Correct Answer: A. Tricalcium silicate (C₃S)

Simple Explanation:

Tricalcium silicate (C₃S) reacts quickly with water and is mainly responsible for the early strength (1–7 days) of cement. Higher C₃S content means faster strength gain.

Detailed Explanation:

Bogue’s Compounds in Portland Cement:

A good quality OPC typically has a higher C₃S content (≥60%) to ensure adequate 28-day compressive strength. For rapid hardening cement, C₃S is increased further and the cement is more finely ground.

Source: SSC JE 2010

Correct Answer: C. 43 MPa

Simple Explanation:

The ’43’ in 43 Grade cement means the 28-day cube compressive strength must exceed 43 MPa. The grades of OPC are 33, 43, and 53 — named after their minimum 28-day strength.

Detailed Explanation:

As per IS 8112:2013, OPC 43 grade must satisfy:

OPC 43 is the most commonly used grade in India for RCC work, plasterwork, and general construction. OPC 53 is preferred where early strength is critical (e.g., prestressed concrete, precast elements).

Source: SSC JE 2012

Correct Answer: B. Greater than 30 minutes

Simple Explanation:

The initial setting time must be at least 30 minutes. This gives workers enough time to mix, transport, and place concrete before it starts to stiffen. The final setting time must not exceed 600 minutes (10 hours).

Detailed Explanation:

Setting time is measured using the Vicat apparatus (IS 4031 Part 5). The needle penetration into cement paste defines the setting stages:

• Initial Setting Time (IST): Time from addition of water until the paste begins to lose plasticity. Minimum 30 minutes for OPC (IS 8112). This gives workable time for placing.
• Final Setting Time (FST): Time until the paste has completely hardened and lost plasticity. Maximum 600 minutes (10 hours) for OPC.

Why is the 30-min minimum important? If concrete sets too fast, it cannot be properly placed and compacted in complex formwork. If it sets too slowly, construction progress is delayed and the site is vulnerable to disturbance.

Gypsum (CaSO₄·2H₂O) is added to clinker during cement manufacturing specifically to retard the rapid-setting reaction of C₃A and ensure sufficient working time.

Source: SSC JE 2011

Correct Answer: B. Hydration of cement

Simple Explanation:

Concrete gains strength through hydration — a chemical reaction between cement and water that produces calcium silicate hydrate (C-S-H) gel. This gel binds the aggregates together and provides strength.

Detailed Explanation:

When water is added to cement, the following chemical reactions occur:

Primary hydration products:

• C-S-H (Calcium Silicate Hydrate) — the main binding gel, responsible for ~70% of the strength
• Ca(OH)₂ — calcium hydroxide (portlandite), a by-product that fills voids

Strength development pattern:

Hydration requires water — which is why curing (keeping concrete moist) is essential. If concrete dries out prematurely, hydration stops and full strength is not achieved.

Source: SSC JE 1 MARCH 2017 Morning Shift

Correct Answer: D. Water-cement ratio

Simple Explanation:

The water-cement (W/C) ratio is the single most important factor governing concrete strength. As per Abrams’ Law, lower W/C ratio → higher strength. More water dilutes the cement paste and leaves pores, reducing strength.

Detailed Explanation:

Abrams’ Law: The strength of fully compacted concrete is inversely proportional to the W/C ratio.

f_c ∝ 1 / (W/C)

Other factors that also affect strength (but water-cement ratio is primary):

• Aggregate quality: Weak or porous aggregates reduce strength
• Cement content: More cement → more C-S-H gel, but W/C still governs
• Degree of compaction: Poor compaction leaves air voids that reduce strength (~5% air void → ~30% strength loss)
• Curing: Improper curing stops hydration early
• Age: Strength increases with time

Typical W/C ratios: 0.45 (high strength), 0.50 (moderate), 0.55–0.60 (low strength). Below 0.38, not enough water for complete hydration; above 0.65, strength drops significantly.

Source: SSC JE 2016

Correct Answer: B. 5%

Simple Explanation:

By definition (IS 456:2000), characteristic strength is the value below which no more than 5% of results fall. This means 95% of test samples must equal or exceed this value.

Detailed Explanation:

As per IS 456:2000 Clause 6.2.1:

“The characteristic strength is defined as the strength of material below which not more than 5% of the test results are expected to fall.”

How it is calculated:

f_ck = f_m − 1.65σ

Where: f_ck = characteristic strength, f_m = mean strength, σ = standard deviation

The 150 mm cube test at 28 days is the standard test method per IS 516. Characteristic strength is measured at 28 days.

Source: SSC JE 2012

Correct Answer: B. Low Heat Cement

Simple Explanation:

Mass concrete generates a lot of heat during hydration. If this heat builds up inside a large concrete mass (like a dam), it causes thermal cracking. Low Heat Cement reduces the heat generated, preventing cracking.

Detailed Explanation:

Why heat is a problem in mass concrete:

In large pours (dams, thick foundations, retaining walls), heat generated by hydration cannot escape quickly. The temperature differential between the interior and surface causes tensile stresses, leading to cracking.

Low Heat Cement (IS 12600):

• Reduced C₃S and C₃A content (these react fastest and generate most heat)
• Higher C₂S content (slower reaction, lower heat)
• Heat of hydration ≤ 250 kJ/kg at 7 days, ≤ 290 kJ/kg at 28 days

Other suitable cements for mass concrete:

Rapid Hardening Cement is the WORST choice for mass concrete — it generates more heat even faster.

Source: SSC JE 2013

Correct Answer: B. Rapid Hardening Cement

Simple Explanation:

Road pavements need to be opened to traffic quickly. Rapid Hardening Cement gains strength faster (it achieves 3-day strength equal to OPC’s 7-day strength), so roads can open sooner.

Detailed Explanation:

Why Rapid Hardening Cement (RHC) for pavements?

• Roads must carry traffic as soon as possible — minimizing closure time reduces economic losses
• RHC achieves high early strength in 1–3 days
• Made by increasing C₃S content and grinding the clinker more finely
• Fineness: ≥ 3250 cm²/g (vs. ~2250 cm²/g for OPC)

Note: RHC generates more heat than OPC, making it unsuitable for mass concrete. It is also used in precast concrete elements, cold weather concreting, and emergency repair work.

Source: SSC JE 2014 Morning

Correct Answer: D. Not interconnected but contain some quantity of gel water

Simple Explanation:

This question distinguishes between gel pores (very small, in C-S-H gel, not interconnected, hold adsorbed gel water) and capillary pores (larger, formed by excess water, interconnected at high W/C ratios). The answer describes gel pores, which are the ones that hold gel water without being interconnected.

Detailed Explanation:

Types of pores in hardened cement paste:

Key fact: At W/C ≤ 0.38, all water is consumed by hydration and capillary pores become disconnected. Above W/C = 0.70, capillary pores remain connected even at full hydration.

Permeability of concrete is largely governed by capillary pore connectivity — lower W/C → disconnected capillary pores → lower permeability → more durable concrete.

Source: SSC JE 2005

Correct Answer: B. Abrams’ Law

Simple Explanation:

Abrams’ Law (1919) states that for a given set of materials, concrete strength depends only on the water-cement ratio. Lower W/C = higher strength.

Detailed Explanation:

Duff Abrams (1919) established that concrete compressive strength (f_c) is given by:

f_c = K₁ / K₂^W/C

Where K₁ and K₂ are empirical constants depending on the materials used.

Limitations of Abrams’ Law:

• Assumes fully compacted concrete
• Assumes the same materials (cement type, aggregate type)
• Does not account for aggregate quality or size
• Valid for W/C ratios between 0.35 and 0.80

Practical application in IS 10262:2019 (Mix Design):

The target water-cement ratio is read off a strength vs. W/C ratio curve for the specific cement and aggregate combination. This is the foundation of concrete mix design.

Source: SSC JE 05/06/2024

Correct Answer: B. Weight of water to weight of cement

Simple Explanation:

W/C ratio = Weight of water / Weight of cement. It is always expressed by weight (mass), not volume, because cement and water have very different densities.

Detailed Explanation:

The water-cement ratio is defined as:

W/C = Mass of water / Mass of cement

Why weight, not volume?

• Cement density ≈ 1500 kg/m³ (bulk) to 3150 kg/m³ (specific gravity)
• Water density = 1000 kg/m³
• Using volume would give a very different (and inconsistent) ratio

Typical W/C ratios per IS 456:2000:

Source: SSC JE 29-01-2018 (Evening Shift)

Correct Answer: B. Weight by weight

Simple Explanation:

W/C ratio is always expressed as weight of water per weight of cement (both in kg or grams). This gives a dimensionless ratio that directly governs concrete strength and durability.

Detailed Explanation:

Water-cement ratio = W(water) / W(cement), both measured by mass.

Example: For a concrete mix using 180 kg of water and 360 kg of cement per m³:

W/C = 180/360 = 0.50

Water-cement ratio vs. water-binder ratio:

In modern concrete with supplementary cementitious materials (SCM) like fly ash, slag, or silica fume, the “water-binder ratio” (W/B) is used:

W/B = Weight of water / (Weight of cement + Weight of SCM)

IS 456:2000 still uses W/C, but IS 10262:2019 (Mix Design) recommends using W/B when SCMs are included.

Source: SSC JE 2014 Morning

Correct Answer: A. To increase permeability

Simple Explanation:

Higher W/C ratio means more free water in the mix. This water eventually evaporates, leaving behind pores and capillary channels that make concrete more permeable (water can pass through more easily).

Detailed Explanation:

W/C ratio and permeability relationship:

As W/C ratio increases → more excess water → more capillary pores → pores become interconnected → higher permeability

Durability implications:

• High permeability allows chlorides, sulfates, CO₂, and water to penetrate → corrosion of reinforcement and chemical attack
• IS 456:2000 limits maximum W/C based on exposure conditions to ensure durability
• Minimum cement content is also specified alongside maximum W/C

At W/C ≤ 0.38, capillary pores become discontinuous and permeability drops dramatically — the threshold for truly impermeable concrete.

Source: SSC JE Civil 2004

Correct Answer: C. Silica, alumina and reactive alkali compounds

Simple Explanation:

Pozzolanas (like fly ash, silica fume, rice husk ash) are rich in silica and alumina. These compounds react with calcium hydroxide (released during cement hydration) to form additional cementite gel, improving strength and durability.

Detailed Explanation:

What makes a material pozzolanic?

A pozzolan is a siliceous (or siliceous-aluminous) material that, by itself, has no cementitious property but reacts with Ca(OH)₂ in the presence of water to form compounds with cementitious properties.

Pozzolanic reaction:

SiO₂ + Ca(OH)₂ + H₂O → C-S-H (additional calcium silicate hydrate gel)

Common Pozzolans:

Benefits of pozzolans in concrete: Reduced heat of hydration, improved long-term strength, reduced permeability, resistance to sulfate attack, and reduced cost.

Source: SSC JE 2010

Correct Answer: D. Reduces the heat of hydration

Simple Explanation:

Fly ash reacts slowly with the Ca(OH)₂ released during cement hydration (secondary/pozzolanic reaction). This slow reaction generates less heat, making fly ash concrete ideal for mass concrete where heat build-up is a concern.

Detailed Explanation:

How fly ash reduces heat of hydration:

Fly ash replaces a portion of cement in the mix. Since fly ash reacts more slowly than cement clinker, the rate of heat release is spread over a longer period — reducing peak temperature rise.

Benefits of fly ash in concrete (IS 3812):

• Reduces heat of hydration (crucial for mass concrete)
• Improves workability due to spherical particles (ball-bearing effect)
• Reduces water demand (some fly ashes)
• Increases long-term strength (pozzolanic reaction continues for months)
• Reduces permeability (denser microstructure)
• Reduces alkali-silica reaction
• Sustainable — uses an industrial by-product

Typical replacement levels: 15–35% of cement (ordinary use); up to 50–60% in High Volume Fly Ash (HVFA) concrete.

Source: SSC JE 23.03.2021 Morning Shift

Correct Answer: D. 50–60%

Simple Explanation:

HVFA concrete uses 50–60% of the cementitious content as fly ash. This significantly reduces cement demand, lowers heat of hydration, and makes concrete more sustainable.

Detailed Explanation:

High Volume Fly Ash (HVFA) Concrete:

Developed by researchers including Dr. V.M. Malhotra, HVFA concrete uses fly ash to replace 50–60% of cement by mass. This is much higher than the standard 15–35% replacement in conventional concrete.

Properties of HVFA concrete:

Applications: Mass concrete pours (dams, thick foundations), sustainable construction, pavement concrete, large industrial floors.

Source: SSC JE 29-10-2020 Morning

Correct Answer: B. Gypsum

Simple Explanation:

Gypsum (CaSO₄·2H₂O) is added to cement clinker during grinding to retard (slow down) the otherwise extremely rapid setting of C₃A. Without gypsum, cement would set within minutes of water addition.

Detailed Explanation:

Why gypsum is essential in Portland cement:

During grinding of clinker, 2–4% gypsum is always added. When cement is mixed with water, C₃A (tricalcium aluminate) would cause a flash set (setting in minutes) without gypsum. Gypsum reacts with C₃A to form ettringite (a needle-like crystal), which coats the C₃A particles and slows down the reaction:

C₃A + 3(CaSO₄·2H₂O) + 26H₂O → Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O (Ettringite)

This ettringite coating retards setting, giving 30+ minutes of workable time.

Note the distinction:

• Gypsum = CaSO₄·2H₂O (dihydrate) — used in cement manufacturing
• Calcium sulphate = CaSO₄ (anhydrite) — also used but less commonly
• Calcium chloride (CaCl₂) = an ACCELERATOR, not retarder

Source: SSC JE 22.1.2018 Morning Shift

Correct Answer: A. Hydroxylated carboxylic acid

Simple Explanation:

Hydroxylated carboxylic acid (like gluconic acid or citric acid) is a chemical admixture classified as a plasticizer (water reducer). It increases workability without adding water, or reduces water content for the same workability.

Detailed Explanation:

Types of Chemical Admixtures (IS 9103):

Plasticizers work by adsorbing onto cement particles and providing a negative charge, causing electrostatic repulsion between particles — dispersing them and releasing trapped water, improving workability.

Source: SSC JE 22.1.2018 Morning Shift

Correct Answer: D. Both (i) and (ii)

Simple Explanation:

CaCl₂ is an accelerator. It speeds up cement hydration (decreases setting time) AND increases drying shrinkage. Both effects are well-established.

Detailed Explanation:

Effects of CaCl₂ on concrete:

Calcium chloride (CaCl₂) is the most commonly used accelerating admixture. When added to concrete:

• ✅ Decreased setting time — accelerates hydration of C₃S and C₃A
• ✅ Increased shrinkage — due to modification of C-S-H gel structure
• ✅ Increased early strength — 1–3 day strength significantly improved
• ✅ Increased heat of hydration
• ❌ Corrosion risk — chloride ions attack steel reinforcement; therefore, CaCl₂ is NOT permitted in RCC or prestressed concrete (IS 456:2000)

Permissible use: CaCl₂ can be used in plain concrete (PCC) only, where there is no reinforcement. It is useful in cold weather concreting to prevent freezing before strength develops.

Maximum dose: 1.5% by weight of cement (if used at all)

Source: SSC JE 2012

Correct Answer: B. Decreases

Simple Explanation:

Higher grade concrete requires lower water-cement ratio to achieve higher strength and durability. As per IS 456:2000, maximum W/C decreases progressively with increasing concrete grade.

Detailed Explanation:

Maximum W/C ratio per IS 456:2000 (Table 5):

This directly demonstrates Abrams’ Law: to achieve higher compressive strength, the W/C ratio must decrease.

Practical impact: Lower W/C requires more cement or the use of superplasticizers to maintain workability. This is why high-strength concrete (M40+) almost always uses superplasticizers.

Source: SSC JE 15.11.2022 Shift-I