Concrete is a foundational construction material, but its transformation from a liquid slurry into a stone-like solid is often misunderstood as a simple drying process, overlooking the complex chemical changes that bind the components together. Concrete is a composite material made from three primary ingredients: Portland cement, water, and aggregates like sand and gravel. The cement and water form a paste that coats the aggregates, and the reaction between these two causes the mixture to solidify and gain strength. Water is actively consumed and permanently incorporated into the new solid structure.
The Chemical Reaction: Hydration
The hardening of concrete is driven by a reaction called hydration, an exothermic process where heat is released. Portland cement is composed mainly of calcium silicates, specifically tricalcium silicate (C3S) and dicalcium silicate (C2S). When water is introduced, these compounds immediately begin to dissolve and react to form new, stable products.
The most important product of this reaction is Calcium Silicate Hydrate (C-S-H), a microscopic, gel-like substance that acts as the physical glue binding the entire matrix. This C-S-H gel grows into a dense, interlocking network of microscopic fibers and sheets. This crystalline structure fills the space between the cement particles and the aggregates, which generates the concrete’s compressive strength.
Tricalcium silicate reacts rapidly and is largely responsible for the strength achieved in the first week, contributing to the initial firmness. Dicalcium silicate reacts much more slowly but continues to hydrate for months or even years, providing the long-term strength and durability of the finished structure. The hydration reaction also produces a secondary byproduct, calcium hydroxide (CH), which does not contribute significantly to the strength but maintains the high alkalinity within the concrete.
Distinguishing Setting from Hardening
After mixing, concrete goes through two distinct physical stages: setting and hardening. Setting describes the initial transition where the mixture loses its plasticity and becomes rigid, typically occurring within the first few hours. This initial set happens when the first layers of C-S-H gel form, stiffening the cement paste enough to hold its shape and defining the point when the concrete can no longer be easily worked. Hardening, in contrast, is the gradual, long-term process of strength gain that follows the initial set, involving the continuous formation and densification of the C-S-H network.
While concrete begins gaining strength immediately upon mixing, its specified strength is conventionally measured at 28 days of age. This standard benchmark is used because the majority of the hydration reaction has occurred by this time, and the rate of strength gain has significantly slowed. The internal chemical reactions continue indefinitely, meaning the concrete’s strength and density will continue to slowly increase over the course of its service life.
The Critical Role of Curing
Curing is the practical process of maintaining sufficient moisture and a favorable temperature around the concrete after it has been placed. Because water is an active reactant, if the concrete surface is allowed to dry out prematurely, the hydration reaction will stop in that area.
A lack of moisture during the early stages results in an incomplete reaction, which prevents the full development of the C-S-H gel. This incomplete hardening leads to a lower final compressive strength, increased risk of surface cracking, and reduced durability. Maintaining a temperature above freezing is also necessary, as the chemical reaction slows dramatically in cold conditions.
Common curing methods focus on retaining the water already in the mix or providing external water. Techniques include:
- Ponding the surface with water.
- Continuously misting with a sprinkler.
- Covering the concrete with wet burlap or plastic sheeting.
- Spraying liquid curing compounds on the surface to form a temporary membrane that seals in the moisture.
Curing allows the concrete to achieve its intended strength and longevity.