Mineral Admixtures
Mineral admixtures (fly ash, silica fume [SF], and slags) are usually added to concrete in larger amounts to enhance the workability of fresh concrete; to improve resistance of concrete to thermal cracking, alkali-aggregate expansion, and sulfate attack; and to enable a reduction in cement content.
Fly ash
-Fly ash is one of largest emissions of the industrial waste residues in current China.
-the emissions of fly ash from coal-fired power factories will increase year by year. A large number of unprocessed fly ash will generate dust and pollute the atmosphere
-discharged into the water system cause the rivers blockage and the toxic chemicals contained in it will be harmful to the human body and life-forms.-the emissions of fly ash from coal-fired power factories will increase year by year. A large number of unprocessed fly ash will generate dust and pollute the atmosphere
-If added to the concretes directly they will save a lot of cement raw materials
Silica Fume
- also known as microsilica, is a byproduct of the reduction of high-purity quartz with coal in electric furnaces in the production of silicon and ferrosilicon alloys.
-collected as a byproduct in the production of other silicon alloys such as ferrochromium, ferromanganese, ferromagnesium, and calcium silicon.
-Before the mid-1970s, nearly all Silica Fume was discharged into the atmosphere, After environmental concerns necessitated the collection and landfilling of Silica Fume, it became economically justified to use Silica Fume in various applications.
-Silica Fume is a highly effective pozzolanic material
-used in concrete to improve its properties.
- improves compressive strength, bond strength, and abrasion resistance; reduces permeability; and therefore helps in protecting reinforcing steel from corrosion.
Slag
-made by intergrinding the granulated slag with portland cement clinker (blended cement), has been used for more than 60 years, the use of separately ground slag combined with portland cement at the mixer as a mineral admixture.
- granular material formed when molten iron blast furnace slag is rapidly chilled (quenched) by immersion in water.
-granular product with very limited crystal formation, is highly cementitious in nature and, ground to cement fineness, hydrates like portland cement.
Chemical Admixtures
Chemical admixtures are added to concrete in very small amounts mainly for the entrainment of air, reduction of water or cement content, plasticization of fresh concrete mixtures, or control of setting time.
Air-Entrainment
-the process whereby many small air bubbles are incorporated into concrete and become part of the matrix that binds the aggregate together in the hardened concrete.
-air bubbles are dispersed throughout the hardened cement paste but are not, by definition, part of the paste
-the formation of minute air bubbles dispersed uniformly through the cement paste increased the freeze-thaw durability of concrete
-ormation can be achieved through the use of organic additives, which enable the bubbles to be stabilized or entrained within the fresh concrete
Water-Reducing
- groups of products that are added to concrete to achieve certain workability (slump) at a lower w/c than that of control concrete
-sed to improve the quality of concrete and to obtain specified strength at lower cement content.
-improve the properties of concrete containing marginal- or low-quality aggregates and help in placing concrete under difficult conditions.
-categorized according to their active ingredients:-
- salts and modifications of hydroxylized carboxylic acids (HC type);
- salts and modifications of lignosulfonic acids (lignins); and
- polymeric materials (PS type).
Set-Retarding
-delay hydration of cement without affecting the long-term mechanical properties.
-used in concrete to offset the effect of high temperatures, which decrease setting times, or to avoid complications when unavoidable delays between mixing and placing occur
-Use of set retarders in concrete pavement construction
1) enables farther hauling, thus eliminating the cost of relocating central mixing plants;
2) allows more time for texturing or plastic grooving of concrete pavements;
3) allows more time for hand finishing around the headers at the start and end of the production day; and
4) helps eliminate cold joints in two-course paving and in the event of equipment breakdown
-to resist cracking due to form deflection that can occur when horizontal slabs are placed in sections
Accelerating
-added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both
-accelerators include some of inorganic compounds such as soluble chlorides, carbonates, silicates, fluosilicates, and some organic compounds such as triethanolamine.
-Among all these accelerating materials, calcium chloride is the most common accelerator used in concrete.
-growing interest in using "chloride-free" accelerators as replacement for calcium chloride has been observed because calcium chloride in reinforced concrete can promote corrosion activity of steel reinforcement, especially in moist environments.
Superplasticizers
-developed in Japan and Germany in the early 1960s
-Superplasticizers are linear polymers containing sulfonic acid groups attached to the polymer backbone at regular intervals. Most of the commercial formulations belong to one of four families:
- Sulfonated melamine-formaldehyde condensates (SMF)
- Sulfonated naphthalene-formaldehyde condensates (SNF)
- Modified lignosulfonates (MLS)
- Polycarboxylate derivatives
17 March 2012 at 21:35
fly ash after added into the concrete, is it stable?
18 March 2012 at 23:06
Admixture is materials other than water, sand, cement into fresh concrete or mortar to change one or more properties of fresh or hardened concrete
Fly ash, silica fume and slag are pozzolan, which possesses pozzolanic characteristic. The main reason these materials classified as pozzolan is due to high reactive silica content in their chemical composition
This reactive silica is important as in finely divided form, with the presence of moisture, pozzolan will react with calcium hydroxide of cement to form compound of cementitious properties. This reaction called as pozzolanic reaction
Si + CaOH = C-S-H (main component for concrete strength)
Replacing fly ash with cement partially will definitely increase the concrete strength in long term due to the continuous pozzolanic reaction in the concrete