Concrete is profoundly basic, or alkaline. The pH scale measures acidity or alkalinity, ranging from 0 to 14, where 7 is neutral, below 7 is acidic, and above 7 is basic. Freshly mixed concrete starts at the high end of this scale, typically exhibiting a pH between 12 and 13.5. This strongly alkaline state results from the chemical transformation that occurs when cement powder is mixed with water, a process known as hydration. Understanding this inherent alkalinity is foundational to grasping how concrete functions, ages, and interacts with its environment.
The Highly Alkaline Nature of Fresh Concrete
The high alkalinity is established immediately during cement hydration, where cement powder reacts with water to form a solid binding matrix. This reaction produces several compounds, the most significant being calcium silicate hydrate (C-S-H) gel, which provides strength, and calcium hydroxide (\(\text{Ca}(\text{OH})_2\)). Calcium hydroxide, also known as portlandite, is a highly soluble compound that saturates the pore water within the concrete structure.
The dissolved calcium hydroxide drives the pH of the fresh concrete pore solution to high levels, generally ranging from 12.5 to 13.5. The cement mixture also contains minor amounts of sodium and potassium oxides, which react with water to form highly soluble sodium and potassium hydroxides. These alkali hydroxides contribute significantly to the initial alkalinity, which is an inherent and desirable property of new concrete.
Carbonation: The Natural Reduction of Concrete’s pH
Over time, this highly alkaline environment changes through a natural chemical process called carbonation. Carbonation occurs when atmospheric carbon dioxide (\(\text{CO}_2\)) slowly permeates the concrete’s porous structure and dissolves in the pore water. The dissolved carbon dioxide forms a weak carbonic acid, which then reacts with the calcium hydroxide inside the concrete matrix.
This reaction converts calcium hydroxide (\(\text{Ca}(\text{OH})_2\)) into calcium carbonate (\(\text{CaCO}_3\)). This conversion neutralizes the pore solution, causing the pH level to decrease from its original value of 13 down to a range of approximately 8 to 9. Carbonation progresses inward from the exposed surface, creating a carbonated front that moves deeper into the material.
The speed and depth of the carbonation front are influenced by the availability of moisture and the porosity of the concrete. Carbonation is fastest when the concrete is neither completely dry nor entirely saturated, as moderate moisture is needed for the \(\text{CO}_2\) to dissolve and react. Denser concrete with a lower water-to-cement ratio carbonates more slowly because carbon dioxide has a more difficult path to penetrate the interior.
Practical Implications of Concrete’s pH
The alkalinity of concrete is a primary defense mechanism that ensures the durability of steel-reinforced structures. When steel reinforcing bars (rebar) are embedded in concrete with a pH above 11.5, the alkaline environment forces the steel to form a thin, stable layer of iron oxide on its surface. This protective layer, known as the passive film, chemically isolates the steel from oxygen and moisture, preventing rust and corrosion.
However, when the carbonation front reaches the embedded rebar, the localized drop in pH causes the breakdown of the passive film. Once the pH at the steel surface falls below a threshold, typically between 9.0 and 9.5, the rebar becomes susceptible to corrosion. The resulting rust occupies a much greater volume than the original steel, creating internal stresses that cause the concrete to crack and spall.
Conversely, the alkaline nature of concrete makes it vulnerable to external chemical aggression from strong acids. When concrete is exposed to acidic substances, such as industrial runoff or natural acids in soil, the acids chemically react with and dissolve the cement hydration products. This neutralization process consumes the alkaline components, leading to the erosion and degradation of the concrete surface. Concrete can rapidly deteriorate when exposed to environments with a pH below 4.0.
The high alkalinity also presents temporary environmental concerns during construction. Fresh concrete runoff that enters local storm drains or bodies of water can temporarily elevate the water’s pH, which may be harmful to aquatic life. Additionally, the surface alkalinity of a concrete slab can impact interior finishes, as high pH levels above 10 can degrade the bonding agents in flooring adhesives, leading to installation failure.