Construction and Building Materials 16(2002) 519–525

0950-0618/02/$ - see front matter� 2002 Elsevier Science Ltd. All rights reserved.PII: S0950-0618Ž02.00034-X

A perspective study on fly ash–lime–gypsum bricks and hollow blocksfor low cost housing development

Sunil Kumar*

Department of Civil Engineering, Harcourt Butler Technological Institute, Kanpur 208002, India

Received 17 April 2001; received in revised form 22 April 2002; accepted 31 May 2002

Abstract

Housing is a great problem in today’s world. The most basic building material for construction of houses is the usual burntclay brick. A significant quantity of fuel is utilized in making these bricks. Also, continuous removal of topsoil, in producingconventional bricks, creates environmental problems. A feasibility study was undertaken on the production of fly ash–lime–gypsum(FaL-G) bricks and hollow blocks to solve the problems of housing shortage and at the same time to build houseseconomically by utilizing industrial wastes. The compressive strength, water absorption, density and durability of these bricks andhollow blocks are investigated. It is observed that these bricks and hollow blocks have sufficient strength for their use in lowcost housing development. Tests were also conducted to study the influence of type of curing on the increase in strength andhardening of the bricks and blocks with time. It was observed that the hot water curing leads to a greater degree of hardeningand higher strength, earlier compared to ordinary water curing.� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Durability; Fly ash; Phosphogypsum; Waste management

1. Introduction

There is a general exodus of rural population to thecities with the rapid industrialization in developingcountries. The infrastructure to support these cities, suchas buildings for housing and industry, mass transit formoving people and goods, and facilities for handlingwater and sewage will require large amounts of construc-tion materials. Enhanced construction activities, shortageof conventional building materials and abundantly avail-able industrial wastes have promoted the developmentof new building materials.The rapid increase in the capacity of thermal power

generation in India has resulted in the production of ahuge quantity of fly ash, which is approximately 50million tons per yearw1x. The prevailing disposal meth-ods are not free from environmental pollution andecological imbalance. Large stretches of scarce land,which can be used for shelter, agriculture or some other

*Corresponding author. Tel.:q91-512-562031; fax:q91-512-545312.

E-mail address: suniljadon@hotmail.com(S. Kumar).

productive purposes, are being wasted for disposal offly ash.Fly ash, lime and gypsum are available in mutual

proximity in many regions. An economical alternativeto conventional burnt clay bricks will be available, ifthese materials can be used to make bricks and hollowblocks of adequate strength. Lime and gypsum areusually available either from mineral sources or may beprocured from industrial wastes.Phosphogypsum is an important by-product of phos-

phoric acid fertilizer industry. It consists of CaSO .2H O4 2

and contains some impurities such as phosphate, fluo-ride, organic matter and alkalies. Approximately 5 mil-lion tons of phosphogypsum is produced each year inIndia and causes serious storage and environmentalproblems w2x. The impurities of phosphogypsum seri-ously restrict the industrial use of phosphogypsum incement industry as a retarderw2–5x.Cementitious binder, FaL-G, finds extensive applica-

tion in the manufacturing of building components andmaterials such as bricks, hollow bricks and structuralconcretesw2–11x. FaL-G technology enables productionof bricks with a simple process of mixing, pressure-free

520 S. Kumar / Construction and Building Materials 16 (2002) 519–525

Table 1Chemical analysis of fly ash

Constituents Percentage

Loss on ignition 5.90SiO2 57.01Al O2 3 23.83Fe O2 3 6.66CaO 3.34MgO 1.77SO3 0.56

Table 3Chemical analysis of calcined phosphogypsum

Constituents Percentage

CaSO .2H O4 2 91.12SiO2 1.05Fe O2 3 0.30

Table 2Chemical analysis of lime

Constituents Percentage

Loss on ignition 5.65CaO 63.25SiOqAl O2 2 3 25.00MgO 4.70

Table 4Mix proportions of FaL-G bricks and hollow blocks

Mix designation Constituent materials(percentage)

Fly ash Lime Calcinedphosphogypsum

BricksM-1 90 5 5M-2 80 15 5M-3 80 10 10M-4 80 5 15M-5 70 20 10M-6 70 15 15M-7 70 10 20M-8 60 30 10M-9 60 20 20M-10 60 10 30

Hollow blocksM-11 80 10 10M-12 70 15 15M-13 60 20 20

moulding and water curing. Due to such appropriatetechnology apart from economy, conservation of energyand pollution control are also achieved.Fly ash bricks are used in multi-storeyed apartment

houses for non-load bearing purposes in making curtainand partition walls of these houses. Use of fly ash bricksin this type of construction is meant mainly to achieveeconomy and make profits. The domestic buildings inthe category of low or middle-income groups mostlyhave single or two-storeyed dwelling units. Therefore,not only the cost effectiveness but also the strength anddurability of fly ash bricks are very important for them.The construction of multi-storeyed complexes, involv-

ing high investments, highlighted the need of completingthe projects fast; in this materials that render fastconstruction are important. This is how the hollowblocks gained their entry in the construction industry,more predominantly in urban areas, with attractive tech-no-economic virtues.Although results of FaL-G bricks and hollow blocks

were promising, the technology could not be imple-mented due to initial consumer resistance in adapting tonew materials. In previous studies on FaL-G bricksw8,10x, limited mix proportions were considered. Amba-lavanan and Rojaw10x in their study of FaL-G bricksutilized waste lime and gypsum with fly ash. They haveobserved that in most cases the use of waste lime doesnot give technically desired results and some improve-ment is needed to increase the strength of FaL-G bricks.The treatment to be given to waste lime increases thecost of FaL-G bricks significantly as compared toconventional bricks, which is hindering the commerci-alisation of this new material.For wider application of FaL-G bricks and hollow

blocks in the housing sector, extensive research is furtherneeded. With this perspective, the properties of bricks

and hollow blocks manufactured with fly ash, calcinedphosphogypsum and mineral lime were investigated.The aim of the present investigation is to produce FaL-G bricks and hollow blocks for low cost housingdevelopment by utilizing the fly ash in the binder to themaximum extent. The results of the cementitious bindercured under water at 23"2 8C and 50 8C are alsoreported.

2. Experimental programme

2.1. Materials

The materials used for FaL-G bricks and hollowblocks were fly ash, lime, calcined phosphogypsum andwater. The chemical compositions of fly ash, lime andcalcined phosphogypsum used in the cementitious binderare given in Tables 1–3.

2.2. Mix proportions

The mix proportions of FaL-G bricks and hollowblocks are given in Table 4.

521S. Kumar / Construction and Building Materials 16 (2002) 519–525

2.3. Process operations

2.3.1. Mixing of raw materialsThe calcined phosphogypsum and fly ash were sieved

through a 4.75-mm sieve. The weighed quantity ofsieved fly ash and calcined phosphogypsum were mixedthoroughly in dry state. The hydrated lime was preparedin the laboratory by adding water to a weighed quantityof unslaked lime. Then, complete slaking of lime for6–8 h was allowed. The slaked lime slurry was sievedthrough a 1.18-mm sieve. The sieved slurry of hydratedlime was added to the mixture of fly ash and calcinedphosphogypsum. Water was added further and the ingre-dients were mixed thoroughly by kneading until themass attained a uniform consistency. Water was addedto the mixture of dry materials and the water contentwas decided as described below.A standard normal consistency test was performed

and the water content for the normal consistency wasdetermined. The water content used in the mix forstrength tests was 90% of that required to produce thestandard normal consistency. In the development of newbuilding materials and test methods, it is essential thatthe test procedures adapted should be, as far as possible,the same as those which were used for traditionalmaterials. In the long run it will help in standardization.It is with this view that the standard normal consistencytest, which is used for cement, is invoked in thisinvestigation as the basis for determining the watercontent to be used.

2.3.2. Preparation of bricksWooden moulds of internal dimension 220 mm=100

mm=75 mm were used. The size of bricks was keptapproximately the same as those of the normal burntclay bricks available in northern India. The FaL-G mixwas placed in moulds in two layers and was properlycompacted on a vibration table.

2.3.3. Preparation of hollow blocksStandard concrete cube moulds of size 150 mm=150

mm=150 mm were used for preparation of hollowblocks. Four wooden battens of cross-sectional area 45mm=45 mm=250 mm long and connected at top wereplaced in cube mould before filling it with FaL-Gcement paste. The mixed FaL-G cementitious binderwas placed in the cube moulds in two layers, each layerbeing compacted on a vibration table. Excess paste washand finished. After approximately 2 h, battens wereremoved and the blocks were finished to shape with 20-mm web and shell thickness, which is above the mini-mum specified by Indian codew12x. These hollow blockshave 36% hollow space.

2.3.4. Method of curingThe bricks and hollow blocks were taken out from

the moulds and were covered with wet gunny bags for

a week. After one week, when the specimens hadattained sufficient strength for handling, these bricksand hollow blocks were transferred to the water filledcuring tanks at 23"2 8C. The durability of FaL-G bricksand hollow blocks was investigated by curing thesebricks in an aggressive environment of sulfate solution.The sulfate solution having sulfate(SO ) concentrationy

4

equal to 10 000 ppm was prepared in the laboratory bymixing 14.79 gm of Na SO in 1 l of water. Identical2 4

samples of mix proportion M-9 given in Table 4 werecured at normal temperature and also at 508C, with aview to assess the effect of curing temperature.

2.3.5. Testing of FaL-G bricks and hollow blocksThe testing procedure recommended in Indian codes

of practice for burnt clay bricks and burnt clay hollowbricks w12–14x were adapted here for FaL-G binder.The bricks and hollow blocks were taken out from waterone day prior to the testing and were tested for com-pressive strength after 24, 72 and 96 days of casting asper the procedure laid down in relevant Indian codes ofpracticesw12–14x. The bricks and hollow blocks curedunder sulfate solution were tested for their compressivestrength after 72 days of casting. The specimens curedfor compressive strength at two different temperatureswere tested after 24, 48, 72 and 96 days of casting. Thetest results of the specimens are expressed in terms ofthe average compressive strength of six specimens. Asper the codew15x, if the individual variation was morethan"15% of the average, that value was not consid-ered in calculating the average value.The specimens were crushed between 3-ply plywood

sheets approximately 3 mm thick. The load was appliedaxially at a uniform rate of 14 Nymm ymin until the2

failure occurred.The strength of FaL-G cementitious binder increases

with age at a faster rate initially and at a relativelylower rate later. In a separate study, which was reportedelsewherew16x, it was observed that almost full com-pressive strength of FaL-G cementitious binder wasreached within the first 4 months. The strength gainbeyond 4 months was not technically significant. Thus,the results presented here for strength up to 96 days ofcasting may be considered sufficient for comparing thisnew building material with the conventional burnt claybricks and blocks. However, long-term studies of theproperties of FaL-G binders certainly need to be carriedout.After 96 days of casting, bricks and hollow blocks

were tested for water absorption. These bricks andhollow blocks were taken out from water curing tanksand were allowed to drain water by placing them on a10-mm wire mesh. Visible surface water was removedwith a damp cloth and immediately specimens wereweighed. After obtaining the saturated weight, thesebricks and hollow blocks were kept in an oven at 105

522 S. Kumar / Construction and Building Materials 16 (2002) 519–525

Fig. 1. Compressive strength of FaL-G bricks(fly ashs80%).Fig. 3. Compressive strength of FaL-G bricks(fly ashs60%).

Fig. 2. Compressive strength of FaL-G bricks(fly ashs70%). Fig. 4. Compressive strength of FaL-G hollow blocks.

8C; the codesw12,14x, however, recommend a dryingtemperature range between 110 and 1158C. They weredried to a constant mass and taken out from the ovenand were weighed at room temperature. From the wetand dry weight of these bricks and hollow blocks, waterabsorption was calculated.The specimens were cooled to room temperature and

their density was obtained by dividing the mass of thebricks or hollow blocks by its overall volume; neglectingthe voids in case of hollow blocks. The nominal volumeof the bricks and hollow blocks based on the dimensionswas used in the calculations.

3. Experimental results and discussion

The experimental results are presented in Figs. 1–6and Tables 5 and 6. The results of compressive strength

of FaL-G bricks are shown in Figs. 1–3. The results ofcompressive strength of FaL-G hollow blocks are givenin Fig. 4. Fig. 5 shows the hydration kinetics of FaL-Gcementitious binder at two different curing temperatures.Water absorption and density of FaL-G bricks andhollow blocks are given in Table 5. Table 6 and Fig. 6give the results of reduction in compressive strength ofFaL-G bricks and hollow blocks cured in sulfatesolution.

3.1. Compressive strength at ambient temperature

In the FaL-G combination, fly ash acts as a source ofreactive silica and alumina, to give silicate and aluminatehydrates, which are responsible for the development ofstrength. Silica, present in fly ash, reacts with lime andforms calcium silicate hydrate. Alumina, together with

523S. Kumar / Construction and Building Materials 16 (2002) 519–525

Fig. 5. Compressive strength of FaL-G specimens(mix M-9) curedat different temperatures.

Table 5Water absorption and density of FaL-G bricks and hollow blocks

Mix designation Water absorption Density(%) (kgym )3

BricksM-1 – –M-2 37.0 1172M-3 36.3 1193M-4 35.2 1224M-5 35.4 1183M-6 33.7 1205M-7 32.4 1220M-8 33.6 1192M-9 30.0 1220M-10 28.9 1223

Hollow blocksM-11 37.2 1196M-12 34.4 1201M-13 31.1 1230

Fig. 6. Reduction in compressive strength of FaL-G bricks and hollowblocks in sulfate solution.

Table 6Reduction in compressive strength of FaL-G bricks and hollow blockscured in sulfate solution

Mix designation Reduction in compressivestrength(%)

BricksM-1 –M-2 33.3M-3 27.3M-4 17.2M-5 23.2M-6 13.9M-7 13.0M-8 13.6M-9 13.5M-10 9.2

Hollow blocksM-11 27.9M-12 17.8M-13 15.9

lime, reacts with gypsum to form calcium trisulfoalu-minate hydratew11x.Figs. 1–4 show the average compressive strength of

each mix proportion with age, based on the average of6 specimens, having individual variation not more than"15% of the average value as per the recommendationof Indian codew15x. Figs. 1–3 show the compressivestrength of FaL-G bricks after 24, 72 and 96 days ofcasting. Fig. 1 corresponds to bricks manufactured with80% fly ash. Figs. 2 and 3 correspond to bricks, whichare manufactured with fly ash contents of 70 and 60%,respectively. It is observed that the strength of thesebricks increases with age. But, the rate of gain incompressive strength is found to be more in initial daysand it decreases with the age.

Fig. 1 shows that by keeping fly ash content constantat 80%, the bricks gain more strength with more phos-phogypsum content. Similar results are also obtainedwith 70 and 60% of fly ash content in the bricks. It canbe anticipated from Figs. 1–3 that the FaL-G bricks willgain more strength even after 96 days of casting.For economy, the fly ash content of FaL-G bricks and

hollow blocks should be kept as high as possible. It wasobserved that in the case of mix proportion with fly ashcontent as high as 90%, i.e. mix M-1, the FaL-G brickseasily break to pieces during handling and completelycrumble even by a free fall from moderate heights. Thecompressive strength of FaL-G bricks with high propor-tion of fly ash, mix M-3, was obtained as 5.9 Nymm2

after 96 days of casting. The minimum average crushingstrength prescribed in the Indian code for burnt claybricks are 3.5 Nymm w17x. Therefore, these bricks can2

524 S. Kumar / Construction and Building Materials 16 (2002) 519–525

easily replace the burnt clay bricks, as they havesufficient strength for their use in low cost housing ornon-load bearing partitions and curtain walls of framedstructures.Fig. 4 shows that the compressive strength of FaL-G

hollow blocks decreases as the fly ash content increases.It was observed that in the case of mix proportion witha high fly ash content of 80%, the compressive strengthof 96 days cured hollow blocks, was less than theminimum average crushing strength prescribed in theIndian codew12x. However, it can be anticipated thatthe FaL-G hollow blocks will gain further strength evenafter 96 days of casting. There is a need to carry outlong-term study of the properties of FaL-G bricksbeyond the age of 96 days.

3.2. Compressive strength at different temperatures

The compressive strength of the FaL-G specimenscured at 23"2 8C and 508C are shown in Fig. 5. Itwas observed that the process of hardening is definitelyinfluenced by temperature. The cementitious bindercured at 508C exhibited a much higher strength thanthat of the binder cured at 23"2 8C, at the same age.Fig. 5 indicates that the strength of FaL-G specimenshas reached maximum within 48 days at 508C whilethe specimens cured at 23"2 8C did not indicate thecompletion of hydration even after 96 days. Thus, thecementitious binder cured at higher temperatures maybe used for making prefabricated products such asbuilding blocks, tiles, boards etc.

3.3. Water absorption and density

Table 5 shows the water absorption and density ofFaL-G bricks and hollow blocks for various mix pro-portions. It was observed that the water absorption ofthe bricks and hollow blocks decreases with the decreasein fly ash content. It can be seen from Table 5 that ingeneral, the density and water absorption are closelyrelated to each other. The general observation is that ahigh degree of inverse correlation exists between densityand water absorption. With the increase in the density,the water absorption of these bricks reduced. Further-more, on comparing Table 5 with Figs. 1–3, it isobserved that the FaL-G bricks with a higher percentageof water absorption have lesser compressive strength.The compressive strength of bricks increases as densityof the bricks increases, irrespective of the mix propor-tion. It was observed that water absorption of FaL-Gbinder, in the present investigation, is between 28.9 and37.2%. As per the codesw12,17x, the water absorptionof ordinary burnt clay bricks or blocks should not bemore than 20% by weight. Clearly, the water absorptionof FaL-G binder with high fly ash content is more

compared to traditional burnt clay products. This aspectcertainly needs further investigation.The mass of clay bricks manufactured in India is

between 1600 and 1920 kgym , whereas, the mass of3

FaL-G cementitious binder was obtained to be between1172 and 1230 kgym . This shows that the use of FaL-3

G binder in bricks and hollow blocks in place of ordinaryburnt clay bricks or hollow blocks will reduce the weightof structures considerably. This reduced weight of FaL-G products will provide a working comfort and ease ofhandling, in addition to reduction in dead weight ofstructures. This reduction in overall weight of the struc-ture, especially in multi-storeyed buildings where FaL-G bricks or blocks may be used in partitions, results inthe economy of the structure.

3.4. Durability

Table 6 shows the loss in compressive strength ofFaL-G bricks and hollow blocks exposed to short-termaccelerated tests for 72 days in aggressive sulfate envi-ronments. Pozzolanic materials are well established asdurable construction materials and FaL-G also belongsto the same class. But, these bricks and hollow blockswith high proportions of fly ash have shown more waterabsorption property and, therefore, there is more ingressof sulfate solution inside the bricks leading to its greaterdamage. However, this weakness is compensated byincreasing the content of phosphogypsum. It can be seenfrom Fig. 6 that the compressive strength loss decreasessignificantly with the increase in the phosphogypsumcontent. In other words, the durability characteristics ofthe material can be controlled by controlling the phos-phogypsum content. Bricks with good strength anddurability may be used in exposed walls while bricksmade with high proportions of fly ash may be usedprofitably in internal walls or partitions.FaL-G bricks and hollow blocks comply with the

criteria for environmentally friendly products, since theingredients of FaL-G utilize the by-products and wastesfrom industries. The manufacturing process, beingdevoid of sintering or autoclaving, is also energy con-servative. In areas where good burnt clay bricks are notavailable or are expensive, and the ingredients of FaL-G are available in mutual proximity, the FaL-G technol-ogy would be an ideal alternative.

4. Conclusions

Based on the experimental investigation reported inthe paper, the following conclusions are drawn:

1. FaL-G is a hydraulic binder that is reactive uponaddition of water.

2. Similar to concrete, the strength of FaL-G bricks andhollow blocks increases at a faster rate in initial daysof curing and subsequently at a relatively lower rate.

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3. Gypsum has more pronounced binding action thanlime.

4. Being lighter in weight, FaL-G bricks and hollowblocks will reduce the dead weight and materialhandling cost for buildings.

5. Water absorption increases with increased fly ashcontent and it decreases with an increase in thedensity of FaL-G bricks and hollow blocks.

6. FaL-G bricks and hollow blocks with suitable phos-phogypsum content have better resistance to strongsulfate environments.

7. Curing temperature influences the process of harden-ing of FaL-G.

8. These bricks and hollow blocks require no skilledlabour and can also be moulded in to any shape andsize depending upon the requirements.

Unique possibility exists for the bulk utilization offly ash in producing FaL-G bricks and hollow blocks ifthermal power plants, phosphoric acid fertilizer indus-tries and limestone mines are available in mutual prox-imity. On account of the transportation cost of fly ash,waste gypsum and other factors, the cost of bricks maybe substantially higher than those of conventional bricksat several places. However, the project should be pro-moted for societal benefits from pollution hazards.It is further needed to develop awareness among

users, professionals and financial supporters for usingthese waste materials for solving the housing problemsin addition to balance economy and achieve energyconservation.

Acknowledgments

The author wishes to thank Mr L.P. Singh, a post-graduate student of Harcourt Butler Technological Insti-tute, Kanpur, India, for providing valuable informationneeded for the present investigation.

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