Concrete continues to gain strength and harden long after it is first poured. Concrete is a composite material created by mixing aggregate, such as sand and gravel, with a paste made of cement and water. The hardening process is not simply a matter of drying out; instead, it is driven by a precise chemical reaction.
The Chemical Process of Hydration
The fundamental reason concrete hardens is a chemical reaction between the cement powder and water, a process known as hydration. Cement is anhydrous, but once water is introduced, the compounds within the cement dissolve and react exothermically, releasing heat. This reaction transforms the wet, moldable paste into a solid, crystalline structure that binds the aggregates together.
The primary product of this reaction is Calcium Silicate Hydrate (C-S-H) gel, often called the “glue” of concrete. Tricalcium silicate (C3S) and dicalcium silicate (C2S), the most abundant compounds in Portland cement, react with the water to form this gel. The C-S-H gel forms a dense, microscopic network that fills the spaces between the original cement particles and the aggregate.
As hydration progresses, this gel network becomes increasingly interlocked and solid, leading directly to the concrete’s strength and durability. While other products like calcium hydroxide (CH) are also formed, the C-S-H gel is the main component responsible for the compressive strength of the material. The reaction of C3S contributes more to the early strength gain, while C2S provides a slower, more gradual strength increase important for long-term performance.
The Timeline of Compressive Strength
The hardening of concrete is measured by its compressive strength, which develops rapidly at first and then gradually slows over time. Immediately following the pour, the concrete enters an initial setting phase, where it loses its plasticity and becomes rigid, typically within a few hours.
The first seven days represent a period of rapid strength gain, where the concrete typically achieves between 60% and 70% of its specified design strength. This early strength is often tested to predict the material’s overall quality and to determine when formwork can be removed safely. By the 28-day mark, the industry standard benchmark, most concrete mixes are considered to have reached their full specified design strength, generally accounting for 95% to 99% of its potential strength.
The 28-day standard serves as a practical compromise for construction timelines, but the hydration process does not cease at this point. As long as moisture is present, the chemical reaction continues, and the concrete gains strength at a much slower rate for months and even years. This long-term strength gain, while incremental, contributes to the material’s ultimate durability.
External Variables That Control Curing
While the hydration reaction is intrinsic to the cement and water mixture, external conditions must be managed carefully for optimal hardening, a practice known as curing. The continuous presence of moisture is paramount because water is a reactant in the hydration process. If the concrete is allowed to dry out prematurely, the hydration reaction stops, which prevents the formation of the C-S-H gel and ultimately compromises the final strength and durability.
Temperature also exerts a significant influence on the rate of strength development. Higher temperatures accelerate the chemical reactions, leading to faster setting and early strength gain. However, excessively high temperatures can result in a weaker final product by causing the rapid formation of a less organized C-S-H structure. Conversely, cold temperatures slow the hydration process, and freezing temperatures can halt it entirely, causing permanent damage to the concrete’s microstructure.
The water-to-cement ratio (w/c) is another factor determined during the mixing phase. A lower w/c ratio generally results in a denser, stronger concrete, provided there is enough water for complete hydration. Extra water is often added to make the concrete more workable, but this excess water eventually evaporates, leaving behind voids and capillary pores that reduce the material’s density and compressive strength.