Test: Cement, Mortar & Lime - 3 - Civil Engineering (CE) MCQ

The fineness of cement is a measure of the size of particles of cement and is expressed in terms of specific surface of cement.
For a given weight of cement, the surface area is more for finer cement than for coarser cement. The finer the cement, the higher is the rate of hydration, as more surface area is available for chemical reaction. This results in the early development of strength.

Air entrainment will reduce damage during freeze- thaw cycles thereby increasing the concrete’s durability. Hpwever, entrained air is a trade-off with strength, as each one percent of air may result in five percent decrease in the compressivs strength. The compounds used for air-entrainment are a number ofaetural wood resins, various sulphonated compounds, and some animal and vegetable fats and oils such as tallow, olive oil and their fatty acids such as stearic and oleic acids.

On increasing fineness setting time decreases. So early gain of strength with decrease in lime content quick setting takes place.
On increasing Gypsum Initial setting time increases.

Explanation:

1. High-strength Portland cement:

 This cement is produced by a special technique called Macro Defect Free (MDF) innovation. In this process 4-7% of one of several water-soluble polymers (such as hydroxypropyl methylcellulose, polyacrylamide of hydrolyzed polyvinyl acetate are added to generate high strength.

C₃S gives high-strength Portland cement

2. Super sulfated cement is resistant to sulfate attack and also to chemical attack.

3. High Alumina cement cannot be used with admixtures

4. Rapid Hardening cement gives a higher rate of heat development due to an increased rate of reaction.

Lime used for mortar may be fat lime (quick or hydrated lime) or hydraulic lime. Slaked fat lime is used to prepare mortar for plastering, while hydraulic lime is used in preparing mortar for masonry construction.

Low-heat Portland Cement: This cement is less reactive than OPC and is obtained by increasing the proportion of C₂S and reducing that of C₃S and C₃A  this is achieved by restricting the amount of calcium and increasing that of silicates in the raw material for manufacture. This reduction in the contents of more rapidly hydrating compounds C₃S and C₃A results in a slow development of strength but the ultimate strength is the same. The initial setting time is about one hour, i.e., greater than that of OPC, and final setting time is about 0 hours. This cement is recommended for the use in mass concrete construction such as retaining walls, bridge piers and abutments, dams, etc., where temperature rise by heat of hydration can become excessive.

The loss on ignition test is carried on portland cement to determine the loss of weight when the sample is heated to 900-1000°C. The loss is weight occurs as the moisture and carbon dioxide which are present is combination with free lime or magnesia evaporate.
The loss on ignition is determined by heating one gram of cement sample is a platinum crucible at a temperature of 900°C-1000°C for minimum of 15 minutes. Normally, the loss, will be in the neighbourhood of two percent.
Maximum allowable loss is four percent.

Heat of hydration is the heat generated when cement reacts with water during the setting process. The compounds mentioned, C3A (tricalcium aluminate), C4AF (tetracalcium aluminoferrite), C3S (tricalcium silicate), and C2S (dicalcium silicate), are the four main components of Portland cement.

The heat of hydration follows the general order:

1. C₃A - Tricalcium aluminate has the highest heat of hydration.
2. C₃S - Tricalcium silicate follows with a significant heat of hydration but less than C3A.
3. C₄AF - Tetracalcium aluminoferrite has less heat of hydration than C3S.
4. C₂S - Dicalcium silicate has the least heat of hydration.

Based on this, the correct order in decreasing heat of hydration is:

C₃A > C₃S > C₄AF > C₂S

So, the correct option is: C₃A > C₃S > C₄AF > C₂S