Concrete Hydration Process Overview

Hydration, Setting, and Hardening of Concrete

• Cement mixed with water forms a plastic mass called cement paste.
• Hydration reaction forms gel and crystalline products.
• Interlocking crystals bind inert particles into a compact rock-like material.
• Solidification comprises setting and then hardening.
• Portland cement becomes a bonding agent through reactions in water-cement paste.

Main Hydration Products

• Calcium Silicate Hydrates (C-S-H)
• Tricalcium aluminate hydrate (C3-A-H6)
• Ca(OH)2

Determining Hydration Progress

• Amount of Ca(OH)2 in the paste
• Heat evolved by hydration
• Specific gravity of the paste
• Amount of chemically combined water
• Amount of unhydrated cement
• Strength of hydrated paste

Stages of Cement Hydration

• Initial mixing (dissolution and hydrolysis)
• Induction (dormant) period
• Acceleration phase
• Deceleration phase
• Steady-state phase (slow reaction)

Stages of Cement Hydration (detailed)

• Ordinary portland cement + water --> calcium silicate hydrate + calcium hydroxide + AFm + Other
• Initial dissolution period
• Induction period
• Acceleration period
• Deceleration period
• Slow continued reaction
• Heat release (over time)
• 10m, 1-4H, 10H, 24H, weeks/months

Setting and Hardening of Concrete

• Setting and hardening are distinct but related processes determining concrete properties.
• Understanding these processes ensures desired strength, durability, and longevity.

Difference Between Setting and Hardening

• Setting: Cement paste transitions from fluid to solid (loss of plasticity and gain in rigidity).
• Hardening: Gaining strength over time after setting (continuous hydration reactions and microstructural changes).

Setting of Concrete

• Stiffening of original plastic mass due to initial gel formation.
• Freshly mixed cement paste (cement + water) transitions from fluid/plastic to rigid/solid state.
• Cement paste loses plasticity and gains early strength during setting.

Setting of Concrete (continued)

• Initial Setting Time: Time after which cement paste loses plasticity (onset of stiffening).
• The paste cannot be molded or worked any longer at this point.
• Final Setting Time: Time for the cement paste to completely lose plasticity and gain enough strength to resist defined pressures.
• Concrete is firm and bearing loads without deformation at this point.

Factors Affecting Setting

• Water-Cement Ratio: Higher water content delays setting times.
• Temperature: Elevated temperatures accelerate setting.
• Cement Composition: C3A and C3S amounts influence setting times.
• Admixtures: Retarders delay and accelerators hasten setting.

Measurement of Setting Time

• Vicat Needle Test: Measures initial and final setting times by determining needle penetration resistance into cement paste.
• Penetrometer Test: Measures resistance to penetration to evaluate setting time, primarily used for concrete mixtures.

Hardening of Concrete

• Development of strength due to crystallization.
• Continuous strength gain in cement paste after setting.
• Ongoing hydration reactions of cement with water leading to calcium silicate hydrate (C-S-H) formation.

Hardening: Phases of Hardening

• Early-Age Hardening (0 to 7 Days): Initial strength development from rapid hydration of C3S and C3A.
• Intermediate Hardening (7 to 28 Days): Continued hydration of C2S (dicalcium silicate) leading to further strength gain.
• Long-Term Hardening (Beyond 28 Days): Slow hydration and further crystallization of hydration products contributing to durability and long-term strength.

Mechanism of Hardening

• Hydration products (C-S-H) form a gel-like structure filling gaps between cement particles, creating a dense interconnected microstructure.
• Increased strength and reduced porosity result from this process.

Factors Affecting Hardening

• Curing Conditions: Proper curing (moisture and temperature maintenance) is crucial for continuous hydration and strength gain.
• Type of Cement: Cement types with higher C2S content show slower early-age hardening but gain strength over time.
• Presence of Supplementary Cementitious Materials (SCMs): Materials (fly ash, slag, silica fume) modify hardening process by influencing hydration kinetics and microstructure development.

Strength Development in Hardening

• Early-age strength is mainly due to rapid hydration of C3S.
• Long-term strength is influenced by slower hydration of C2S and ongoing chemical changes.

Practical Implications in Construction

• Setting Time: Determines the workable period for placing and finishing concrete.
• Hardening: Indicates when concrete has enough strength to be loaded and perform structural role.

Challenges and Solutions

• Premature Setting: Difficulties in placement and compaction—use retarders or modify mix design.
• Delayed Setting: Prolonged construction times—use accelerators or optimize curing conditions.
• Inconsistent Hardening: Long-term structural issues—ensure proper curing and control environmental factors.

Summary and Key Takeaways

• Recap of hydration, setting, and hardening.
• Key factors influencing each process.
• Best practices for achieving desired concrete properties; understanding and controlling setting and hardening processes essentials for high-quality concrete.

This quiz delves into the hydration, setting, and hardening of concrete, outlining key processes such as the formation of cement paste and the role of hydration products. It covers stages of cement hydration, factors determining hydration progress, and the importance of compounds like Calcium Silicate Hydrates. Test your understanding of these fundamental concepts in concrete technology.