--- title: "Setting, setting time, admixture vs additive" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:setting-time-admixture-additive-diff" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/setting-time-admixture-additive-diff site: BCT Engineering --- # Setting, setting time, admixture vs additive Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2080 Baisakh, 3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/setting-time-admixture-additive-diff - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question What do you think is the basic difference between i) setting and hardening, ii) initial and final setting time, and iii) admixture and additives? ## Short answer Setting is the change from plastic to rigid state in hours; hardening is subsequent strength gain. Initial setting is when paste loses plasticity (Vicat needle penetrates 5±0.5 mm from bottom, min 30 min); final setting is when needle leaves imprint but no full penetration, max 600 min. Admixtures are added at mixer in small doses (<5% cement); additives are ground with clinker. ## Answer #### i) Setting and hardening - **Setting**: the change of the fresh paste from a plastic to a rigid (but weak) state; it takes hours and is tested with the Vicat apparatus. - **Hardening**: the gain of strength after setting, taking days to years; it is tested by compressive strength tests. #### ii) Initial and final setting time - **Initial setting time**: the time from adding water to the point where the paste begins to lose its plasticity (the Vicat needle of 1 mm square section penetrates only to 5 ± 0.5 mm from the bottom of the mould). IS 269: not less than 30 minutes (OPC). Concrete must be placed and compacted before this time. - **Final setting time**: the time at which the paste has become hard enough that the needle makes an impression on its surface but the annular attachment fails to do so. Not more than 600 minutes (10 h). #### iii) Admixture and additive - **Admixture**: added to the concrete (or mortar) at the mixer, in small amounts (usually below 5% of the cement mass), to change its fresh or hardened properties; e.g. superplasticiser, retarder. - **Additive**: added to the cement at the factory (during grinding), or in large quantities by weight of cement, e.g. gypsum, fly ash, slag; it becomes part of the cementitious material. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement correctly separates setting from hardening? A. Setting is the change from plastic to rigid but weak state; hardening is the later strength gain B. Setting is strength gain over years; hardening is loss of plasticity in hours C. Both are the same process, tested by Vicat apparatus D. Setting is tested by compressive strength; hardening by Vicat apparatus Answer: A. Setting is the hours-long stiffening tested by Vicat; hardening is the later strength gain tested by compressive strength. (Key point: Setting vs hardening) 2. Which describes the final setting time of cement paste? A. Needle penetrates to 5 mm from the bottom of the mould B. Paste is hard enough that the annular attachment leaves an impression but the cutting edge leaves no mark C. The time when the paste has reached 28-day strength D. The time at which water is first added Answer: B. Final set is reached when the needle with annular attachment makes an impression but the cutting edge does not. (Key point: Initial and final setting time) 3. What best distinguishes an admixture from an additive? A. An admixture is added at the cement factory in large amounts B. An additive is added in small doses at the mixer to change properties C. An admixture is added at the mixer in small amounts; an additive is added to cement at the factory or in large amounts and becomes part of the binder D. They differ only in name Answer: C. Admixtures (e.g. superplasticiser) are small-dose mixer additions; additives (gypsum, fly ash, slag) go into the cement or binder. 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