Concrete is a fundamental building material used globally in various structures, from roads and bridges to homes and skyscrapers. Its longevity and strength depend on its chemical properties, particularly its pH level. The pH scale measures how acidic or alkaline a substance is, ranging from 0 to 14, where 7 is neutral, values below 7 are acidic, and values above 7 are alkaline. Concrete typically exhibits a high pH, which is significant for its performance and durability.
The High Alkalinity of Fresh Concrete
Freshly poured concrete typically has a pH ranging from 12 to 13, and can sometimes be even higher, reaching up to 13.5. This characteristic high pH stems from the chemical reactions that occur during the hydration of cement, a primary component of concrete. Cement is largely composed of calcium oxide (CaO), derived from limestone.
The process of cement hydration involves calcium silicates within the cement reacting with water. This reaction forms several compounds, notably calcium silicate hydrate (C-S-H) gel, which provides concrete its strength, and calcium hydroxide (Ca(OH)₂), also known as portlandite. Calcium hydroxide is a highly alkaline substance, and its significant presence in the concrete pore solution is the main reason for the elevated pH of fresh concrete. Additionally, other alkali metal ions like sodium (Na+) and potassium (K+) present in the cement can further increase the concentration of hydroxide ions, contributing to the overall high pH.
Concrete pH Over Time
The pH of concrete does not remain constant; it gradually changes after placement and curing due to a process called carbonation. Carbonation occurs when atmospheric carbon dioxide (CO₂) infiltrates the concrete’s pores and reacts with the calcium hydroxide within the cement paste. This chemical reaction converts the highly alkaline calcium hydroxide into calcium carbonate (CaCO₃) and water.
As carbonation progresses from the surface inwards, the pH of the affected concrete can drop significantly, typically from its initial range of 12-13 down to approximately 8.5 to 9. This is a natural and slow process, with the rate influenced by factors such as the concrete’s density, moisture content, and the concentration of carbon dioxide in the surrounding air.
The Significance of Concrete pH
The pH of concrete holds considerable importance, particularly for the long-term durability of reinforced concrete structures. Initially, the high alkalinity of fresh concrete provides a protective environment for embedded steel reinforcement bars (rebar). This alkaline condition causes a thin, passive layer of iron oxide (Fe₂O₃) to form on the surface of the steel, which acts as a barrier and prevents the rebar from corroding.
However, when the concrete’s pH drops due to carbonation, this protective passive layer can break down. Research suggests this protective effect is lost when the pH falls below a certain threshold, typically 9.5 to 11.5. Once the rebar is de-passivated, it becomes susceptible to corrosion, especially in the presence of moisture and oxygen. As steel corrodes, it forms rust, which can expand up to six times its original volume. This expansion creates internal pressure within the concrete, leading to cracking, spalling (flaking of the surface), and delamination of the concrete cover. Such damage compromises the bond between the rebar and the concrete, reducing the structural integrity and load-bearing capacity of the entire structure.