Admixtures. Lecture No. 12

Transcription

1 Admixtures Lecture No. 12

2 Set-Retarding This type of chemical admixtures decreases the initial rate of reaction between cement and water and thereby retards the setting of concrete. It functions by coating the surface of C3S (Tri calcium silicate) components, thus, delaying this reaction with the water. Reaction products are slow to form as such the setting and hardening of concrete are delayed reducing early compressive strengths. Retardation in setting time up to 8-10 hours is possible by suitable use of retarders.

3 Set-Retarding The main ingredients of retarders are as follows: Lignosulphonic acids and their salts. e.g. Na, Ca or NH4, Hydro-carboxylic acids and their salts. Carbohydrates including sugar. Inorganic salts based on flourates, phosphates, oxides, borax and magnesium salts.

4 Set-Retarding: Advantages Improves workability, cohesion and extends setting time, provides protection against delays and stoppages and facilitates keeping workable concrete for extended period. In the large construction, good workability of the concrete throughout the placing period and prevention of cold joints is ensured by adding retarders in the concrete. Extended setting time minimise risks of long distance delivery in hot weather, improves pumpability of concrete by extended setting period and improved workability of concrete.

5 Set-Retarding: Advantages Reduces bleeding and segregation where poor sand grading are unavoidable. Reduces adverse environmental effects of various nature on concrete and embedded steel by considerable reduction in permeability.

6 Effect of use of Retarder Retarding admixture forms a film around the cement grain that prevent or delays the reaction with water. After some times, this film breaks and normal hydration takes place.

7 Accelerating Admixtures These admixtures when added to concrete, mortar or grout increases the rate of hydration of hydraulic cement, shortens the time of set, accelerates the hardening or development of strength of concrete/ mortar. These admixtures function by interaction with C3S (Tricalcium silicate) component of the cement thus increasing the reaction between cement and water

8 Accelerating Admixtures Many substances are known to act as accelerators for concrete. They include Alkali Hydroxides, Silicates, Fluoro- Silicates, Organic Compounds, Calcium Formates, Calcium Nitrates, Calcium Thio Sulphates, Aluminium Chlorides, Potassium Carbonates, Sodium Chlorides & Calcium Chlorides.

9 Accelerating Admixtures: Advantages Shortens the setting time of cement and therefore increases the rate of gain of strength. Enables earlier release from precast moulds thus speeding production. Reduces segregation and increase density and compressive strength. Cures concrete faster and therefore uniform curing in winter and summer can be achieved. Reduces water requirements, bleeding, shrinkage and time required for initial set.

10 Effect of use of Accelerating Admixtures

11 Mineral Admixtures Mineral admixtures are finely divided siliceous materials which are added to concrete in relatively large amounts, generally in the range 20 to 70 percent by mass of the total cementitious material.

12 Mineral Admixtures

13 Ecological Advantage Power plants using coal as fuel and metallurgical furnaces producing cast iron, silicon metal, and ferrosilicon alloys are the major sources of byproducts that are being produced at the rate of millions of tonnes every year in many countries. Dumping of these by-products into landfills and streams amounts to a waste of the material and causes serious environmental pollution. Disposal as concrete aggregate or for roadbase construction is a low-value use which does not utilize the pozzolanic and cementitious potential of those materials.

14 Ecological Advantage With proper quality control, large amounts of many industrial by-products can be incorporated into concrete, either in the form of blended portland cement or as mineral admixtures. Whenever a pozzolanic and/or cementitious byproduct can be used as a partial replacement for portland cement in concrete, it represents significant energy and cost savings.

15 Advantage It has been amply demonstrated that the best pozzolans in optimum proportions mixed with Portland cement improves many qualities of concrete, such as: Lower the heat of hydration and thermal shrinkage; Increase the watertightness; (Reduce the alkali-aggregate reaction; Improve resistance to attack by sulphate soils and sea water; Improve workability; Lower costs.

16 Pozzolanic Reaction Pozzolana + Calcium Hydroxide + Water C S H (Gel) The mechanism by which pozzolanic reactionexercises a beneficial effect on the properties of concrete is the same irrespective of whether a pozzolanic material is added to concrete in the form of a mineral admixture or as a component of blended portland cement. Pozzolanic (e.g., low-calcium fly ash), Cementitious (e.g., granulated iron blast-furnace slag), Both cementitious and pozzolanic (e.g., highcalcium fly ash)

17 Classification Natural materials: have been processed for the sole purpose of producing a pozzolan. Processing usually involves crushing, grinding, and size separation; in some cases it may also involve thermal activation. By-product materials: are not the primary products of the industry producing them. Industrial byproducts may or may not require any processing.

18 Natural Pozzolans Volcanic glasses: Santorini Earth of Greece, Bacoli Pozzolan of Italy, and Shirasu Pozzolan of Japan are examples of pozzolanic materials which derive their lime-reactivity characteristic mainly from the unaltered aluminosilicate glass. Volcanic tuffs: Pozzolans of Segni-Latium (Italy), and trass of Rheinland and Bavaria (Germany), represent typical volcanic tuffs. Calcined clays or shales: clay and shales will not show appreciable reactivity with lime unless the crystal structures of the clay minerals present are destroyed by heat treatment

19 Natural Pozzolans Diatomaceous earth: This group of pozzolans is characterized by materials of organic origin. Diatomite is a hydrated amorphous silica which is composed of skeletal shells from the cell walls of many varieties of microscopic aquatic algae. The largest known deposit is in California.