Hydration, Setting, and Hardening of Concrete PDF

PROCESSES: Hydration, Setting, and SETTING HARDENING Hardening of Concrete HYDRATION GENERAL Cement when mixed with water forms a plastic mass called cement paste. During hydration reaction, gel and crystalline products are formed. The inter-locking of the crystals binds...

PROCESSES: Hydration, Setting, and SETTING HARDENING Hardening of Concrete HYDRATION GENERAL Cement when mixed with water forms a plastic mass called cement paste. During hydration reaction, gel and crystalline products are formed. The inter-locking of the crystals binds the inert particles of the aggregates into a compact rock like material. This process of solidification comprises of (i) setting and then (ii) hardening Cement Hydration The reactions by which Portland cement becomes a bonding agent take place in a water- cement paste. The main hydration products:  Calcium Silicate Hydrates (C-S-H)  Tricalcium aluminate hydrate (C3-A-H6)  Ca(OH)2 Cement Hydration The progress of hydration of cement can be determined by different means: ▪ The amount of Ca(OH)2 in the paste. ▪ heat evolved by hydration. ▪ the specific gravity of the paste ▪ the amount of the of chemically combined water ▪ the amount of unhydrated cement present ▪ indirectly form the 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 Setting and Hardening of Concrete The setting and hardening of cement are two distinct but closely related processes that determine the mechanical and physical properties of concrete. Understanding these processes is crucial for ensuring that concrete achieves the desired strength, durability, and longevity. Difference Between Setting and Hardening Setting is the process where cement paste transitions from a fluid to a solid state, primarily reflecting the loss of plasticity and gain in early rigidity. Hardening is the process of gaining strength over time after setting, driven by continuous hydration reactions and microstructural changes. Setting of Concrete Setting is defined as stiffening of the original plastic mass due to initial gel formation. Setting is the process by which a freshly mixed cement paste (cement + water) transitions from a fluid or plastic state to a rigid or solid state. During this stage, the cement paste starts losing its plasticity and begins to gain early strength. Setting of Concrete Initial Setting Time: The time period after which the cement paste starts losing its plasticity. It marks the onset of stiffening and occurs when the paste can no longer be molded or worked. Final Setting Time: The time taken for the cement paste to completely lose its plasticity and acquire enough strength to resist certain defined pressures. It indicates that the concrete is firm enough to bear loads without deformation. Setting of Concrete Factors Affecting Setting: Water-Cement Ratio: Higher water content results in delayed setting times. Temperature: Elevated temperatures accelerate the setting process. Cement Composition: The amount of C3A (tricalcium aluminate) and C3S (tricalcium silicate) influences setting times. Admixtures: Retarders, such as gypsum, delay setting, while accelerators, like calcium chloride, hasten it. Setting of Concrete Measurement of Setting Time: Vicat Needle Test: Measures the initial and final setting times by determining the penetration resistance of a needle into the cement paste. Setting of Concrete Measurement of Setting Time: Penetrometer Test: Measures the resistance to penetration to evaluate the setting time, primarily used for concrete mixtures. Hardening Hardening is development of strength, due to crystallization. Hardening refers to the process of continuous strength gain in the cement paste, which starts after the setting stage. It is primarily driven by the ongoing hydration reactions of cement with water, leading to the formation of calcium silicate hydrates (C-S-H) and other products. Hardening Hardening is development of strength, due to crystallization. Hardening refers to the process of continuous strength gain in the cement paste, which starts after the setting stage. It is primarily driven by the ongoing hydration reactions of cement with water, leading to the formation of calcium silicate hydrates (C-S-H) and other products. Hardening Phases of Hardening: Early-Age Hardening (0 to 7 Days): Initial strength development due to the 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 During hardening, the hydration products form a gel-like structure known as C-S-H, which fills in the gaps between the cement particles, creating a dense and interconnected microstructure. This process results in increased strength and reduced porosity. Factors Affecting Hardening Curing Conditions: Proper curing (maintaining moisture and temperature) is crucial for continuous hydration and strength gain. Lack of curing can lead to reduced strength and durability. 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 like fly ash, slag, and silica fume modify the hardening process by influencing hydration kinetics and microstructure development. Strength Development in Hardening Early-age strength is mainly due to the rapid hydration of C3S. Long-term strength is influenced by the 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 the concrete has gained sufficient strength to be loaded and to start performing its structural role. Challenges and Solutions Premature Setting: Can lead to difficulties in placement and compaction. Use retarders or modify the mix design to mitigate. Delayed Setting: Results in prolonged construction times. Employ accelerators or optimize curing conditions. Inconsistent Hardening: Can cause long-term structural issues. Ensure proper curing and control of environmental factors. Summary and Key Takeaways  Recap of hydration, setting, and hardening.  Key factors influencing each process.  Best practices for achieving desired concrete properties. Questions and Discussion Understanding and controlling the setting and hardening processes of cement is essential for producing high-quality concrete that meets performance expectations and project requirements. Any Questions??