Fly Ash for Bricks, Cement and Concrete – The Indian Perspective

Authored by; Dr. N. Bhanumathidas & N. Kalidas

India is a resourceful country for fly ash generation with an annual output of over 220 million tons, registering almost 50% escalation during the last decade. Though utilisation of fly ash has been a subject of great concern in India for the past two and half decades, the utilization has picked up during the last 15-20 years, recording 40-50% use. Nevertheless, the challenge do continue with continuous generation of fly ash out of all the coal based power plants that summons for more aggressive promotional efforts at all levels.

Indian fly ash is one of the best lots in the world but the confidence level in the country is low for various reasons in the past and present. This paper discusses the transition in the Indian fly ash scenario.


The promotional issues of fly ash utilization in India are altogether different for the peculiar conditions prevailed in the country such as:

  • Brick market is traditionally attached to the age-old clay bricks, vetoeing the chances for any alternate materials to penetrate on price logistics as well as traditional attachments to clay bricks;
  • Till the advent of FaL-G technology, autoclave curing being the predominantly known art, no approach was economical to manufacture walling material with fly ash;
  • The poor quality of fly ash generation during 70’s and the unscientific use of such fly ash in some parts of cement industry had effected the credibility of pozzolanic cements with regard to durability that haunts the confidence level in certain segments even today;
  • The ban on the use of pozzolan cement for structural applications, imposed by the government during fall of 80’s, and reiterated in 1999, has given more room for consumers’ apprehensions, making the task tough to the industry while exploring the market;

Since the beginning of the 1990s, the need for the utilization of fly ash has attained new thrust from all segments of the country buttressed by many positive developments. Though India has to go a long way on fly ash utilization, the stage, set in the country, is poised to accomplish the target of over 200 million tons in the near future.


Use of pulverised coal:

The installation of PCB (Pulverised Coal Boilers) and FBC (Fluidised Bed Combustion) systems rapidly since 80’s in Indian thermal plant operations has contributed to change the quality of fly ash drastically. The coal is segregated from clay lumps and pulverized to attain a fineness of 75 micron (70% passed). Such fineness facilitates total combustion within short residential time and also the effective phase transformation of the mineral matter of the coal.

Economiser and Electrostatic Precipitator (ESP)

The fly ash, thus generated, travels along with the flues and undergoes sudden temperature drop in economizer attaining more reactivity. Then the fly ash is segregated from the flue gas and collected in ESPs (Electrostatic Precipitators) or bag filters. The fineness of fly ash improves as it passes through field after the field; the field at the boiler-end collecting the coarsest fly ash and the field at the chimney-end collecting the finest.

It is beyond dispute that the optimization of thermal plant operations, with heat recovery systems such as economizers, helped to generate the most qualified fly ash in India. The following data show the difference in PAI (Pozzolanic Activity Index) of two fly ashes, collected from the same field, with reference to the impact of economizer:

Soluble fraction%PAI
Fly ash not subjected through economizer:4.3094.00
Fly ash subjected through economizer:8.80103.00

The following data show the improvement in fineness, soluble fraction and PAI of the fly ash of a typical super thermal plant, as the field progresses:

Field No.Fineness– 25 micronSiO2Al2O3SolublePAI %
cm2/gm%%%fraction %7-d28-d
II  340354.2062.5230.996.917792

Pneumatic Collection and Conveying Systems

The fly ash utilization has remarkably picked up with the installation of dry ash collection systems at thermal plants, whereby, the fly ash collected in ESP is pneumatically transferred to silos for onward delivery to the users. The 1.6 km long pneumatic conveying system, one of the longest in the world, installed at Ropar Thermal Plant in Punjab, has been facilitating two giant cement industries to consume about 360 thousand tons annually. Similar arrangements are underway at many other plants. At certain power plants, the cement industry is installing transmitters to the ESP hoppers, at their investment, in order to tap the required grade of fly ash.


Circular from Ministry of Power

Considering the enormous quantity of fly ash generated in thermal power stations associated with serious disposal and ecological problems, the government has felt the imperative to promote the utilisation with a sense of urgency.  Pursuant to the deliberations by a working group set up by the department of Power consisting of members from various concerned ministries, Planning Commission and representatives of various autonomous and NGO bodies, a circular was issued on 10th September, 1991, consisting of a lot of promotional programs and incentive packages.

Fiscal Incentives in the Union Budget

In order to give fillip to the circular of Power Ministry, Ministry of Finance has accorded excise duty exemption for fly ash products in Union Budget 1991-92. But this incentive was withdrawn in the budget of 1997-98 without having achieved the target. Realising the mistake, the government has reintroduced the duty waival in 1998-99 budget.

Gazette Order by Ministry of Environment

Ministry of Environment & Forests has drawn much cohesive action plan vide a Gazette Notification dt 22.5.1998, giving thrust on use of fly ash towards cement, concrete and other building materials. It has also summoned the user departments and construction agencies to include ash and ash based products in construction systems in their respective schedules of specifications, including appropriate standards and codes of practice. Noticing the passivity of response, the Delhi High Court has responded to a Public Interest Litigation (PIL) and issued an Order on 25.8.1999 directing the Central Government to publish a final Notification. Ultimately, the Ministry of Environment & Forests has issued a Notification dt 14.9.1999 consisting of more specific directions to all agencies concerned.

Not satisfied with above, MOEF has come out with draft amendments No. 948/6.11.2002 leading to Amended Notification No. 979 (E)/27.08.2003. There after, pursuant to the Writ Petition(C)2145/1999 and orders of High Court of Delhi /5.08.2004, another draft amendment No. S.O.513(E) / 3.04.2007 followed by a Stake Holders Meeting on 28.01.2008 to understand the barriers for 100% fly ash utilization, lead to the issuance of Draft Amendment No. S.O.2623(E) / 6.11.2008 that contained following anomalies:

  • Due lobbying was made by power producers in enlarging the scope of fly ash, covering all types of ash such as bottom ash and pond ash.This has resulted in claiming of undue figures inutilization by power plantsin meeting the targets of fly ash utilisation specified by MOEF. For example, certain power plants claimed 100% utilization by adding the use of pond ash for formation of bund to ash ponds.
  • Without understanding the process needs and chemistry of fly ash, the amendment has specified minimum content of fly ash for each building product that has lead to various legal tussles at Excise/GST department in defining applicable tax structure.
  • The potential of fly ash use in brick segment is undermined resulting to allocate meager 20% of output against the demand of 180 million tons. This has dampened the proliferation of fly ash brick units for non-availability of fly ash.
  • By permitting to sell fly ash, MOEF encouraged rat race in commercialization of fly ash rather than serious efforts for its 100% use.
  • The recommendation of the Authors, to permit the sale of fly ash by power producers only upon reaching 100% utilization, was not included in the Amendment.

Subsequent notifications by MOEF S.O.2804(E) / 03.11.2009; S.O.1396(E) / 25.3.2015; S.O.254(E) / 25.01.2016 and G.S.R 157(E) / 25.2.2019 have also gone at tangent to the ground realities by not adhering steadfast to the requisites of 100% utilization.

Pursuant to the follow up by PMO under National Clean Air Program giving thrust to utilization of fly ash and NitiAayog to harness better utilization of fly ash, another draft  Notification No. GSR 285(E) / 22.3.2021 is issued which appeared to have failed in implementation of its directions in letter and spirit. The crux is that all these documents are helping to support the claim of utilization in order to satisfy the judiciary rather than implementing the action plan to effective utilization.

Fly ash Mission

In order to buttress the promotional efforts, it is proposed to promote a coordinating body that has given way to the constitution of ‘National Fly ash Mission’ under the aegis of Ministry of Science & Technology.  But, surprisingly, this Mission has precluded the cement and building material from its scope and, ultimately, added the same in its programme at the fag end of its term, thereby not being able to do justice to the utilization to its potential in construction sector.

CBIP Seminars

Central Board of Irrigation & Power (CBIP) has taken up the promotion of fly ash utilisation through education programs and conducting seminars. Despite its conviction on the subject, CBIP could not influence to vacate the ban on the use of PPC for structural applications that has been brought in by Ministry of Surface Transport.


Advent of FaL-G Technology

Fly ash-lime reactions are known for their slow chemistry resulting in feeble strengths at early age. Hence, autoclave is an indispensable production unit to manufacture fly ash-lime products wherein, at a high temperature of around 180 oC and pressure of about 8-12 bar, the chemistry is augmented. This cost intensive equipment and utility inputs made the activity prohibitive as far as Indian economic and marketing logistics are concerned.

FaL-G, a mix of fly ash lime and gypsum, a name christened to the product after its ingredients, changed the scenario completely within the last 9 years. As against a handful of autoclaved plants in India, there are over 700 FaL-G brick plants, manufacturing more than one billion bricks or 2 million cubic meters of blocks annually. The product is well accepted for rendering two to four fold strength, but at parallel price to that of clay brick.

FaL-G is the extension of work based on the theory of Crystallo-Mineral Combination of Setting behaviour, postulated and presented by the authors in 1986[1] to define the improved behaviour of gypsum towards increased strength and water resistance. Thereby the weak phases of calcium aluminate hydrates of fly ash-lime mixtures have been tapped of their potential towards calcium sulpho aluminate hydrates, resulting in the mixture of FaL-G.

The further research to optimize FaL-G as a cementitious binder, oriented the attention of INSWAREB towards pozzolanic cements and concrete. At this juncture the ban on PPC imposed in 1987 towards structural applications has caught their attention. To counter this issue, extensive research work has been undertaken and the efficacy of Indian fly ashes has been established with pozzolanic activity index to as high as 115 to 120%, proving them as one of the best lot in the world.

To prove the issues in the field, INSWAREB has executed various RCC structures such as, 2000 sft of slab with FaL-G and 2000 sft of another slab with Portland: FaL-G. To take the work further, the 3-storeyed institute building has been built, from the pile foundation to 3000 sft of slab in Portland: FaL-G, where the strengths were recorded over 40 MPa. Inspired by the HVFC development at CANMET, two more floors of 3000 sft each have been added to the same structure in HVFC, containing 40% fly ash, achieving a 60-day strength of 60 MPa.

In infrastructure segment, 3 km of footpaths have been laid in the beach road with FaL-G, one km of pavement in black cotton soil, partly in PCC of Portland: FaL-G and remaining part with brick on edge (Khadanza) with FaL-G blocks, all in Visakhapatnam.

In order to enlarge the raw material base for FaL-G brick, INSWAREB has roped in various byproducts such as lime grit from milk-of-lime plant, flue gas desulphurised gypsum from the ‘sintered-pet coke’ industry and magnesia slag from the magnesium plant. In all these approaches, as observed by Mehta, INSWAREB has disregarded the standard chemical and physical requirements and developed tailor-made blends to produce products of adequate strength on normal curing with a check on their durability criteria in standard test methods.

Building built by NTPC with NACA at Deepenjalinagar, Simhadri Power Plant, Visakhapatnam

Fly ash in Concrete:

Notwithstanding FaL-G concrete, taking advantage of 4th generation chemical admixtures, the authors have extended consultancy to various clients on fly ash blended concrete replacing OPC by as high as 60-70% but achieving strengths at 40-60MPa.

Not satisfied by replacement of cement with fly ash, based on the primary research in Japan during 80s-90s, the authors have developed in 2004 Self Compacting Concrete (SCC) using fly ash as micro aggregate also. The typical SCC contained as low as 212 kg of OPC and as high as 502 kg of fly ash. The cementitious to aggregate ratio is 1:1.87 rendering strengths at 40 MPa. This concrete was used with due modifications based on site conditions, both in housing and infrastructure projects.

Invention of Nano Concrete and NAC Aggregate

The studies by Authors on strength and durability of concrete have drawn their attention to transition zone ie., the bond between coarse aggregate and cement mortar paste, which pronounces limitation to the concrete. Thereby the Authors were encouraged to conceive a concrete without aggregate, leading to the development of No Aggregate Concrete that has been christened as Nano Concrete (NAC) which was duly patented in March 2010. Some field level structures were built with NAC for understanding its behavior in site conditions.

While extending consultancy to Bangladesh, the Authors have noticed that they crush bricks of 15-20 MPa strength into coarse aggregate for its use in structural concrete. This has encouraged them to crush NAC into coarse aggregate (NACA) and undertake relevant studies. This work has been compiled and submitted as entry to National Grand Challenge-2019, organized by NTPC to identify technologies for mass scale fly ash utilization, bagging the 1st Prize. As a follow up, a two storeyed building was built at NTPC-Simhadri using NACA totally as coarse aggregate right from foundation concrete.

Natural stone aggregate Vs Nano Concrete Aggregate

Classification of Fly ash

Notwithstanding the ASTM C 618 classification of fly ash, based on the boiler operations, INSWAREB has further classified the fly ash with two distinct identities:

  • LT fly ash: Generated out of combustion temperature below 900 oC
  • HT fly ash: Generated out of combustion temperature above 1000 oC.

This classification has a great significance in pronouncing the reactivity and applications. Thus the distinct characteristics of LT and HT fly ashes have been studied, more so with reference to reactive alumina, and their avenues of use have been pronounced.

SourceStrength N/mm2
Fly ash + LimeFaL-G
LT Fly ash 1917.92532
LT Fly ash 21115.82025.8
HT Fly ash
HT Fly ash

In either ash, the autoclave could be dispensed away and the FaL-G brick activity has been brought within the reach of small and tiny sector entrepreneurs. It is studied that, for FaL-G production, LT fly ash proves good in lime route and HT fly ash proves good in OPC route as shown in the table:

Strength: N/mm2Lime routeOPC route
OPC route3-day7-day28-day3-day7-day28-day
LT fly ash12.416.022.814.017.420.0
HT fly ash2.57.822.06.418.433.8

The Role of Aluminate Chemistry in FaL-G and PPC

As shown in the above, HT fly ash is the ideal input for the production of PPC. INSWAREB strongly advocates the need to tap the aluminate phase of fly ash in association with gypsum, towards the formation of sulpho-aluminate hydrates to achieve rapid setting and high early strengths. Thus, it is possible to use high volumes of fly ash and still meet or exceed the strengths of control mortar by 28-day and beyond, as shown by the data:

Dome cast with Nano Concrete over FaL-G Mansion (Authors’ Residence)

Compressive strength of mortar (N/mm2)

OPC:7 day14 day28 day60 day270 day
Fly ash(a)(b)(a)(b)(a)(b)(a)(b)(a)(b)
50 : 50
40 : 60
100 : 0
(control mortar)

(a) :  Added with anhydrite commensurate to fly ash quantity

(b) :  Without anhydrite

For this purpose, the role of gypsum is distinctly described in every blend and thus defined as a set-accelerator for FaL-G and set-retarder for NPC. The chemistry of cement with fly ash is a two-stage mechanism. First NPC hydration starts independently wherein, the chemistry of C3A is retarded. This follows instantaneously with the commencement of secondary mineralogical reactions wherein, the reaction of alumina of fly ash, with the released lime of hydration reactions, is accelerated towards calcium sulpho-aluminate hydrates. It is not proper to consider that the slow setting of blended cement gets further retarded if gypsum is added. Gypsum manifests as the accelerator when added commensurate to reactive alumina phase in fly ash.

A study to manufacture prestressed railway concrete sleepers, replacing cement by 25% fly ash, has been undertaken. While the compressive strength is almost parallel, the flexural strength has exceeded by 10-18%, as shown in the table:

Strength (tons)
Data on Railway Concrete SleepersRS1RS2Centre top
With Control concrete22.622.06.5
With PPC concrete containing 25% fly ash25.026.06.6

FaL-G as the Coarse Aggregate

While attempting to break the stone-like hardened FaL-G blocks into workable pieces, ideas have emerged to avail the broken pieces as coarse aggregate. Upon achieving positive results, the outcome is viewed as an approach to agglomerate quarry dust that has missed its journey to concrete by getting disintegrated into dust.

The MOR and Young’s Modulus have shown totally identical behaviour to that of control concrete despite low compressive strength of FMA (FaL-G Mortar Aggregate). This is attributed to the strong bonding of cement paste with FMA in the transition zone on account of the cementitious nature of the latter product.

Mix Design:1:2:4 by volume at Zero slump

Strength (N/mm2)
With NSAWith FMA
Compressive strength:3 day25.622.6
7 day32.828.5
28 day40.637.4
Modulus of Rupture28 day5.45.4
Young’s Modulus28 day0.25×1050.25×105

Development of Non-Autoclaved Aerated Concrete (NAAC)

This process, developed and patented by INSWAREB, does not use autoclave or foaming agents, but gives an aerated concrete product with impressive pores and pore refinement, at lesser cost of plant and utilities.

The object of this invention is to rationalise the plant cost by avoiding the cost-intensive equipment such as autoclave, as well as the cost of utilities such as steam or foaming agent. Such approach makes aerated concrete production sustainable in practice and affordable in price, more so in second and third world countries. This could be made feasible by integrating the principles of FaL-G technology with parameters of age-old aeration process. By doing so, the energy consumption could be slashed down from 250 K. Cal to 75 K.Cal/Kg.

Two compositions, as prepared in the above process, give the following strength and density:

Constituent %: Fly ashOPC+Lime
Sample I 70300.1560882
Sample II68320.1552950

The studies on water absorption indicate that the product has pores but relatively with a lesser grade of interconnectivity to result in lower water absorption. The following data substantiate this observation:

Density kg/cu.mWater absorption
Sand lime AAC74038%

Fly ash – Clean Development Mechanism (CDM)

The Kyoto Protocol is enshrined with CDM in order to ensure the participation of second and third world countries towards the minimization of green house gases. INSWAREB has developed certain mathematical models and conducted base-line studies to define the fly ash based brick and cement plants as qualified CDM projects. INSWAREB has projected these baseline studies of carbon credits for various cement and building material projects during their attendance to the COP-6 (The Hague; November 2000) for which UNFCC has accredited them as the Observer Organisation. For the tangible performance-indicators projected, the PCF

(Prototype Carbon Fund) of the World Bank has offered a tie up of US$ 10 million that can be encashed by operative FaL-G brick plants against carbon credits. But the Government of India could not clear the proposal for not becoming a signatory to Kyoto Protocol. But, one can be quite sure and optimistic of interfacing the incentive of carbon credits to the fly ash utilisation program, thus making it more lucrative and sustainable to the industry and exchequer with impact on GDP (Gross Domestic Product).

INSWAREB believes in promoting the knowledge of cement studies in simple and tangible explanations.  For this purpose, they have selected to interpret the cement hydration behaviour and related transformations in parallel to the factors of biological phenomena in the human system. For example, they draw the parallel of four principal mineralogical phases of cement to that of four basic genetic codes of life system, A, T, G and C that ultimately decide the behavioural pattern and personification of the human system. Similarly they compare the surplus hydrated lime of a concrete system to that of surplus sugar in the human body. This way they believe to give a deep imprint in the minds of laymen and marketing folk who have the ultimate say in the success of a product marketing.


1996-99 is the crucial period for the marketing segment of cement in India. Price-wars have badly beaten the industry to maintain low prices. At this juncture, one industry from South India has given thrust to the promotion of blended cements, selecting PPC as the starting product. The phenomenal success of this company to register profits, despite spending massive amounts for advertisement and educational campaigns on blended cements. This has sensitised the cement industry to go for blended cements aggressively, raising their production from 22 million tons to 35 million tons within a span of three years ie., from 1998 to 2001.

The industry has realised that, in the environment of price constraints, blended cement is the only avenue, whereby one can offer the product of better durability and, at the same time, maximising the profit margins. While blast furnace slag has limited quantity (10million tons) of availability, fly ash availability is huge (90 million tons) and scattered throughout the country. Thus PPC is proving to be the sustainable path for the cement industry to grow and flourish.

The scope of fly ash is realised even by the Ready Mixed Concrete (RMC) industry. The features of retardation in setting time, improvement in workability and pumpability, make fly ash as the most favourite complementary input that can relatively rationalise the use of chemical admixtures.


The codes have remained as the major stumbling block for quite a long time. IS:1489, the code for PPC has confined the input of fly ash to 25% that has been revised to 35% very recently.

IS:456, the code of practice for concrete, was formulated in 1953 giving thrust on structural factors, disregarding the importance of material science. However, pursuant to various government notifications and demand from the industry, a lot of thrust has been given to the application of blended cements on the plank of durability in the fourth revision of the code in 2000. Disregarding all these positive developments at national and international level to the use of fly ash blended cement and concrete as a practice of durability, CPWD has not upgraded their code of practices as evident by the reiteration of the ban in 1999 on the use of pozzolanic cements towards structural applications, that was primarily imposed in 1987. With the use of fly ash blended concrete in Delhi-Metro project, Bandra-Worli sea link project and many other infrastructure projects by leading construction agencies the embargo by CPWD has diluted progressively.


It is beyond doubt that a distinct shift is evident in the Indian scenario of fly ash utilisation from pessimism to optimism if the increased production of PPC is any indication. However the mind set in government construction agencies has to be necessarily changed for the furtherance of the trend. There is a need to converge the commitments and actions of various State governments into action.

Maximum sources of Indian fly ash are some of the best lots in the world. Without giving credence to this fact, any interpretation to discourage the utilisation through reduction-approach is unfair and disservice to the Nation.

Fly ash based brick and cement are far superior in engineering properties over their conventional competitors. This knowledge needs to be disseminated globally, more so in second and third world countries, through tangible technical explanations.

The opportunity to abate CO2 is over 50 million tons in cement and 72million tons in brick by using fly ash in both the segments in India. Fly ash utilisation proved as the money-spinner in ‘green trading’ point of view, for achieving CO2 abatement. The Authors have transferred 451,590 tons of Carbon credits to the World Bank during 2007-14 to the contract value of over USD 4.76 million and, distributed the fund to 112 fly ash brick manufacturers registered in various bundles under Clean Development Mechanism (CDM), after meeting administrative costs and Community Development programs.

Very few technologies can assimilate E4 indicators ie., Ecology-Environment-Economy-Empowerment, in a single go.

Fly ash utilisation is the unique opportunity at global plateau in serving these multiple indicators of Sustainable Development Goals (SDG) collectively.

About Authors

Dr Bhanumathidas did her Masters in Physics and doctorate in chemical engineering from Andhra University. She has specialized in cement chemistry and concrete technology. Mr. Kalidas is the founder Director of Institute for Solid Waste Research & Ecological Balance (INSWAREB), an NGO promoted in association with his spouse, Dr N Bhanumathidas, the founder Director General of INSWAREB.

Mr. Kalidas is a technocrat-scientist, having specialized in cement chemistry and concrete technology. He has been pursuing the subject for the last 23 years. He worked for a couple of German companies, promoting their building material technologies in India before starting his own enterprise for development and promotion in the same field.

They have jointly authored and presented several papers at various national and international fora. They also authored a comprehensive text book, Fly ash for Sustainable Development, which is a compendium of various data right from fly ash generation upto its role in concrete durability and sustainable development. They have also served on various government bodies in their individual capacities and as the nominee of INSWAREB. National Waste Management Council, Ministry of Environment & Forests; National Fly ash Mission, Task force on fly ash are some of the prominent bodies among them.


The authors thankfully acknowledge the various articles of Prof P.K. Mehta, Professor Emeritus of Civil & Environmental Engineering, University of California, Berkeley. Housing & Urban Development Corporation (HUDCO) deserves special mention for its funding to R&D laboratories at INSWAREB which has facilitated this study.

Based on the TECHNICAL PAPER PRESENTED IN THE EVENT OF RECEIVING CANMET/ACI AWARD in 2001, added with inputs of subsequent developments.

Letter of Endorsement for utilisation of fly ash

A letter of endorsement was given for INSWAREB having successfully catalysed the promotion of over 21,000 fly ash brick units in India to resolve total fly ash utilisation.


Constrofacilitator congratulates them for this unique feat achieved and wishes good luck for their future endeavors on initiations reducing carbon footprint with fly ash utilisation.