Wissem GallalaPhD student, Earth SciencesDepartment, faculty of Science,

Mohamed Essghair GaiedAssociate Professor, EarthSciences Department, faculty

Ali TliliAssociate Professor, EarthSciences Department, facultyof Science, Sfax, Tunisia

Mabrouk MontacerProfessor, Earth SciencesDepartment, faculty ofScience, Sfax, Tunisia

Proceedings of the Institution ofCivil EngineersConstruction Materials 161February 2008 Issue CM1Pages 25–30doi: 10.1680/coma.2008.161.1.25

Paper 700012Received 16/03/2007Accepted 25/10/2007

Keywords:buildings, structure & design/geology/research & development

Sfax, Tunisia of Science, Sfax, Tunisia

Factors influencing the reactivity of quicklime

W. Gallala, M. E. Gaied, A. Tlili and M. Montacer

This paper describes the factors influencing the reactivity

of quicklime; namely the chemical composition of the raw

material, the calcination and decrepitation processes to

manufacture calcium oxide (CaO) and the slaking rate of

lime in water. The water composition was also

investigated. It was found that the presence of impurities,

the tendency to decrepitate and overburning of the lime

during calcination dramatically decreased its reactivity.

Furthermore, the water chemistry has an important

influence on the slaking rate of lime: the presence of

chloride in the slaking water increases the hydration rate

whereas sulfate and magnesium ions decrease this rate

and delay the slaking process.

1. INTRODUCTION

Nowadays, the use of lime in its various forms is steadilyincreasing. This study investigated quicklime, the use of which isgaining in importance throughout the world in situations such aspollution control, sodium carbonate manufacture and inagriculture, in order to decrease soil acidity, etc.

The preparation of quicklime (Fig. 1) requires limestonecalcination at a temperature that exceeds 9008C,1 producingcarbon dioxide and calcium oxide according to the followingchemical equation (equation (1))

CaCO3 Cheat % CaOCCO2

ðcalcium carbonateÞCheat ðcalcium oxideÞCðcarbon dioxideÞ

limestone quicklime

1

Equation (2) shows that 56 weight units of CaO (pure) will hydratewith 18 weight units of water. Conversely, 320 litres of waterwould be needed to hydrate 1 t of CaO.2

CaOCH2O % CaðOHÞ2 Cheat

ðcalcium oxideÞCwater ðcalcium hydroxideÞCheat

quicklime hydrated lime

2

In addition, quicklime is an air binder whose heating is doneaccording to the following reaction (equation (3))

CaðOHÞ2 CCO2%CaCO3 CH2O3

Quicklime can be distinguished by reactivity, hydration ratemeasurement, or by a rate that produces an aqueous suspensionin hydroxyl ions (OH). Many studies have dealt with the factors

Construction Materials 161 Issue CM1 Factors in

influencing both the reactivity and the slaking process,including: the effect of storage and production on reactivity;3 theeffects of limestone characteristics (microstructure and texture)and calcination temperature;4 the influence of H2O and CO2;

5 andthe effect of chloride and sulfate ions on the slaking rate.6,7

Hosten and Gulsun8 showed that item size and the dolomitecontent of the limestone were the most influential parameters.The specific area and alumina content have also beenexperimentally proven to be key factors in the progress of thereactivity.9

Quicklime reactivity is influenced by many factors that canaffect the quality of the quicklime and the slaking process. Theaims of this study were: the characterisation of Tunisiancalcrete limestone; the evaluation of the decrepitationtendency; the determination of the quicklime reactivity with orwithout CaCl2; and elucidation of the factors influencingquicklime reactivity.

2. EXPERIMENTAL

2.1. Materials

The test was carried out on three samples of calcretes from theTunisian Sahel: two Early Pleistocene calcretes (JB1 and HEN3)from Jebiniana and Hencha; and one Middle Pleistocene calcrete(GR1) from Graba. These calcretes were selected in order to assessthe characteristics of quicklime, which is calcined in traditionallimekilns and most commonly used as a binder.

The calcrete was used as raw material to produce quicklime.Mineralogically, it consisted of calcite (80–85%), quartz (11%)and traces of clays (Fig. 2). Chemically speaking, the CaO had avalue between 45 and 50%.

Samples of calcretes were calcined at four different temperatures(900, 1000, 1100 and 12008C) for 24 h for the production ofquicklime. One hundred grams of quicklime was hydrated in400 ml of distilled water with a temperature adjusted to 258C andthe stirring rod in the flask revolving at 400 G 50 rpm.

2.2. Analytical methods and techniques

(a) The X-ray diffraction (XRD) method was employed tocharacterise the mineral constituents of the calcrete samples,employing a PHILLIPS diffractometer using CuKa radiationand a nickel filter, and the diffraction interval was between 3and 608.

fluencing the reactivity of quicklime Gallala et al. 25

H2O

H2O

H2O

CO2

CO2

CaO

CaCO3

Ca(OH)2

Calciumcarbonate

Quicklime

Lime putty

Aggregate

Mortar

+ heat

Fig. 1. Lime cycle

Dial thermometer

Stopper

Stirringrod

Covers

Dewar flaskwith base

Ring standapparatus detail

Fig. 3. Lime slaking apparatus

26

(b) Atomic absorption spectroscopy (Perkin Elmer). Thistechnique was used to determine the percentage of majorchemical elements present.

(c) Transmission light microscopy (Olympus) was carried out onpolished thin sections of the calcrete in order to identify thetexture, shape and size of the grains. The morphology ofcalcrete was also observed with a PHILLIPS scanningelectron microscope (SEM) using gold-coated samples.

(d) The adsorption of nitrogen was performed on the quicklimein order to assess the value of the specific surface area byphysical sorption isotherm data according to the method ofBrunauer–Emmet–Teller (BET); the apparatus used was aBET 2000.

(e) Mercury intrusion porosimetry was used to estimate thetotal porosity of the quicklime.

( f ) A pycnometer was used to measure the bulk density of thequicklime.

(g) In order to determine the performance of quicklime in termsof its slaking characteristics, the following equipment wasused (Fig. 3).(i) A mechanical stirrer capable of 400 G 50 rpm and

fitted with a special stainless steel stirring rod as shown.(ii) A modified Dewar flask, 665 ml, fitted with special

rubber gasket covers.

C

C C CC

CC

C

C: calcite

C

602q

50403020100

C

Q

Q Q

Q QQ Q QQ

Q: quartz

C

Fig. 2. X-ray diffraction patterns of oriented powder mountsfrom bulk sample of calcrete

Construction Materials 161 Issue CM1 Factors influ

(iii) Thermometers (dial type), range 0–1008C in 18increments.

(iv) Weight scale.(v) Sieve: 3$35 mm.

3. RESULTS AND DISCUSSION

3.1. Calcrete characteristics

The calcrete was generally in crystalline form and was dominatedby micritic to microsparitic calcite. Two different types of cementwere suggested to reflect the differences in environmentalconditions during the cementing period. The first one wasconstituted during the replacement of the sediments under wetconditions with high rates of evapotranspiration, whereas opaquecements represented less replacement and contained a higherpercentage of the original grains.10 Moreover, themicromorphological analyses of the calcrete presented apolycyclic and polyphasic development of calcite (micrite1–4 mm, microsparite 4–20 mm, sparite higher than 20 mm)around quartz grains (Fig. 4(a)). The micrite and allochem can beaffected by crack networks (Fig. 4(a)), which were filled withlimpid microsparite resulting from late diagenesis. Theabundance of these cracks or veins reflected the withdrawalinfluence of the process,11 which can be due to seasonaltemperature variation. Porosity in this hardpan was present underdissolved allochem, which would be replaced by microsparite.The important phenomenon affecting the quartz grains was thesilica corrosion and replacement by calcite (Fig. 4(a)). Theinteraction of water with the grains caused dissolution.12

The results of the study using optical microscopy were inagreement with X-ray analysis. Indeed, the presence of the twocardinal minerals was noted, namely calcite and quartz; feldsparappeared incidentally (Fig. 4(b)).

encing the reactivity of quicklime Gallala et al.

(a)

(c)(d)

(b)

ms M

Sp

50 µm 20 µm

0.1 mm

Q

Q

F

C

Fig. 4. Microphotographs of calcretes: (a) badly graded quartz, giving a packstone structure,presence of late filling of sparite in cracks ( JB1); (b) presence of macled feldspar with the otherelements of calcrete (GR1); (c) scanning electron micrographs illustrating calcrete proprieties:micrite-dominated cement with minor quartz grains ( JB1); (d) presence of fibrous clay (palygorskite)in the calcrete (GR1). M, micrite; ms, microspar; Sp, sparite; C, calcite; F, feldspar; Q, quartz

The SEM illustration shows the compact texture of the calcretewith micritic calcite (Fig. 4(c)). Impurities occur when porosityand cracks are developed; in this case the cavities were occupiedby fibrous clay (Fig. 4(d)). These impurities will influence thequality of the quicklime produced.

The chemical analyses confirmed the mineralogical analyses. TheSiO2 rate, which formed the main part of the quartz and clays,moved in an opposite direction with the CaO, Al2O3, TiO2, Fe2O3

contents to evolve in the same direction as SiO2 (Table 1).

The Ca/Mg ratio indicated a high predominance of calcite in allcalcretes. The MgO percentages did not exceed 2%, although thehighest values recorded, testified to the presence of palygorskite,which was identified by X-ray diffraction (Fig. 5). The formationof authigenous clays, such as palygorskite in the calcretes(Fig. 4(d)), was the result of an enrichment of groundwaterwith magnesium after magnesium calcite precipitation in thecalcrete.13 The presence of a high rate (about 40% of SiO2 C

Al2O3 C Fe2O3) was due first to the presence of free quartzand second to the presence of clays at the chalky anddiscontinuous levels.

LOI CaO MgO SiO2 Fe2O3 Al2O3

HEN1 Raw material 40$37 46$20 0$93 9$24 0$33 0$68Quicklime 9$64 71$80 1$40 14$64 0$45 1$06

JB1 Raw material 38$74 49$28 0$57 10$15 0$29 0$45Quicklime 13$66 72$80 1$22 9$78 0$46 0$76

GR1 Raw material 39$84 45$40 0$65 9$58 0$40 0$91Quicklime 8$74 67$20 0$89 18$91 0$58 1$28

Table 1. Chemical composition of investigated calcretes and quicklime produced

Construction Materials 161 Issue CM1 Factors influencing the reactivity of quicklime

3.2. Calcination and

decrepitation of calcretes

The decrepitation of limestoneis a measure of itssusceptibility to disintegrationduring calcination. A simpletest to measure the tendency ofthe stone to decrepitate can beundertaken by sieving thecalcined material andevaluating the material passingthrough the sieve relative tothe total weight, to obtain thepercentage decrepitation.14 InFig. 6 it can be seen that theGR1 sample has the mostimportant decrepitationwhereas JB1 and HEN1represent a low degree ofdecrepitation. This could bedue to the hardness andcompactness of the rocks aswell as the size of the particles.In general, there is a positivecorrelation between the degreeof decrepitation and thecrystals’ size in the calcareousrock. It was noted that for the

same sample the decrepitation percentage was higher as thecalcination temperature became more significant.

After crushing, the particles used must be between 10 and 20 mmin size as the heat does not quite penetrate to the core of larger-sized stones; therefore, the centre of these pieces remains ascalcium carbonate whereas the outside is converted into CaO.These cores are referred to as grit. For the smaller stones, the heatreaches the core rapidly and the outside layer is overheated,forming a hard outer shell that water cannot penetrate; therefore,the slaking process is greatly retarded or prevented.15

3.3. Quicklime hydration

The degree of reactivity is normally measured by the rate of thelime hydration (slaking) reaction. It is evaluated by a relativelysimple slaking test,16 which measures the heat of hydrationproduced by a known weight of lime in a known volume ofwater.14

Each curve represents a temperature level (Fig. 7). The curvespresent a rather steep slope, indicating a very fast exothermicreaction and the plateau indicates the end of this reaction.

Na2O K2O Total

0$07 0$15 97$970$10 0$12 99$210$04 0$14 99$660$09 0$13 98$900$06 0$17 97$010$12 0$30 98$02

Gallala et al. 27

P: Palygorskite,S: Smectite,K: Kaolinite,I: Interstratified clay

P

P

a

b

c

05 10 15 20 25 30

KK

I

I S

2q

Fig. 5. Oriented clay powders: (a) after heating at 5008C for 2 h;(b) air-dried at 258C; (c) after adding solvated glycol

JB1

HEN1

GR1

45

40

35

30

25

20

15

10

5

0

0

012345678

9

10

20

30

40

50

0

0 5 10 15 20 25 30

5 10 15 20 25

0 5 10

Time : min

15 20 25

Tem

pera

ture

diff

eren

ce: °

CTe

mpe

ratu

re d

iffer

ence

: °C

Tem

pera

ture

diff

eren

ce: °

C

900°C

1000°C

1100°C

1200°C

900°C

1000°C

1100°C

1200°C

900°C1000°C1100°C1200°C

Fig. 7. Slaking rates of different quicklimes

28

The texture of the quicklime depended initially on the burningtemperature. According to the maximum temperature it waspossible to distinguish between softly burnt quicklime,moderately burnt quicklime and hard burnt quicklime.4,15

Figure 7 provides the reactivity behaviour of the four levels ofburning (900, 1000, 1100 and 12008C). Soft burnt quicklimes,which were produced at a lower temperature, were characterisedby a larger reactivity in comparison with the hard burnt onesproduced at a higher temperature.

This high reactivity is due to fact that the crystal size was smaller;the specific area was larger, the insulated pores had smalldimensions in comparison with the total volume of larger poresand the apparent density was lower (Table 2).

The loss of reactivity was caused by overburning or by heatingquicklime in the kiln at a higher temperature for a long period.When quicklime was calcined, the release of CO2 left pores in it,thus creating extra surface area on which the reaction couldoccur. Overburning resulted in the collapse of these pores andreduced the ability of the quicklime to react with water.

Overburning will also result in impurities in the quicklime(mainly silica, alumina and iron) by forming cement clinkerminerals, which further reduces the calcium content available forwater reaction.2

GR1

JB1

HEN1

120011001000900

20

15

10

5

0

Dec

repi

tatio

n: %

Temperature: °C

Fig. 6. Decrepitation percentages of calcretes

Construction Materials 161 Issue CM1 Factors influ

The slaking rate of quicklime was significantly influenced byproduction and storage under wet conditions. Both hard burningas well as air-slaking and recarbonation, significantly decreasedthe slaking ability of the quicklime. Air-slaking had a moredramatic effect than recarbonation on the lime slaking rate andthe former seemed to precede and catalyse the latter. Regardlessof the rate at which the lime slaked, it seemed that the amountthat eventually slaked remained approximately the same.3

The results of this test showed that the most reactive quicklimewas burnt at a temperature of 10008C. The reactivity was alsoinfluenced by the nature of the raw material.

Quicklime that originated from the Early Pleistocene calcretesdisplayed a greater rate of reactivity than that produced from theMiddle Pleistocene calcretes. Consequently, the degree ofreactivity was probably negatively correlated with that of

Soft burntquicklime

Moderatelyburntquicklime

Hardburntquicklime

Bulk density: g/cm3 1$5 to 1$8 1$8 to 2$2 O 2$2Total porosity: % 46 to 55 34 to 46 ! 34Specific surface: m2/g O 1 0$3 to 0$1 ! 0$3

Table 2. Physical properties of quicklime under different degreesof burning

encing the reactivity of quicklime Gallala et al.

25

20

15

10

5

00 5 10 15 20 25

Concentration: g/l

Rea

ctiv

ity

Fig. 9. Relationship between reactivity and CaCl2 concentration

decrepitation: namely the limestone that presented a lessertendency to decrepitate produced higher reactive quicklime. Thespecific surface area can be a reliable factor for the estimation ofquicklime reactivity. The greater the specific surface area, themore reactive the quicklime.4

3.4. Reactivity of quicklime in the presence of CaCl2

The experiments were carried out on lime produced at 10008C,from sample JB1.

Figure 8 shows that as soon as the calcium chloride was added,the slaking time decreased. For example, the reactivity accordingto the CaCl2 concentration was estimated during a period of2 min and it was noted that there was a linear relation betweenthese two parameters (Fig. 9). The increase in the CaCl2concentration involved an increase in reactivity (Fig. 9). Indeed,the chlorides contained in the water by hydration increased thesolubility of lime, in contrast to the sulfates and magnesiumcarbonates, which acted as inhibitors of reactivity.5,8 Matscheiet al.9 showed that the amount of reactive CaCO3 showed theopposite behaviour to the amount of SO3. When the amount ofSO3 decreased, the most reactive CaCO3 was obtained.

4. CONCLUSION

The production of high-quality quicklime depends on thecalcination conditions and the nature of the raw materials. Thequality of the quicklime was always controlled by the reactivityand specific surface measurements. Other factors such as theporosity, the apparent density, the loss on ignition and hardnesscan also be evaluated. Moreover, the tendency for the stone todecrepitate during the calcination is always a factor.

The aim of the present study was to control the quality of thequicklime produced according to temperature.

At low calcination temperature, the products formed duringburning contained quicklime and unburnt carbonate. When thetemperature was increased, high reactivity quicklime wasobtained and after remaining at high temperatures, low reactivityquicklime was obtained.

A highly reactive quicklime was produced when the heat reachedthe cores and transformed all of the carbonate present toquicklime. The product of high reactivity was relatively soft,contained small quicklime crystals and had an open porousstructure.

Diff

eren

ce o

fte

mpe

ratu

re: °

C

25

20

15

10

5

00 5 10 15 20 25

0 g/l4 g/l

10 g/l20 g/l

Time: min

Fig. 8. The effect of CaCl2 in the slaking water on the slakingprocess

Construction Materials 161 Issue CM1 Factors in

The optimum quicklime properties were: high reactivity, a largespecific area and a low density. The increase in degrees ofburning associated with increasing calcination temperature riseand/or long residence time in the kiln would be harmful. Underthese conditions, crystals tend to increase, to agglomerate and tosinter. Consequently, there will be a specific surface reduction aswell as a reduction in porosity and reactivity as the bulk densityincreases (Table 2). As a result the densest quicklime was the leastreactive.

The degree of quicklime reactivity is important in manyindustries. This property is measured by the hydration reactionrate. The results of the present investigation of quicklimereactivity degree of Quaternary calcrete from the Sahel (easternTunisia) suggest that the Lower Pleistocene calcrete is moreadequate for the manufacture of this type of binder, which is amuch needed product in the chemical industry and environment,in addition to its traditional applications.

ACKNOWLEDGEMENTS

The authors are grateful to Raouin Hassen (Office National desMines) for chemical analysis and to Hachicha Olfa for the SEMoperation. They wish to thank Mr Bejaoui Hafedh who helpedproofread this paper. They are also grateful to the anonymousreviewers for their helpful comments and constructive reviews onan earlier version of the paper.

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encing the reactivity of quicklime Gallala et al.