More About Ash
Properties of Ash
Pulverised Fuel Ash (PFA) and Furnace Bottom Ash (FBA) each have unique physical and chemical properties, as a result of the combustion process and the chemical composition of the coal.
Physical Properties
PFA particles, particularly those below 50 microns, are spherical in shape, as a result of the way in which they are formed. As the coal is burned at temperatures approaching 1400°C, the minerals associated with it become molten and form spherical shapes. On cooling they solidify as amorphous, glassy material.
PFA particles - generally silt size - are so fine they become airborne and are swept upwards in the flue gases. FBA particles, which are sand sized fall into the grates.
| Property | Typical value(s) |
| Compacted bulk density | 1300 - 1500 Kg/m3 |
| Relative Density (oven dry) | 2.0 - 2.4 |
| Specific Heat capacity | 0.8 - 0.7J/Kg/ºC |
| Water Permeability - compacted fly ash | 1.0 x 10-6 - 7.0 x 10-7 |
| Electrical conductivity | 0.09 W/mK |
| Loss on Ignition | 6 - 10% |
Chemical and mineralogical properties
Fly ash has three main elements, silicon, aluminium and iron, the oxides of which account for 75 - 85% of the material. Fly ash consists principally of glassy spheres together with some crystalline matter and unburned carbon.
| Element | Typical range of values for fly ash |
| Silicon (% as SiO2) | 38 - 52 |
| Aluminium (% as Al2O3) | 20 - 40 |
| Iron (% as Fe2O3) | 6 - 16 |
| Calcium (% as CaO) | 1.8 - 10 |
Uses & Benefits
Using PFA, or fly ash, in place of conventional construction products results in many benefits from environmental to long term strength and durability gains.
PFA in Cement and Concrete
PFA can be used as a cement addition in concrete, for example in ready mixed concrete, concrete products and block manufacture.
It can also be used in cement manufacture, as a raw material in place of clay or shale, as a Minor Additional Constituent (MAC), and to manufacture cements to BS EN 197-1 CEM II and CEM IV.
PFA, or fly ash, is a pozzolana - a material that reacts with lime to form a hardened mass. During the hydration of Portland cement, lime is released. The PFA reacts with this to produce cementitious hydrates, providing additional strength.
The key benefits of using PFA as a cement enhancer are: -
- Increases strength and durability due to pozzolanic reaction
- PFA improves sulphate resistance and reduces the risk of alkali-silica reaction
- Reduces the water required for equal workability, which reduces permeability
- Reduces the heat of hydration
- Improves pumpability
- Increases the quality of the surface finish
- Reduces creep and shrinkage
- Reduces efflorescence in concrete products
PFA for grouting
When mixed with cement and/or lime, PFA grout can be used for filling fissures, voids and cavities such as redundant mines, pipes and behind tunnel segments.
Benefits
Grouts are thin fluid mortars, used to reduce ground permeability, increase the shearing resistance of the ground and fill inaccessible voids.
They may be categorised as chemical, suspension or emulsion systems and include cement, sand/cement, clay/cement, slag/cement, resins and others. The use of PFA in combination with these materials offers a number of important technical, rheological (flow), durability and economic advantages, including:
- Reduced water/solids ratios, which improves flow, reduces bleed and increases durability
- Increased compressive strength
- Low shrinkage
- Reduced permeability
- Improved pumpability
- Higher resistance to chemical attack
- Increased yield
Lightweight Fill
PFA and FBA can be used as a load -bearing fill in embankments, sub-bases for roads and airfields.
Benefits
PFA has been used successfully as a fill material for many years. It is acknowledged to have a number of important benefits;-
- It is lightweight, which reduces the load on weak ground, and increases the yield per tonne
- It is self hardening - free lime within the PFA starts the pozzolanic reaction, offering many advantages over natural materials.
- It offer low permeability
- Mixed with lime, it forms a stabilised capping layer.
Lightweight Aggregate
PFA and FBA can be used in the manufacture of lightweight concrete blocks including aerated/autoclaved blocks.
Benefits
Sintered fly ash aggregate (Lytag) and Furnace Bottom Ash (FBA) have a number of benefits:-
- Excellent insulating properties
- Produce cost effective building products
- Reduce transport costs and manual handling problems
- Higher strength in comparison to other lightweight aggregates
PFA as an industrial filler
PFA and Cenospheres (hollow, lightweight particles with low thermal conductivity, which float to the surface of ash lagoons) are used in the manufacture of Plastics, Rubbers, refractory products and bituminous materials.
Benefits
Many different markets require inert fillers ranging from foods to high technology products, all of which seek different properties. The benefits of PFA and Cenospheres as fillers include:-
- Higher workability index in asphalts resulting in lower mixing and laying temperatures
- Low coefficient of thermal conductivity for use in ceramics and refractory products
- Lightweight increases yield of manufactured products
- Increases resistance to acids
- Spherical shape reduces induced stress concentrations
- Reduction in compound viscosity
- Increasing stiffness
- Improving dimensional stability
PFA in Stabilisation/Solidification Products
The stabilisation/solidification of contaminated land and soils uses very specialised cementitious products, which often incorporate PFA. We manufacture various products of this type, including EnviroCems and RSA/PT, many of which are custom blended to client specifications. See Waste Stabilisation or Recycled Roads (PDFs) and project case studies.
