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.
Efflorescence is a deposit of salts, usually white, on the surface of bricks and blocks after being laid. The salts usually come from ground water or out of the mortar, but may also, come from within the masonry units themselves.
Shotcrete is mortar or very fine concrete deposited by jetting or impacting it with high velocity (pneumatically projected or sprayed) on to a prepared surface. Shotcrete is frequently more economical than conventional concrete because of less formwork requirements, requiring only a small portable plant for manufacture and placement.
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.
Back hoe or hoe can be used for digging trenches where precise control of depth is needed. Power shovels can be used to excavate the earth and load it into the trucks or other hauling equipments waiting nearby. They are capable of excavating all classes of earth except the solid rocks without loosening.. A dragline is designed to excavate below the level of the machine and need not to go into a pit or hole for excavating the earth. When the excavated earth is to be deposited on nearby banks or dams, it is better to use dragline with a long boom enough to dispose of the earth in one operation, eliminating the need of hauling units thereby reducing the cost of handling the earth. A dragline is excellent for excavating trenches without shoring. The clamshell bucket is designed to excavate material in vertical direction and it is primarily used for handling loose materials such as sand, gravel, crushed stone, coal, etc.
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.
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.
Unsoundness is cement is due to excess of lime, excess of magnesia or excessive proportion of sulphates Le chatelier test detects unsoundness due to free lime only. Autoclave test is used to detect unsoundness due to magnesia and lime.
One of the potential ways of bulk disposal and gainful use of fiyash in an economical manner is to convert the fiyash into more usable and manageable form such as lightweight aggregate. Sintered fiy as lightweight aggregate is manufactured by sintering the fiyash pellets in a vertical shaft kiln. The aggregate is suitable for use in production of structural lightweight concrete and precast lightweight concrete units for the use as load bearing and nonload bearing units.
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.