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Fire Safety Journal 43 (2008) 610–617

www.elsevier.com/locate/firesaf

Development of a high-temperature-resistant mortar by usingslag and pumice

Serdar Aydın�

Department of Civil Engineering, Dokuz Eylul University, Buca 35160, Izmir, Turkey

Received 30 April 2007; received in revised form 10 December 2007; accepted 7 February 2008

Available online 19 March 2008

Abstract

The effects of high temperatures up to 900 1C on the mechanical properties and the microstructure of cement-based pumice mortars

incorporating different amounts of ground granulated blast furnace slag (GGBFS) were investigated in this study. The residual

compressive and flexural strength of mortar specimens were determined after exposure to high temperatures. The results have indicated

that the effect of GGBFS incorporation on high-temperature resistance of pumice mortar is shown significantly at 900 1C. At this

temperature level, the mortar containing 80% GGBFS exhibited only 23% and 28% compressive strength loss when cooled in air and

water, respectively, where as mortars without GGBFS lost almost 70% of their strength. Furthermore, none of the GGBFS incorporated

mortar specimens showed compressive strength loss up to 600 1C when cooled in air. The most severe conditions in terms of strength loss

due to high temperatures were flexural loading and water cooling case.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: High temperature; Mortar; Pumice; Granulated blast-furnace slag; Mechanical properties; Microstructure

1. Introduction

One of the most important physical deteriorationprocesses that influence the durability of concrete struc-tures is high temperature. Nevertheless, it is possible tominimize the harmful effects of high temperature onconcrete by taking preventive measures, such as choosingthe right materials. Material properties, such as propertiesof aggregate, cement paste and aggregate–cement pastebond, and thermal compatibility between aggregate andcement paste, greatly affect the high-temperature behaviorof concrete [1–4].

A high-temperature-resistant insulation material properfor plastering jobs may be developed by using mineraladmixtures, and thermal stable and porous aggregate inconventional cement mortar mixtures. The aggregateshould also produce a strong temperature-resistant bondwith the cement paste.

The beneficial effect of mineral admixtures, such as flyash and ground granulated blast furnace slag, with respect

e front matter r 2008 Elsevier Ltd. All rights reserved.

esaf.2008.02.001

2 412 7044; fax: +90 232 412 7253.

ess: serdar.aydin@deu.edu.tr

to high-temperature resistance arises from stabilizing ofCa(OH)2 from cement by means of pozzolanic reaction.The detrimental effects of Ca(OH)2 are that, the micro-cracks appear first in the areas of Ca(OH)2 concentrationat about 300 1C [5], and the decomposition of calciumhydroxide into lime and water vapor above 350 1C.Decomposition of calcium hydroxide is not critical interms of strength loss during heating. However, it may leadto serious damage due to lime expansion during the coolingperiod [6]. Also, this damage can be observed during fire-extinguishing process, CaO reacts with water and turnsCa(OH)2 with a significant expansion.Ground granulated blast furnace slag (GGBFS) is

obtained during steel production as a by-product. It hasbeen reported that, slag generation is about 10% of thetotal production. The generation of GGBFS for Turkeywas about 300,000 tones years in 2003. The usage ofGGBFS in concrete has some advantages such as improv-ing the workability, strength (especially at later ages),thermal insulation and high-temperature resistance. It alsoreduces bleeding of fresh concrete, heat of hydration,permeability and porosity of concrete. The useful con-tribution of GGBFS on high-temperature resistance was

ARTICLE IN PRESSS. Aydın / Fire Safety Journal 43 (2008) 610–617 611

proved by numerous researchers [2,7–9]. A detailedresearch on compressive strength, elastic modulus, andcomplete stress–strain curves of high-performance concretewith GGBFS at elevated temperatures up to 800 1C werereported by Xiao et al. [10].

The porosity and mineralogy of the aggregate seem toexercise an important influence on the behavior of concreteexposed to fire. Siliceous aggregates containing quartz,such as quartz sand, sandstone and flint, may cause distressin concrete at about 573 1C. A similar distress may beginabove 700 1C as a result of the decarbonation reaction incarbonate rocks. The loss of strength is considerably lowerwhen the aggregate does not contain silica, e.g. withlimestone, basic igneous rocks, and particularly withcrushed brick and blast furnace slag. In addition topossible phase transformations and thermal decompositionof the aggregate, the response of concrete to fire isinfluenced in other ways by aggregate mineralogy. Forinstance, the aggregate mineralogy determines the differ-ential thermal expansions between the aggregate andcement paste, and thus ultimate bond strength of thetransition zone [11]. Therefore, the first criteria to beconsidered when selecting an aggregate for concrete at hightemperatures is its thermal stability, both physical andchemical. Concrete with a low thermal conductivity has abetter fire resistance so that, for instance, lightweightconcrete stands up better to fire than ordinary concrete[12]. Lightweight aggregates such as pumice, foamed slag,and expanded clay products have in themselves highresistance to fire, and concrete made from them has alow heat conductivity [13]. All of these aggregates wereexposed to high temperatures during their formation andproduction stages. Due to this fact, these aggregates havethemselves a high resistance to volume expansion anddecomposition due to the elevated temperatures [14]. Hertz[15] has reported the results of experimental studies on fire-resistant concrete, which are realized by Nekrasov andTarasova [16] and Muff [17]. Nekrasov and Tarasova [16]showed that concrete based on a light aggregate of burnedchamotte and a matrix obtained by adding pulverized

Table 1

Physical, chemical and mechanical properties of cement and GGBFS

Chemical composition (%) Cement GGBFS

SiO2 19.30 35.71

Al2O3 5.57 14.52

Fe2O3 3.46 0.80

CaO 63.56 32.13

MgO 0.86 9.39

Na2O 0.13

K2O 0.80

SO3 2.91

LOI 2.78

Insoluble residue 0.42

Free CaO (%) 1.22

chamotte to the cement as a pozzolona did not suffer fromfire exposure up to at least 800 1C. Muff [17] replaced thepulverized chamotte by a natural powder of a specialDanish clay called mo-clay and the aggregates are gravel ofburned mo-clay. In case of chamotte, almost no strengthloss up to 800 1C was recorded while mo-clay showed 70%strength loss at 800 1C.Pumice is essentially an aluminum silicate of igneous

origin with a cellular structure formed by a process ofexplosive volcanic activity. Due to its cellular structure,lightweight and insulating properties, pumice has beenextensively used as a construction material. High-tempera-ture (600 1C) performance of pumice, brick powder andriver sand aggregates incorporated mortars were investi-gated by Aydın and Baradan [18]. Pumice aggregatemortar revealed the best performance with only 5%compressive strength loss while the mortar incorporatednatural river sand aggregate lost 40%. Another studyrevealed by Aydın and Baradan [19] showed that pumiceaggregate mortar incorporating 60% FA did not exhibitany loss in compressive strength at 900 1C when cooled inair, as a result of strong aggregate–cement paste interfacialtransition zone and ceramic bond formation.The purpose of this study is to improve a high-

temperature resistant mortar by using pumice aggregateand GGBFS.

2. Materials and experimentation

The cement used in this study, CEM-I (42.5N) procuredfrom Cimentas- Cement Plant, Izmir, Turkey. The chemicaland physical properties of the cement and GGBFSprocured from Iskenderun steel plant have been presentedin Table 1. The pumice was provided from Menderesregion of Izmir. Its gradation, physical and chemicalanalyses are given in Table 2.Cement was replaced with up to 80% GGBFS, and the

pumice to binder (cement and GGBFS) ratio of 3 was keptconstant for all mixtures. The compositions of all mixturesare given in Table 3. All test batches were mixed by using

Physical and mechanical properties of cement

Specific gravity 3.15

Specific surface (Blaine) (m2/kg) 352

Initial setting time (min) 145

Final setting time (min) 275

Volume expansion (mm) 1.00

Compressive strength (MPa) of cement

2 days 27.2

7 days 42.4

28 days 52.7

Pozzolanic activity index (%) of GGBFS

7 days 98

28 days 108

Specific surface of GGBFS (Blaine) (m2/kg) 485

ARTICLE IN PRESSS. Aydın / Fire Safety Journal 43 (2008) 610–617612

an electrically driven mechanical mixer conforming to therequirements of ASTM C305 [20]. Initially, cement,GGBFS and pumice were dry mixed for 3min and thenwater was gradually added while mixing continued forabout 5min. The water was added at different ratios toprovide constant fluidity of about 11071mm. The flowtest was performed according to ASTM C230 [21]. Freshmixtures were cast into prismatic (40mm� 40mm� 160mm) steel molds. The specimens were kept in themolds for 24 h at room temperature of about 20 1C. Afterdemoulding, the specimens were left for 27 days in a curingroom at a temperature of 20 1C and relative humidity of9075% until testing. After the curing period, 12 specimensfrom each mixture were exposed to 300, 600, and 900 1Ctemperatures for 3 h in oven. The heating rate was set at10 1C/min. It should be emphasized that this heating rate is

Table 2

Grading, physical and chemical properties of pumice

Chemical composition (wt%) Physical properties

SiO2 75.51 Bulk specific gravity 2.03

Fe2O31.10 Water absorption (%) 6.38

Al2O3 9.94 Unit weight (kg/m3)

CaO 0.25 Loose 1220

MgO 0.04 Compacted 1380

Na2O 2.04 Sieve size (mm) % Passing

K2O 5.12 4 100.0

TiO2 o0.01 2 91.5

P2O5 o0.001 1 75.4

LOI 4.27 0.5 52.5

0.25 27.4

Table 3

Mix design of all mixtures

Mixture GGBFS

(%)

Water/

binder

Flow (mm) Unit

weight in

fresh state

(kg/m3)

GGBFS0 0 0.72 111 1968

GGBFS20 20 0.71 110 1944

GGBFS40 40 0.70 110 1941

GGBFS60 60 0.70 110 1940

GGBFS80 80 0.70 110 1942

Table 4

Compressive and flexural strength of all mixtures

Mixture Compressive strength (MPa)

20 1C 300 1C 600 1C 900 1C

SC FC SC FC SC

GGBFS0 45.6 55.9 37.0 43.7 31.5 14.4

GGBFS20 41.1 55.2 35.0 45.5 28.4 11.4

GGBFS40 40.7 57.6 35.4 47.5 31.2 18.0

GGBFS60 32.0 45.8 27.3 43.7 23.2 16.5

GGBFS80 20.5 28.1 14.7 25.6 13.1 15.7

very high for large specimens due to thermal stressformations. For example, a rapid fire exposure of 10 or20 1C/min in an oven will cause a considerable temperaturedifference of approximately 400 1C between surface andcore giving rise to thermal stresses, which will damage thespecimen before it is tested [15].Afterwards, the hot mortar specimens were cooled in

two ways. One group of specimens were left in laboratoryconditions for slow cooling while the others were soaked inwater (�20 1C) for fast cooling. After the cooling period,the prismatic specimens were subjected to flexural strengthtest according to ASTM C348 [22]. Six specimens weretested at each stage and average values were reported. Thespecimens were loaded from their mid span and the cleardistance between simple supports was 120mm. Thecompressive strength tests were performed following theflexural tests on two broken pieces left from flexural testaccording to ASTM C349 [23]. The flexural and compres-sive strength test results were compared with the test resultsof unheated mortar specimens.

3. Results and discussion

Stressed, unstressed, and unstressed residual strength testmethods are used to determine the residual strength ofconcrete at elevated temperatures. The details of these testmethods are thoroughly explained in Refs. [19,24]. The firsttwo types of test methods are suitable for assessing thestrength of concrete at high temperatures, while the latter isproper for determining the residual properties after thehigh-temperature exposure [24,25]. It is well known that,the last method gives the lowest strength values and istherefore more appropriate for determining the limitingvalues and due to this fact the third method has beenselected for this research.The effects of temperatures over the range of 20–900 1C

on compressive and flexural strength of slow cooled(SC) and fast cooled (FC) mortar specimens are given inTable 4. As seen in Table 4, GGBFS replacement resultedin decrease of both compressive and flexural strength ofmortar specimens at room temperature. The decrement isdramatic above 40% GGBFS replacement, and at 80%GGBFS replacement, compressive and flexural strengthreduction was reached about 55% and 38%, respectively.

Flexural strength (MPa)

20 1C 300 1C 600 1C 900 1C

FC SC FC SC FC SC FC

12.8 9.0 8.6 5.6 6.1 3.6 1.8 1.0

10.4 9.1 8.8 4.6 5.3 3.1 1.4 0.8

16.2 8.1 8.9 4.1 5.5 3.1 2.5 1.2

14.9 7.2 7.0 2.9 4.2 2.4 2.3 1.0

14.7 5.6 3.9 1.5 2.5 1.3 2.2 1.1

ARTICLE IN PRESSS. Aydın / Fire Safety Journal 43 (2008) 610–617 613

3.1. Residual compressive strength

The variation of residual compressive strength of SC andFC mortar specimens are presented in Fig. 1. As shown inFig. 1, the compressive strength increase for SC mortarswas between 23% and 43% at 300 1C compared withcontrol specimens. At this temperature, maximum strengthincrease was obtained above 40% GGBFS replacement.The strength gain up to 300 1C may be explained partiallydue to the closer configuration of hydrated cement pasteafter the evaporation of free water, which leads to greatervan der Waal’s forces as a result of the cement gel layersmoving closer to each other [1,26,27]. Since transportationof moisture in mortar is rather gradual, residual moisturein mortar allowed accelerated hydration at the early stageof heating mortars to high temperatures. Further hydrationof cementitious materials is another important cause of thehardening of hydrated cement paste. For GGBFS0 mortar,the temperature increase leads to additional hydrationproducts from the unhydrated cement grains. For GGBFSincorporated mortars, besides unhydrated cement, unhy-drated GGBFS particles can react with calcium hydroxideand produce C–S–H like gels [1,27,28]. At 600 1C, GGBFSincorporated mortar did not show any strength loss.However, GGBFS0 mortar showed only a slight strengthloss of 4%. For pumice aggregate mortar, only 5%compressive strength loss for 600 1C was reported byAydın and Baradan [18]. A compressive strength gain of41% at this temperature was also reported by Yazıcı et al.[29]. Turker et al.’s [14] studies showed that pumiceaggregate mortar did not show compressive strength lossup to 500 1C. The superior performance of pumice mortaris closely related to strong aggregate–cement paste inter-face, and similar behavior of cement paste and aggregate athigh temperatures. Pumice is lightweight aggregate withshrinking properties under high temperatures in contrastwith normal aggregates that have expanding characteristics[30]. Cement paste also shrinks at temperatures above

0

20

40

60

80

100

120

140

160

0GGBFS, %

Res

idua

l com

pres

sive

str

engt

h, %

604020

Fig. 1. Relative residual compressive

149 1C [31]. Nevertheless, when the temperature is elevatedto 900 1C, GGBFS0 specimens showed severe strength lossabout 68%, while GGBFS80 mixture loosed only 23%compared with control specimens. The severe strength lossof GGBFS0 specimens between 600 and 900 1C is due tothe decomposition of C–S–H gel [6,7,11,32]. The beneficialeffect of GGBFS was observed clearly at 900 1C. At thistemperature, the relative residual compressive strength ofthe specimens increased with GGBFS content. Further-more, the maximum residual compressive strength wasobtained for GGBFS40 specimens with a value of 18MPa(Table 4).The compressive strength of FC mortar specimens

decreased by about 13–28% at 300 1C and 23–36% at600 1C as shown in Fig. 1. After exposure to 900 1C, theincrement of GGBFS above 20% results in decrease ofstrength loss significantly similar to SC mortars. At thistemperature, the strength loss of GGBFS0 specimens isabout 72% while for GGBFS80 specimen the loss is only28%. For 300 and 600 1C, with the increasing GGBFScontent above 40%, the relative residual compressivestrength is tend to decrease slightly while at the 900 1Cthe relative residual compressive strength increased con-tinuously. Nevertheless, at 900 1C, GGBFS40 mixture stillhad the highest strength with a value of 16.2MPa.Rapid cooling in water caused additional compressive

strength losses compared with slow cooling in air at all testtemperatures. This result is attributed to conversion of freecalcium hydroxide to CaO (quick lime) by losing waterabove 400–500 1C. When CaO gets in contact with water, itrehydrates to form Ca(OH)2 accompanied by an expansionin volume. In addition, the degree of water saturation ofthe specimen will increase [2]. Both of these phenomena arepotentially detrimental to strength. However, strengthdifferences between SC and FC were diminished with theincrease of temperature up to 900 1C. This is due to most ofthe microcracks having take place at relatively hightemperatures of 600 1C and beyond [3]. Also, Ca(OH)2

y = -0,0073x2 + 0,7707x + 122,54 R=0.9964

y = -0,0071x2 + 0,9914x + 94,686 R=0.9335

y = 0,0086x2 - 0,1257x + 30,857 R=0.9861

y = 0,0087x2 - 0,155x + 27,4 R = 0.9838

y = -0,0054x2 + 0,3986x + 67,314 R=0.8445

y = -0,0068x2 + 0,4529x + 80,171 R=0.9702

300 °C (SC)

600 °C (SC)

300 °C (FC)

900 °C (FC)

600 °C (FC)

900 °C (SC)

80

strength of SC and FC mortars.

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y = -0,0159x2 + 1,0164x + 91,486 R=0.9373

y = -0,0054x2 + 0,1986x + 64,914 R=0.8612

y = 0,0018x2 + 0,1371x + 18,029 R=0.9226

y = -0,0023x2 - 0,2093x + 60,343 R=0.9780

y = -0,0029x2 + 0,0686x + 38,114 R= 0.8911

y = 0,0018x2 - 0,0329x + 10,229 R=0.91190

20

40

60

80

100

120

140

160

0GGBFS, %

Res

idua

l fle

xura

l str

engt

h, %

300 °C (SC)

600 °C (SC)

900 °C (SC)

300 °C (FC)

600 °C (FC)

900 °C (FC)

80604020

Fig. 2. Relative residual flexural strength of SC and FC mortars.

500

400

300

200

100

500

400

300

200

100

010 20 30 40 50 60 70

Theta-2Theta (deg)

I (cp

s)I (

cps) Q

uartz

Qua

rtzQ

uartz

Feld

spar

Feld

spar

Cal

cite

Cal

cite

900°C

20°C

Fig. 3. X-ray analysis of GGBFS0 SC mortar at 20 and 900 1C.

S. Aydın / Fire Safety Journal 43 (2008) 610–617614

formation might be beneficial due to the filling effect ofcracks and voids in case of water cooling and leads to anincrease in residual strength [15].

3.2. Residual flexural strength

The deteriorating effect of elevated temperatures onflexural strength was more severe than compressivestrength case. Although, all tested SC mortars gainedcompressive strength at 300 1C, most of them have lostsome flexural strength even at this low temperature. Theexistence of cracks reduces a valid area of cross-sectionsand the existence of tensile stress causes expansion ofcracks [33–35].

Fig. 2 shows the variation of relative residual flexuralstrength of SC and FC mortar specimens. The maximumrelative residual flexural strength for SC mortars wasobtained for GGBFS40 specimens at 300 and 600 1C, andfor GGBFS80 specimens at 900 1C. Higher GGBFScontents result in increase of relative residual flexuralstrength at 900 1C. Similar to compressive strength, themaximum strength loss at 900 1C was observed forGGBFS0 mixture, as 80%. As shown in Table 4, themaximum residual flexural strength for SC mortars wasobtained from GGBFS40 specimens with a value of2.5MPa.

Relative residual flexural strength of FC mortar speci-mens is lower than SC, similar to compressive strengthresults. At 300 and 600 1C, strength loss increases with theincreasing GGBFS content, however at 900 1C, the relativestrength increases with the increasing GGBFS content.

3.3. Microstructure investigations

X-ray analyses were realized on air-cooled GGBFS0 andGGBFS60 mortars. Besides, to determine alteration ofpumice with temperature, the X-ray analyses were alsomade on unheated pumice (�20 1C), and pumice exposed

to 900 1C for 3 h. Analyses showed that pumice consist ofquartz and feldspar at 20 and also 900 1C. In other words,no visible change has been observed in pumice at 900 1C asexpected. The X-ray analyses of GGBFS0 at 20 and 900 1C,and GGBFS60 at 20, 300, 600, and 900 1C are given inFigs. 3 and 4, respectively. GGBFS0 mortar has beenformed by quartz, feldspar and calcite phases at 20 and900 1C (Fig. 3). As shown in Fig. 4, GGBFS60 mortar isformed by quartz, feldspar and calcite phases at 20 1C.Nevertheless, at 300 and 600 1C calcite phase disappear andthe structure consists of quartz and feldspar. At 900 1C,gehlenite formation was observed beside quartz, feldspar,and calcite. Gehlenite formation was also observed oncement paste incorporated fly ash at 900 1C [19], thisindicates that when the temperature has been elevated to900 1C, glassy phases that are molten appear. This moltenphase fills in the pores and compressive strength of mortarincreases after the mortar become cold. But hot compres-sive strength of mortar decreased [36].

ARTICLE IN PRESS

600

400

200

600

400

200

600

400

200

600

400

200

010 20 30 40 50 60 70

Theta-2Theta (deg)

I (C

PS

)I (

CP

S)

I (C

PS

)I (

CP

S)

Qua

rtz

Feld

spar

Qua

rtzFe

ldsp

arQ

uartz

Feld

spar

Qua

rtz

Feld

spar

Cal

cite

Cal

cite

Geh

leni

te 900°C

600°C

300°C

20°C

Fig. 4. X-ray analysis of GGBFS60 SC mortar at 20, 300, 600, and 900 1C.

Fig. 5. SEM analysis of GGBFS0 SC mortar at 900 1C: (a) cement paste, (b) ITZ.

S. Aydın / Fire Safety Journal 43 (2008) 610–617 615

SEM investigations were realized on air-cooled GGBFS0and GGBFS60 mortars at 900 1C. The SEM observationsof GGBFS0 mortars at 900 1C are shown in Fig. 5. All thehydrated phases including C–S–H and CH appeared asamorphous structures loosing their characteristic crystallinestructure. Small, rounded formations began to be observedinstead of C–S–H crystals after exposure to 900 1C(Fig. 5a). Crystals with rounded shape may be b-C2S, whichis one of decomposition products of C–S–H at elevatedtemperatures, as confirmed in the literature [14]. Further-more, cement paste was separated from the aggregatethereby creating gaps. Increasing of temperature to 900 1Cleads to cracks in the cement paste (Fig. 5b).Fig. 6 shows the SEM observations of GGBFS60 mortarat 900 1C. At this temperature, although the space ratio incement paste increased, the matris phase was not damagedas much as GGBFS0. The bond between aggregate andcement paste is also damaged (Fig. 6b).

4. Conclusions

The effect of GGBFS incorporation on high-tempera-ture resistance seems to be dependent on amount ofGGBFS, temperature level, and the cooling methodaccording to test results. GGBFS incorporated mortarsshowed no compressive strength loss up to 600 1C in case ofair cooling. The relative residual flexural strength of air-cooled mortar showed a slightly decreasing trend above40% GGBFS replacement at 300 and 600 1C. For the caseof water cooling, the relative residual flexural strength ofmortars decreased with the increasing replacement ratio.However, at 900 1C increasing GGBFS replacement ratiosalways positively affect the high-temperature resistance forboth cooling methods. For 80% GGBFS replacement, theresidual compressive and flexural strength ratios at thistemperature level were superior for air cooling case as 77%and 39%, respectively. For non-GGBFS incorporated

ARTICLE IN PRESS

Fig. 6. SEM analysis of GGBFS60 SC mortar at 900 1C: (a) cement paste, (b) ITZ.

S. Aydın / Fire Safety Journal 43 (2008) 610–617616

mortar, these ratios were 32% and 20%, respectively. Thepositive contribution of GGBFS at 900 1C may be relatedto gehlenite formation.

The cooling method, and type of stress acting on thespecimen (compression, flexural, etc.) have remarkableeffect on high-temperature resistance of cement-basedmaterials. Water cooling results in higher strength lossescompared with air cooling especially at relatively lowtemperatures, and flexural strength loss is much higherthan compressive strength loss. In other words, cement-based materials, those subjected to bending are moresensitive to high temperatures than the members subjectedto compressive loading.

The developed pumice–GGBFS-cement mortar seems tobe a promising multipurpose insulation material forstructures that are exposed to high temperatures duringtheir service life.

Acknowledgments

This study is a part of the research project (project no.IC- TAG I679), which is financially supported by theScientific and Technological Research Council (TUBI-TAK) of Turkey. Author is gratefully acknowledged forthis support.

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