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

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)

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

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

Fig 1.

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.

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)

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

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

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)

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,

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

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

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

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.

• 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

• 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

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

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.