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Technical Bulletin Number 2 - Hydration & Curing

(PDF Version)

January 2010

Understanding Key Technical Terms

Hydration and Curing

Introduction – The essential element in all good communications is that the parties involved must understand the vocabulary of the language used by one another in expressing their ideas and thoughts. If an idea or thought is expressed that holds two different interpretations, then communication will break down, be misunderstood, or at the very least be confusing and perplexing to all parties. In an attempt to prevent this miscommunication, the National Plasterers Council will address the often-misused terms of hydration and curing.

General Description – In the NPC Technical Manual, 6th Edition, hydration is defined as....”the chemical reaction between hydraulic cement and water forming new compounds most of which have strength producing properties.” The American Concrete Institute defines hydration as, “formation of a compound by the combining of water with so me other substance: in concrete, the chemical reaction between hydraulic cement and water.”1,2

The Portland Cement Association, in its engineering bulletin (book) titled “Design and Control of Concrete Mixtures” describes how, “Hydraulic cements set and harden by reacting with water. During this reaction, called hydration, cement combines with water to form a stone-like mass.”3

Many people in our industry erroneously describe abnormal or staining discoloration issues in cementitious based interior finishes as “hydration”. The NPC Technical Manual, 6th Edition, in Section 6.5.1 through 6.5.2 describes the difference between normal, abnormal, and staining discoloration.4

Another common misconception is the erroneous use of the term “curing” in our industry’s language. Curing is often applied to cementitious interiors to describe the maturation process or hydration process. Curing (of plaster) in the NPC Technical Manual is defined on page 31 as, “the act or process by which the cementitious surface coating continues hydration. Curing is typically done by immersing the cementitious coating in water as soon as possible after final set. The hydration of the cementitious compounds will continue underwater.”5

In the American Concrete Institute, Concrete Primer, curing is explained as follows:

Question – “What is meant by curing”?

Answer – “The term curing is used in reference to the maintenance of a favorable environment for the continuation of these chemical reactions; that is, the retention of moisture within or supplying moisture to the concrete and protection against extremes of temperature. It is through early curing that the internal structure of the concrete is built up to provide strength and water tightness. While simply retaining moisture within the concrete may be sufficient for low to moderate cement contents, mixes that are rich in cement generate considerable heat of hydration, which may expel moisture from the concrete in the period
immediately after setting. With such concrete, water curing should begin as soon as possible with free water kept on the concrete to replace any lost moisture and to help dissipate heat”6

ACI simplifies this statement with the definition of curing as – “the maintenance of a satisfactory moisture content and temperature in concrete during its early stages so that desired properties may develop. The Portland Cement Association defines the objectives of curing are to 1) prevent (or replenish) the loss of moisture from concrete and 2) maintain a favorable concrete temperature for a definite period of time.7,8

By immersion or “ponding” curing of a swimming pools’ cementitious surface, the hydration process is allowed to proceed without the accelerated loss of moisture from evaporation, wind, and other factors. This “ponding” curing will also greatly reduce the tendency of the surface to develop shrinkage cracking.

Summary: Simply put, hydration is the chemical reaction of hydraulic cement with water that forms new cement compounds, ultimately producing a stone-like mass. Curing is the process of providing the proper environment for the hydration or maturation process to proceed until the pool’s cementitious interior finish achieves its desired properties. Interior cementitious finishes begin to hydrate during the mixing process. The new finish is cured by filling the pool with water after the surface has hardened to a point that it can be immersed.


National Plasterers Council Technical Manual, 6th Edition, February 2008. 1,4,5

Concrete Terminology for the Legal Profession, American Concrete Institute, International, CT-1. 2,6,7

Design and Control of Concrete Mixtures, 13th Edition, Steven Kosmatka and William Panarese, Portland Cement Association, Skokie, Illinois, 1990. 3,8