Presentation on curing and hardening mechanism of cement

Cement

1. Cement Cement is the backbone of modern infrastructure. It binds aggregates to form concrete. Concrete is a key material in construction. Its strength and durability are critical. These factors ensure structural integrity.Cement finely ground material that, when mixed with water, hydrates to form a hard, stone-like substance.

2. Chemical composition of cement Tricalcium Silicate (C3S) Dicalcium Silicate (C2S) Tricalcium Aluminate (C3A) Tetracalcium Aluminoferrite (C4AF) Major components

3. hydration products Calcium Silicate Hydrate (C-S-H) Calcium Hydroxide (CH) Gypsum as a minor component classification Based on Application Ordinary Portland Cement (OPC) High-Strength Cement Rapid Hardening Cement Low Heat Cement Sulphate Resisting Cement White Cement Colored Cement Air-Entraining Cement Hydrophobic Cement

4. Based on Chemical Composition Portland Cement Blended Cements Based on Setting Characteristics Normal Setting Cement Quick setting cement Setting of cement The setting of cement refers to the process by which cement paste changes from a fluid to a solid state, ultimately hardening and gaining strength. The setting process is influenced by the chemical reactions that occur when water is added to cement, primarily involving the hydration of cement particles. Initial Setting (or Setting Time ) This is the period when the cement starts to stiffen and loses its plasticity. It typically begins within a few minutes of adding water to the cement.During this stage, the hydration reaction begins, and the cement paste starts to become less workable.The initial setting time is important to determine how much time you have to work with the cement before it begins to set and harden. This time is usually around 30 minutes to 60 minutes, depending on the type of cement and environmental conditions.

5. Final Setting (or Hardening) This stage marks the point when the cement has hardened sufficiently to be handled without deformation . The final set occurs when the cement reaches a certain level of rigidity, and the chemical hydration processes continue to take place, causing the material to gain strength over time. The final setting time is typically around 6 to 12 hours but can vary based on factors like cement composition, water-to-cement ratio, temperature, and additives. Factors Affecting Cement Setting Water-to-Cement Ratio: More water can delay the setting, while less water can cause quicker setting . Cement Type: Different types of cement (e.g., Portland cement, rapid-setting cement) will have varying setting times.Temperature : Higher temperatures generally accelerate setting, while lower temperatures can slow it down . Additives and Admixtures: Chemical admixtures like retarders or accelerators can either slow down or speed up the setting process.

6. Hardening of cement The hardening of cement is the process by which cement gains strength and becomes a solid mass after setting. Hardening happens due to chemical reactions between water and the cement particles, primarily through a process called hydration. It is a gradual process, and the cement continues to gain strength over time, even after the initial setting has occurred. Hydration of Cement When water is added to cement, it reacts chemically with the compounds in the cement, primarily the tricalcium silicate (C₃S) and dicalcium silicate (C₂S). The hydration reactions produce a gel-like substance called calcium silicate hydrate (C-S-H), which is responsible for the development of strength. The general hydration reactions are as follows : Tricalcium silicate (C₃S) + Water → Calcium silicate hydrate (C-S-H) + Calcium hydroxide (Ca(OH)₂ ) Dicalcium silicate (C₂S) + Water → Calcium silicate hydrate (C-S-H) + Calcium hydroxide (Ca(OH)₂ ) These reactions form the solid structure that makes the cement hard. The calcium hydroxide (Ca(OH)₂), also known as lime, contributes to the alkalinity of the cement, but it is less effective in strength development than C-S-H.

7. Stages of Hardening Early Hardening (First Few Days): In the initial stages, cement starts to set and gains some strength relatively quickly. This is when the majority of the heat of hydration is released, and the cement begins to stiffen . Continued Hardening (Over Weeks and Months): Hardening continues for an extended period after the cement has set. The cement continues to hydrate, albeit more slowly, for months and even years. This is why cement can continue to gain strength over time as long as it has moisture available for hydration. Factors Influencing Hardening Water-Cement Ratio: A lower water-to-cement ratio generally results in stronger and more durable concrete, as there’s less water to dilute the cement paste. However, too little water can make it difficult to mix and work with.Temperature : Warm temperatures speed up the hydration process, while colder temperatures slow it down. In extreme cold, cement may not harden effectively . Cement Composition: Different types of cement (e.g., Ordinary Portland Cement, Rapid Hardening Cement) have varying rates of hardening due to differences in chemical composition. Some are designed to set and gain strength more quickly, while others develop strength more slowly but over a longer period . Admixtures : Chemical admixtures can either speed up or slow down the hardening process. For example, accelerators can be added to speed up the hardening, while retarders can delay it.

8. Strength Development Cement doesn’t achieve its maximum strength immediately after it has set. Typically, it reaches around 60-70% of its ultimate strength within the first week, and the remaining strength develops over a period of months or even years. 1 Day: Cement will have gained around 25% of its ultimate strength.7 Days: Around 60-70% of its ultimate strength . 28 Days: The strength typically reaches its design strength, around 90-95 %. Long-Term Strength: Cement will continue to harden and gain strength slowly over time, especially if it is kept moist. Hardening and Curing Curing plays an important role in the hardening process. Curing refers to maintaining adequate moisture and temperature conditions during the hardening of cement to ensure the proper hydration process. Without proper curing, the cement may not harden effectively, leading to weak concrete . Curing methods include:Water curing: Keeping the cement surface wet . Covering with wet burlap: Often used for large slabs or pavements . Curing compounds: These form a film that retains moisture and helps the hydration process.

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