, salt water, reactive minerals trapped inside the mix, and even pure soft water can all break concrete down over time. Some of these processes dissolve the calcium compounds that hold concrete together, while others generate pressure from within until the material cracks apart.
Bacteria That Produce Sulfuric Acid
The most dramatic biological threat to concrete comes from colonies of sulfur-oxidizing bacteria. These microorganisms are common in sewer systems, wastewater treatment plants, and other environments where hydrogen sulfide gas is present. The process, called microbiologically induced concrete corrosion, works in stages. First, sulfate-reducing bacteria in wastewater convert sulfates into hydrogen sulfide gas. That gas rises and settles on exposed concrete surfaces above the waterline, where a second group of bacteria colonizes the surface and begins converting the sulfur compounds into sulfuric acid.
The most aggressive strains, including species like Acidithiobacillus thiooxidans, can drop the pH of a concrete surface to around 2, which is roughly as acidic as lemon juice. At that level, the acid reacts with calcium hydroxide in the concrete paste and converts it into gypsum, a soft mineral that has none of concrete’s structural strength. The gypsum then reacts further with other compounds in the cement to form ettringite, a crystal that expands as it grows and accelerates cracking. Over years, this bacterial attack can eat through inches of concrete pipe wall, turning solid infrastructure into soft, chalky mush.
Acids From Industry and Nature
Concrete is naturally alkaline, with a pH around 12 to 13. That high pH is what makes it vulnerable to any acid. Hydrochloric acid, sulfuric acid, acetic acid (from food processing), and carbonic acid (from carbon dioxide dissolved in rainwater) all attack the calcium compounds that bind concrete together. The acid dissolves calcium out of the hardite paste, increasing porosity and reducing compressive strength. In lab tests, cement paste exposed to hydrochloric acid at a pH of 1 showed measurable decomposition of its key binding gel within just 10 days.
In practice, this means concrete floors in chemical plants, dairy facilities, and breweries face constant acid exposure. Acidic soils can attack foundations. Even ordinary rainwater, which is slightly acidic from dissolved CO2, slowly leaches calcium from exposed concrete over decades. The damage typically appears as surface pitting, softening, and a white powdery residue where calcium has been pulled to the surface and deposited.
Sulfates in Soil and Groundwater
Sulfate ions dissolved in groundwater or present in certain clay soils can penetrate concrete from the outside and trigger a destructive chain reaction. When sulfate ions reach the calcium aluminate compounds inside the cement paste, they form ettringite crystals within the concrete’s pore network. These crystals generate what engineers call crystallization pressure as they grow, filling the smallest capillary and gel pores first, then expanding outward.
When the internal pressure exceeds the concrete’s tensile strength, cracks form. Those cracks allow more sulfate solution to penetrate deeper, accelerating the cycle. The surface of sulfate-damaged concrete often looks like it’s scaling or flaking in layers, and the damage tends to start at the base of foundations or retaining walls where contact with sulfate-rich soil is greatest.
Concrete That Attacks Itself From Within
Sometimes the threat is baked into the concrete from the start. Alkali-silica reaction (ASR) occurs when the highly alkaline pore solution inside concrete dissolves reactive silica minerals in certain types of aggregate (the gravel and sand mixed into the concrete). The dissolved silica reacts with alkalis like sodium and potassium to form an amorphous gel that absorbs water and swells. This gel generates solidification pressures between 6 and 13 megapascals, more than enough to crack concrete from the inside out.
ASR damage is slow, often taking 5 to 20 years to become visible. The telltale sign is a pattern of irregular “map cracking” on the surface, sometimes accompanied by white or translucent gel seeping from cracks. The reaction requires three ingredients: reactive silica in the aggregate, high alkalinity in the cement, and moisture. Remove any one of those three and ASR stops, which is why engineers test aggregates before use and sometimes add materials like fly ash to reduce alkalinity.
Salt Water and Chloride Damage
Saltwater doesn’t dissolve concrete directly the way acid does, but it causes devastating indirect damage. Chloride ions from seawater or road deicing salts penetrate through the concrete’s pore structure over time. Once they reach the steel reinforcement bars (rebar) embedded inside, the chlorides break down the thin protective oxide layer that normally keeps the steel from rusting in concrete’s alkaline environment.
Once corrosion starts, the rust that forms occupies several times the volume of the original steel. This expansion cracks the surrounding concrete from inside, creating the characteristic spalling you see on old bridges, parking garages, and coastal structures: chunks of concrete popping off to reveal corroded rebar underneath. In marine tidal zones, reinforced concrete elements have shown 15 to 18 percent loss of load-bearing capacity within just 6 to 12 months of active corrosion.
Pure Water Is Surprisingly Destructive
This is counterintuitive, but extremely pure or soft water is more damaging to concrete than hard water. Soft water contains very few dissolved minerals, so it’s chemically “hungry” and aggressively dissolves calcium out of concrete’s pore structure. The process, called calcium leaching, strips away portlandite (calcium hydroxide) first, then attacks the deeper binding compounds that give concrete its strength.
In accelerated testing, concrete exposed to soft water lost about 2% of its mass over 200 days, nearly matching the rate caused by completely demineralized water. The degradation penetrated 2.7 millimeters deep after 300 days, compared to just 1.6 millimeters for specimens in hard water. Over years, leaching increases the concrete’s porosity, weakens its mechanical strength, and can eventually compromise structures like dams, water treatment tanks, and pipes carrying mountain runoff or distilled process water.
Fire and Extreme Heat
Heat doesn’t “eat” concrete in the biological sense, but it systematically dismantles its internal chemistry. The calcium silicate hydrate gel that serves as concrete’s primary binding agent begins losing its weakly bound water at relatively modest temperatures. This first stage of dehydration causes the most significant shrinkage. As temperatures climb, the process intensifies: strongly bound water is driven off, hydroxyl groups break apart, and the gel’s structure progressively collapses.
Below about 800°C (1,470°F), the damage is primarily dehydration. The concrete loses strength and develops a network of internal microcracks as different components shrink at different rates. Above 800°C, the binding gel transforms into wollastonite, a completely different mineral with none of the original cement paste’s binding properties. At this stage, the gel’s characteristic foil-like structure changes to rounded, drop-like particles, and the concrete is essentially destroyed as a structural material. This is why concrete structures that survive fires often look intact on the surface but have lost a significant portion of their load-bearing capacity.
Plants and Surface Growth
Tree roots are well known for cracking sidewalks and foundations, but this is mechanical force rather than chemical attack. The roots grow into existing joints and cracks, then expand as they thicken, wedging concrete apart over years. Mosses, lichens, and algae commonly colonize concrete surfaces, especially in damp, shaded areas. Despite their appearance, these organisms generally do not damage the concrete they grow on, according to the Royal Horticultural Society. Their main practical concern is making surfaces slippery. However, by trapping moisture against the surface, they can modestly accelerate freeze-thaw damage in cold climates, where water repeatedly freezes and expands in the concrete’s surface pores.