Relevance of rice husk ash for the sustainability of construction industry

Rice husk ash (RHA), used as a pozzolanic admixture in cement and concrete, is obtained from the combustion of rice husk (RH) under certain conditions of the surrounding environment, temperature and residence time in a combustor, and subsequent size reduction. It contains low unburned carbon, and silica is mostly in amorphous form. RHA manufactured in the modern fluidized or cyclonic bed reactors has high surface area of the order of 20–40 m²/g, comparable with that of silica fume (SF).

It is a material with proven pozzolanic characteristics and is added to cement and concrete as a partial replacement of Portland cement. However, its application has not been widely commercialized as yet, mainly on account of the non-availability of RHA of the desired pozzolanic characteristics on a large scale on the one hand and the lack of awareness about the potential for RHA as a mineral admixture on the other.

The RH produced in farms and rice mills is conventionally employed as a fuel or dumped as waste. Many researchers found that the use of properly manufactured and treated RHA improves the performance and the durability of concrete.

RHA possesses the potential to replace SF in high-strength and high-performance concrete. RHA manufactured through controlled burning of RH shows performance comparable with that of SF, when added to concrete in binary (Portland cement [PC] + RHA) or tertiary (PC + RHA + FA) blends, in terms of strength and reduced permeability toward the external deteriorating agents.

The major characteristics of RHA are its high water demand and coarseness in comparison to SF. In order to improve these characteristics, RHA needs to be ground finer into particle size range of 4–8 μm (1 μm = 10^-6 m) and a superplasticizer is added to reduce water requirement.

RH is presently considered as an agricultural waste and used as fuel, as mentioned earlier, where its pozzolanic value lies unutilized. Thus, the incorporation of RHA in concrete as a mineral admixture adds value, both from the economical and ecological point of view.

Rice, produced from paddy, is a cereal grain and the most important staple food for a large part of the world’s human population, especially in tropical Latin America, the West Indies, and east, south, and southeast Asia.

According to one estimate, the world paddy production is expected to touch 847–915 × 10⁶  ton by the year 2030, from the current (2008) level of 683 × 10⁶  ton; out of which around 600–774 × 10⁶  ton paddy and from that around 120–155 × 10⁶  ton RH shall be produced in the Asian countries. The abundant availability of RH in the rice producing countries  provides us a huge scope to recover its heat value to generate power and to use the RHA produced in cement and concrete on a large scale.

The production of RHA with cogeneration of power as well as its application in cement and concrete, both contribute toward the reduction of green house gas (GHG) emissions. It is found that the generation of power through the combustion of RH reduces carbon emissions, in comparison to coal, oil, and natural gas.

RHA is added to cement and concrete as a partial replacement of cement to the extent of 30%. Thus, it reduces the consumption of PC and to that extent contributes toward the reduction of CO2 emission, which is a GHG, in the manufacture of PC.

The reduction of GHG through such practices has been provided with incentives under the United Nations (UN) framework. The Kyoto Protocol is part of the United Nations Framework Convention on Climate Change (UNFCCC) and has set an agenda for reducing global GHG emissions. If CO2 emissions can be shown and verified to be reduced due to different practices, then Certified Emission Reductions (CERs) are issued under the Clean Development Mechanism (CDM) of UNFCCC. These CERs are tradable in the primary and secondary market and generate revenue for the CERs holding party. The readers are advised to go through the UNFCCC documents to obtain more information on the subject.  When RHA is used in cement and concrete manufacture as a cement substitute, there is potential to earn CERs.

There are other environmental benefits of substituting Portland cement with RHA. The need for quarrying and mining primary raw materials and fuel is reduced, namely, limestone, clay, and coal, and thus overall reduction in emissions of dust, CO2, and acid gases is attained. As the cement and concrete industry uses RHA with amorphous silica, the health issues, mainly associated with the fine crystalline silica, are minimal.

The large-scale application of RHA in the construction industry requires industrial and economic policy planning and efforts in the following areas:

a) Creation of general awareness about the benefits of using RH in power generation and RHA in cement and concrete. In India, the government took lead promoting the utilization of pulverized fuel ash (PFA) in cement and concrete, through the Fly Ash Mission. It is time that similar missions are taken up to create awareness about

the less known mineral admixtures, such as RHA.

b) RH is produced by the farmers in their paddy fields. The RHA producing unit will require continuous supply and adequate storage of RH. Thus, a viable method of collection and transportation of RH from the paddy fields to the RHA producing unit will have to be put in practice.

c) Identification of a techno-economically feasible method to produce and process RHA along with the cogeneration of power to suit the local conditions.

d) Formulation of national Standards on the quality assessment and the use of RHA in cement and concrete.

In summary, the application of RHA in the construction industry shows tremendous potential for the rice-producing countries, both from the point of view of promoting the sustainable development of construction industry and as a valuable input for the economic growth of these countries.

Ref: “Mineral Admixtures in Cement and Concrete”, CRC Press (http://www.crcpress.com/product/isbn/9781439817926). Author: Dr J D Bapat (http://www.drjdbapat.com)

Written for engineers, book focuses on making more workable and durable concrete using mineral admixtures. For each mineral admixture, book looks at manufacturing and processing, physical characteristics, chemical and mineralogical composition, quality control, and reported experiences. It also examines the provisions of national standards.It encourages engineers to more effectively use these and other wastes in cement and concrete to support more sustainable growth of industry. Buy this book online to obtain 20 % discount and free shipping. Download details: http://bit.ly/online_purchase

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Sustainable Construction through Application of Pulverized Fuel Ash (PFA) or Fly Ash (FA) in Concrete

The “pulverized fuel ash” (PFA) or the so-called fly ash (FA), used as a mineral admixture in cement and concrete, is a product of the pulverized coal firing system, through conventional boilers, mostly used in the thermal power plants. While carbon burns in oxidizing surroundings, the inorganic mineral matter gets sintered and liquefied at high temperature. The melt flows down the walls of the furnace and about 25% gets collected as “bottom ash” (BA). It is crushed before disposal. The rest, PFA or FA, gets entrained in the up-flowing hot gas in the form of fine particles, which get trapped in the economizer, air-preheater, mechanical separator, and, finally, battery of electrostatic precipitators (ESP).

As a general practice in many countries, PFA and BA are mixed with water and transported to ash ponds/lagoons. The ash thus deposited in lagoons is called “lagoon ash” (LA) or “pond ash.” It causes problems besides occupying huge stretches of agricultural land. Notwithstanding the greater utilization of PFA (and BA) in recent times in cement and concrete, in bricks, and for land filling, a large quantity of ash still lies unutilized.

Application of fly ash in cement and concrete improves long term strength and durability of structure and also contributes to the sustainability. 

As per several estimates, the cement industry contributes about 5% of the global generation of carbon dioxide. The cement industry’s sustainable program developed by the World Business Council for Sustainable Development (WBCSD) prepared an “Agenda for Action” for a 5 year period from 2002 to 2007, endorsed by the leading cement manufacturers of the world. The agenda addressed the issues of (a) climate protection, (b) fuels and raw materials use, and (c) emission reduction besides other issues.

Ref: “Mineral Admixtures in Cement and Concrete”, CRC Press (http://www.crcpress.com/product/isbn/9781439817926). Author: Dr J D Bapat (http://www.drjdbapat.com)

Written for engineers, book focuses on making more workable and durable concrete using mineral admixtures. For each mineral admixture, book looks at manufacturing and processing, physical characteristics, chemical and mineralogical composition, quality control, and reported experiences. It also examines the provisions of national standards.It encourages engineers to more effectively use these and other wastes in cement and concrete to support more sustainable growth of industry. Buy this book online to obtain 20 % discount and free shipping. Download details: http://bit.ly/online_purchase

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Reducing carbon footprint of concrete

As I understand on date, there is no carbon neutral or carbon negative concrete mix design available, as all concrete contains cement. The production of one tonne of cement emits nearly the same amount of carbon dioxide in the atmosphere. It is interesting to know that fresh concrete absorbs some of that CO2 back during carbonation which takes place during the setting and hardening process. One effective way to reduce the carbon footprint of concrete is to minimise the use of cement, partially replacing it with the mineral admixtures like fly ash, blast furnace slag, rice husk ash, which are industrial and agricultural wastes. These admixtures, besides giving long term strength to concrete also enhance its durability.

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Using building code to improve construction quality

In the last few years , two major changes have taken place in the cement and construction industry in India. Firstly, the emphasis of construction industry has shifted from high – strength to high – performance concrete . The realisation has come on account of the fact that nearly 65 % of the total cement sales in the country presently go towards the repairs of old structures , most of which are built with OPC . Secondly, the mineral admixtures, namely the fly ash (FA), silica fume (SF) and the blast furnace slag (BFS), are being increasingly used to improve the long term strength and the durability characteristics of cement and concrete. The need to build durable structures is felt not only from the point of view of economy , but also for the conservation of resources , energy and environment . The present study reviews the provisions of the Building Code IS 456 – 2000 towards building durable structures

The durability of concrete incorporates , besides strength , its capacity to resist the effect of the internal and external deteriorating factors , such as sulphate attack , chloride attack manifested in the corrosion of the reinforcement , carbonation , alkali – aggregate reaction , freezing and thawing , so as to give a satisfactory performance during the economic life , for which it is designed . Some of the important provisions in the Building Code, related to quality and durability are as follows :

a/ Clause 5.2: The following mineral admixtures, conforming to relevant Indian Standards, are permitted in the concrete: fly ash, silica fume, rice husk ash, metakaoline and ground granulated blast furnace slag (BFS).

b/ Clause 5.5 and 10.3.3: These two Clauses give various provisions on chemical admixtures. The dosage of retarders, plasticisers and superplasticisers has been restricted to 0.5, 1.0 and 2.0 % respectively by weight of cementitious materials.

c/ Clause 7: In the revised Code, the workability of concrete has been expressed in terms of slump only, unlike old Code wherein it was expressed in terms of Vee-bee time/compacting factor.

The expression in terms of compacting factor is recommended only in case of ‘very low’ workability category, such as pavement quality concrete. The ‘very high’ workability category has been newly introduced in the revised Code, applicable to tremie concrete and the workability measurement by flow has been recommended there. It will also be applicable to self-compacting concrete. It is well known that it is always better to express workability in one particular unit i.e. slump, compacting factor or flow, as these units are not always compatible with each other.

The maximum water-cement ratio for reinforced concrete has been reduced (example, 0.55 instead of 0.6, for M-20 concrete) and the assumed standard deviation (Clause 9.2.4.2) has also been reduced for higher grades of concrete. It is observed, in line with the reduced water-cement ratio, that the values of slump allowed by the Code are also low for normal type of construction.

The Code is thus indirectly expecting a change towards mechanisation of placing and consolidation of concrete.

d/ Clause 8.1.1: The permeability of concrete to the ingress of deleterious agents has been identified as one of the major characteristics affecting the durability. The factors influencing the durability have been delineated as environment , cover to embedded steel , type and quality of construction materials , cement content and water-cement ratio of the concrete , workmanship to obtain full compaction and efficient curing and shape and size of the member

e/ Clause 8.2.2.1: The general environment , to which the concrete will be exposed during its working life , is classified into five levels of severity , namely mild , moderate , severe , very severe and extreme. In comparison to the old Code, two more intermediate weather conditions, with less cement content, have been added in the revised Code.

f/ Clause 8.2.2.3: The air – entraining admixtures have been recommended for use in the concrete where freezing and thawing actions under wet conditions exist

g/ Clause 8.2.2.4: Recommendations have been given for the type of cement , maximum free water-cement ratio and minimum cement content , to develop adequate resistance in the concrete exposed to different sulphate concentrations. It is stated that PSC conforming to IS 455, with slag content more than 50 %, exhibits better sulphate resisting properties. Under conditions where chloride is encountered along with sulphates, the Code recommends the use of OPC with C₃A in the range of 5-8 % instead of SRC, PSC with more than 50 % slag or a blend of OPC and slag

h/ Clause 8.2.4.1: Recommendations have been given on the minimum cement content, maximum free water – cement ratio and minimum grade of concrete , for different exposure conditions. The minimum grade of concrete for footings or elements under non-aggressive soil or ground water (moderate environment) has been specified as M- 25. Thus the design mix is obligatory even for small buildings/structures.

i/ Clause 8.2.4.2: The upper limit of the cement content , not including FA and GGBS , has been kept at 450 kg / m³ , considering the increased risk of cracking due to drying shrinkage in thin sections or early thermal cracking and the increased risk of damage due to alkali-aggregate reaction, at the higher cement contents. It is hoped that the provisions for maximum cement content made in other standards, like IS 1343 on prestressed concrete structures, will also be brought in line with that in IS 456 – 2000.

j/ Clause 8.2.5.2: The total amount of chloride content ( as Cl ) in the concrete , at the time of placing , has been specified . The maximum chloride content of 0.6 kg/m³ of concrete has been stipulated, for reinforced or plain concrete containing embedded metal. The maximum limit has been rationalised and revised upward, as that specified in the earlier Code was difficult to realise in practice.

k/ Clause 8.2.5.3: The maximum total water – soluble sulphate content of the concrete mix , expressed as SO₃ , has been specified as 4 % by mass of the cement in the mix

l/ Clause 8.2.5.4: As a precaution against alkali – aggregate reaction , recommendations have been given on the constituent materials, like use of non-reactive aggregates and low alkali Portland cement ( < style=""> Na₂O equiv.) , partial replacement of cement with the mineral admixtures , use of impermeable membranes to reduce the degree of saturation of concrete during service and limiting the cement content of concrete .

m/ Clause 8.2.8: Recommendations have been given on the concrete constructions in sea - water or directly exposed along the sea - coast , with respect to the Grade of the concrete , type of cement , mix design and the use of pre-cast members . The use of slag or pozzolana cement has been recommended under such conditions.

n/ Clause 9.1.2: The Code stipulates the following information to be included while specifying a particular grade of concrete: type of mix (design or nominal), concrete grade, cement type, maximum nominal size of aggregate, minimum cement content for design mix, maximum w/c ratio, workability, mix proportions for nominal mix, exposure conditions (as per the Code), maximum placing temperature, method of placing and the degree of supervision

o/ Clause 10.1: This Clause on Quality Assurance Measures has been incorporated to emphasise the good concreting practices.

p/ Clause 10.2: The use of ready-mixed concrete or concrete from on/off site

batching and mixing plant has been recommended for large and medium size projects.

q/ Clause 10.3: The Clause on concrete mixing makes an important new provision. It stipulates that the concrete mixers shall be provided with water measuring (metering) devices. The provision will go a long way controlling water-cement ratio, especially in site mixed concrete. The old provision of hand mixing the concrete with 10 % extra cement in case of breakdown of mixer, work in remote areas or when concrete quantity is very small, has been removed in the revised Code.

r/ Clause 12.3.2: The old provision of negative tolerance on the cover has been removed. Under the new provision, use of PVC cover blocks has been permitted.

s/ Clause 13.4: The provisions under this Clause on “Construction Joints and Cold Joints” have been improved

t/ Clause 13.5: The use of proper and adequate curing techniques has been stressed, to reduce the permeability of the concrete and enhance its durability by extending the hydration of cement. The difference has been maintained between recommended minimum period of curing for concrete containing ordinary Portland cement and that containing cement with mineral admixtures

u/ Clause 16: The acceptance criteria for strength requirement has been totally revised and divided into two parts, namely that for (I) compressive strength and (II) flexural strength of concrete

v/ Clause 21: The concept of fire resistance of concrete has been newly introduced and discussed.

w/ Clause 26.4: The minimum values for the nominal cover (a new term introduced in the Code) have been specified , to meet the durability requirements. The minimum cover for footings has been specified as 50 mm.

While going through these provisions, it becomes clear that besides improving the construction quality, concrete strength and durability, the revised Code also puts emphasis on conservation of building materials.

Reference: Bapat J. D., “ IS – 456 : 2000 and Further ” Indian Cement Industry Desk Book 2002, December 2002, pp 30-39