Over the past several decades, the use of fly ash in concrete has had a successful track record. The performance benefits that fly ash provides to mechanical and durability properties of concrete have been well researched and documented in actual structures. Rao lists down the benefits and parameters to assess and ascertain the quality of the material.
Fly ash, also known as flue-ash, is one of the residues generated in combustion, and comprises of fine particles that rise with the flue gases. Fly ash material solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify rapidly while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 ?m to 300 ?m.
Fly ash can significantly improve the workability of concrete. Recently, techniques have been developed to replace partial cement with high-volume fly ash (50 per cent cement replacement). Due to the spherical shape of fly ash particles, it can increase workability of cement while reducing water demand.
We have used fly ash successfully in the Shantigram Township project at Ahmedabad, which includes The Meadows, Water Lily, Golf Club Building, Bungalows, Aangan, Pratham, STP and the Corporate House.
Fly ash use in our concrete blends
We are using fly ash (35 per cent to 18 per cent) in our concrete production – Concrete grade M10 to M40. We are conducting Workability test, Compressive Strength as well as Durability test by means of water permeability tests at our own laboratory.
Benefits of blending fly ash in concrete
There are two major benefits of using fly ash in concrete.
Environmental benefits
a) Use of fly ash in concrete imparts several environmental benefits and thus it is eco-friendly.
b) It saves the cement requirement for the same strength thus saving raw materials such as limestone, coal, etc., required for manufacture of cement. Manufacture of cement is very energy intensive. In the manufacturing of one tonnes of cement, about 1 tonnes of CO2 is emitted in the atmosphere. Less requirement of cement means less emission consequently reducing green house gas emissions.
c) Due to low calorific value and high ash content in Indian coal, thermal power plants in India are producing huge quantities of fly ash. This huge quantity is being stored/disposed-off in ash pond areas. The ash ponds acquire large areas of agricultural land. Use of fly ash reduces area requirement for pond, thus saving good agricultural land.
Engineering benefits
d) Adding fly ash to concrete will reduce cement consumption and thereby heat of hydration, resulting in production of sound concrete, which is also economical for project.
e) Fly ash blended cement not only safeguards against environment hazards but also improves quality of concrete structures in terms of durability.
f) Leaching in concrete is reduced. Excess Ca(OH)2/free lime is consumed by high reactive silica (HRS) (additional cementitious material). It also increases durability and strength over a period of time.
g) The values of water permeability are lower for fly ash blended concrete due to pore refinement, due to greater secondary reaction products, because of more number of reactive fly ash particles and their desirable particle size distribution.
h) It has been observed that the compressive strength of fly ash blended concrete at later stage is more than the normal concrete.
i) It helps to control differential temperature of concrete mass.
Quality of fly ash
To utilise fly ash as a Pozzolana in cement concrete and cement mortar, the Bureau of Indian Standards (BIS) has formulated IS: 3812 Part-1, 2003. In this code quality requirement for siliceous fly ash (Class F fly ash) and calcareous fly ash (Class C fly ash) with respect its chemical and physical composition have been specified. The major constituents of most of the fly ashes are silica, alumina, ferric oxide and calcium oxide. There is wide range of variation in the principal constituents – silica (25-60 per cent), alumina (10-30 per cent) and ferric oxide (5-25 per cent). When the sum of these three principal constituents is 70 per cent or more and reactive calcium oxide is less than 10 per cent – technically the fly ash is considered as Siliceous fly ash or Class F fly ash. If the sum of these three constituents is equal or more than 50 per cent and reactive calcium oxide is not less than 10 per cent, the fly ash will be considered as Calcareous fly ash or Class C fly ash.
Fly ash must conform to IS3812 Part 1: 2003 standards. Generally we use quality of fly ash, which has:
- High lime reactivity
- Low carbon content, and
- High fineness.
Good quality fly ash has following mineralogical composition:
- SiO2 + Al2O3 + Fe2O3 (per cent by mass) – 70 per cent min
- SiO2 (per cent by mass) – 35 per cent min
- Magnesium Oxide (per cent by mass) – 5 per cent max
- Total Sulphur as SO3 (per cent by mass) – 3 per cent max
- Loss on ignition (per cent by mass) – 5 per cent max, and
- Total Chloride (per cent by mass) – 0.05 per cent max.
Good quality fly ash has following Physical characteristics:
- Fineness; specific surface (m2/kg) – 320 min
- Lime reactivity; average compressive strength (N/mm2) – 4.5 min
- Particle retained on 45 micron (per cent) – 34 per cent maximum, and
- Compressive strength at 28 days (per cent) – Not less than 80 per cent of the strength of corresponding plain cement mortar cubes.
Availability of good quality fly ash in country
Any countrys economic and industrial growth depends on the availability of power. In India coal is a major source of fuel for power generation. About 60 per cent power is produced using coal as fuel. Indian coal has low calorific value (3000-3500 Kcal) and very high ash content (30-45 per cent), resulting in huge quantities of ash generated in coal based thermal power stations.
As per the Bureau of Indian Standards, IS: 3812 (Part-1), this type of ash is termed as Pulverised Fuel Ash. Fly ash produced in modern power stations of India is of good quality as it contains low sulphur and very low unburnt carbon, i.e., less loss on ignition. In order to make fly ash available for various applications, most of the new thermal power stations have set up dry fly ash evacuation and storage systems. In this system fly ash from Electrostatic Precipitators (ESP) is evacuated through pneumatic system and stored in silos. From the silos, it can be loaded in open truck/closed tankers or can be bagged through suitable bagging machines.
The coarse particles of fly ash are collected in first fields of an ESP. The fineness of fly ash particles increases in subsequent fields of the ESP.
As of now in Gujarat (Ahmedabad), we purchase good quality fly ash from Mundra, Gandhinagar and Wanakbori thermal power plants for our projects. By consuming fly ash we are saving environment by reducing the CO2 emissions and as well as increasing the durability of structures.
Any country’s economic and industrial growth depends on the availability of power. In India coal is a major source of fuel for power generation. About 60 per cent power is produced using coal as fuel. – K Madhusudana Rao, Adani Realty
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