Improving Workability – Effect of common mineral/chemical additions Which of the following additions most reliably improves the workability of concrete (for the same water content), considering typical behavior of each material?

Introduction / Context:
Workability describes how easily fresh concrete can be mixed, placed, and compacted. Various mineral and chemical additions influence flow and cohesiveness. Recognizing which additions consistently enhance workability at a given water content supports better mix design and site control.

Given Data / Assumptions:

• No high-range water reducers are considered; the focus is on the listed additions.
• Water content is held constant for comparison.
• Standard concreting conditions are assumed.

Concept / Approach:
Fly ash, due to its spherical particle shape (particularly Class F), often improves workability and pumpability by acting like microscopic ball bearings, reducing interparticle friction and paste demand for a given slump. Hydrated lime primarily improves plasticity and water-retentivity in mortars and plasters; its effect on concrete workability is not as consistently beneficial as fly ash for the same water content. Calcium chloride is an accelerator; while it may sometimes appear to increase fluidity initially, it is not reliably used for workability enhancement and has corrosion implications for reinforcement. Bentonite is an extremely fine, highly absorptive clay that increases water demand and typically reduces concrete workability if water is unchanged.

Step-by-Step Solution:
Assess fly ash: spherical morphology → improved flow at constant water → positive.Assess hydrated lime: benefits mortars; limited, mix-specific effects in concrete.Assess calcium chloride: accelerator with corrosion risks; not a workability admixture.Assess bentonite: increases viscosity and water demand; reduces workability at constant water.Therefore, select “Fly ash”.

Verification / Alternative check:
Field experience and pumpability guidance consistently cite fly ash as a means to improve cohesiveness, reduce bleeding, and enhance workability without increasing water.

Why Other Options Are Wrong:

• Hydrated lime: primarily a mortar plasticizer; concrete effects vary.
• Calcium chloride: used for acceleration; not recommended for RC due to corrosion risk.
• Bentonite: reduces slump at constant water.
• “All of the above”: incorrect because effects are not uniformly positive.

Common Pitfalls:
Assuming any fine addition increases workability; many fines increase water demand. Always confirm with trial mixes.

Final Answer:
Fly ash