3.cementing matrials
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construction materialTRANSCRIPT
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IntroductionCementing materials
•These are Materials with additive and cohesive properties•They have capable of uniting and bonding solid particles together•These are: Lime, gypsum, and Portland cement
Cement•Is additive substance •Which when mixed with water form paste
The paste •Is temporarily plastic and be molded or deformed•And later it sets and harden to rigid mass•Cement of this kind are known as calcareous cement
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Calcareous cement
• Non hydraulic cement
– Are cement which are not able to set and harden (e.g., Non hydraulic lime) or stable (e.g. gypsum plaster) in water.
• Hydraulic cement
– Are cement which are able to set and harden in water
– And give a solid mass that dose not disintegrated.
– Remain stable in water (e.g. Portland cement)
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Lime• It is found in its natural form as rock of varying degree
of hardness• Mainly composed of calcium oxide(CaO)• Lime deposits are generally found mixed with
impurities (CO2, FeO3 and MgCO3 ), this acquire different colors
Production of lime• It involves burning of the row materials and then
Slaking• Depending on the amount of heat and the method of
slaking, the product is hydraulic or non-hydraulic lime
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A. Quick lime/ commercial lime– Burning the natural stone in some of vertical kilns
to a temperature of 1000 oC.
– The CO2 is driven off, leaving the CaO i.e., quick lime or caustic lime
– White in color and having a specific gravity of about 3.40
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Fig 1.
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B. Hydrated/ slaked lime• Quick lime can never be used for construction
purposes, but must mixed with water. The processes is called slaking or hydration of lime.
• The resulting product is calcium hydroxide
(Ca (OH)2) – Slacked or Hydrated lime
• It is ready to be made into plaster or mortar by adding water and sand to form a temporary plastic mass.
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Type of slacking (depending on the amount of water added)
• Wet-slacking • Dry-slacking
Wet-slacking • At the building site by mixing quick lime with an excess
of water• Passed through a fine sieve to remove slow slaking
particle• Left to mature for several days • Stirred by shovel or stick this - reduce all anhydrated
particles (causes popping, pitting and disintegration)
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Dry-slacking
• It is manufactured in a factory under controlled condition.
• Sufficient water is used to hydrate the quick lime
• then the lumps break in to a dry powder known as dry hydrate or hydrate lime
• Depending on the impurities present the specific gravity of hydrated lime varies from 2.08 - 2.4
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Preserving quicklime• Is has much affinity for water and quickly absorb
moisture from atmosphere and air – slacked and loses its cementing qualities
• Therefore kept in dry storage and carefully protected from dampness
Setting and hardening of lime• Slaked lime hardens or sets by gradually losing its
water through evaporation and absorbing CO2
from the air, thus changing back from Ca(OH)2 to CaCO3 or limestone
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The cycle is completed through• Burning
CaCO3 + heat CaO +CO2 (quick lime)
•Slaking CaO + H2O Ca(OH)2 + heat (slaked lime or Hydrated lime)
•Setting Ca(OH)2 + CO2 CaCO3 + H2O (dehydrated lime)
Two type of lime1.High calcium lime
•Pure, white, reach•High degree of plasticity(workability)
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Fig 2. Completed cycle in lime used as a cementing material
The Use of lime• It is used mortar for building brick • Stone masonry•Plastering walls of buildings
2. Manganese lime(lean)poor ,gray,
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Gypsum plasters• It is used in arts and in building construction• A combination of sulphate of lime with water of
crystallization• It occurs as
– Hydrous sulphate of lime (CaSO4.2H2O) 75% and 24%
– Unhydrous sulphate of lime little or no water of crystallization
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Gypsum rock contain
• Silica, alumina, lime carbonate, carbonate of magnesia, iron oxide and other impurities
• At least 65% by weight must be Hydrous sulphate of lime (CaSO4.2H2O)
• Pure gypsum is known as alabstor and
• It is a white translucent crystalline mineral
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Generally gypsum is
• Soft that it can be scratched with the fingernail.
• Fire sensitive
• Sets quickly
• Light in weight
• Never used in places exposed to the weather, moisture, underground moist
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Manufacture of plaster
• Gypsum plasters are manufactured by heating the row materials gypsum at either moderate or high temperature, the result being plaster of paris or hard–finish plaster
A. plaster of paris (stucco)-calcined gypsum
– gypsum a good deal of water of crystallization(24% by wt.)
– If some of H2O is driven off at a temperature just above the boiling point of water 100 0C, a semi-hydrated plaster is obtained which is known as plaster of Paris.
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• CaSO4.2H2O (moderate heat) CaSO4.1/2H2O +1/2H2O
Plaster of paris:
• is white powder having a specific gravity of 2.57
• known as low-temperature gypsum derivative or semi-hydrated plaster.
Fig 3. completed cycle in gypsum used as plaster
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• Sets rapidly (5-10 minuets)• No chemical change but recombination of the semi
hydrous sulphat with H2O take place• Solid mass of fine interlocking crystal is formed • It attain ultimate or final strength on drying Disadvantages:
A. ShrinkageB. Rapid settingC. Difficult to workD. Used for ornamental works due to A & B
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B. Hard-finish plaster• By burning gypsum to a considerably higher
temperature
• Unhydrous sulphate or unhydrous plaster or high-temperature gypsum derivative is produced
• CaSO4.2H2O (High heat) CaSO4. + 2H2O
• When mixed with water, plaster of paris sets very rapidly whereas hard-finished plaster sets too slowly
• It is reluctant to reabsorb water
• Less soluble in water
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Setting retardant s
a. Flour, glue
b. Borax or Alum– is added to finely ground calcinated gypsum to further improve warkability and accelerate the eventual setting
Calcination- the process of drying of water of crystallization by heat.