Materials and Design 35 (2012) 374–383

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Materials and Design

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Mechanical and microstructural properties of heat cured alkali-activatedslag mortars

Serdar Aydın ⇑, Bülent BaradanDepartment of Civil Engineering, Dokuz Eylül University, Buca 35160, Izmir, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 August 2011Accepted 1 October 2011Available online 12 October 2011

Keywords:A. ConcreteC. Heat treatmentsE. Mechanical propertiesF. Microstructure

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.10.005

⇑ Corresponding author. Tel.: +90 232 412 7044; faE-mail address: serdar.aydin@deu.edu.tr (S. Aydın

The effects of steam and autoclave curing on the mechanical properties and microstructure of alkali acti-vated slag mortars (AAS) were investigated within the scope of this study. Slag was activated by the mix-tures of sodium hydroxide and sodium silicate, in different Na2O and Ms (SiO2/Na2O) ratios. Test resultsshowed that high strength mortars can be produced with very low alkali content under autoclave curingas a result of better pore size distribution, higher rate of hydrated parts of slag grains which leads to abetter microstructure formation. A compressive strength value of 70 MPa has been reached by incorpo-ration of 2% Na2O under autoclave curing. Nevertheless, steam curing presents similar mechanical perfor-mance with autoclave curing for AAS mortar activated by the solutions with high Ms values. Both curingmethods were significantly effective in terms of reducing drying shrinkage of AAS mortars.

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1. Introduction cement concretes [18,19]. When the disadvantages of alkali acti-

Alkali-activated cements are new binders produced by the acti-vation of different industrial by-products. Fly ash, metakaolin, blastfurnace slag, kaolinitic clays, rice husk and red mud were used forraw materials [1–4]. Some natural pozzolans have also been usedfor this purpose recently [5]. Various alkali activators also play amajor role in producing this binder [2–4]. Researches on this typeof binders have been increased substantially within the last dec-ades due to environmental, economical and technical advantagesof alkali-activated cements.

Alkali-activated slag cements (AAS) seem to be one of the mostattractive binders due to some technological advantages compareto ordinary Portland cements such as, lower hydration heat [4],the development of earlier and higher mechanical properties [6,7],better resistance to chemical attack [8,9] and freeze-thaw cycles[10], and stronger aggregate-matrix interface formation [11]. Theyalso present some problems such as rapid setting periods [12], highshrinkage values [13], and higher formation of salt efflorescences[7]. There are contradictive results about carbonation [14,15] andalkali-aggregate reaction resistances [16,17].

In precast concrete industry, atmospheric and high-pressuresteam curing methods are employed to obtain a sufficiently highearly strength so that the concrete products can be handled as soonas possible after casting. Besides, high pressure steam curing (auto-claving) provides less creep and shrinkage related problems, elim-ination of efflorescence and better sulfate resistance for Portland

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vated binders such as, efflorescence and high drying shrinkagetaken into consideration, heat treatment seems to be a better cur-ing method in solution of these problems.

Some researchers from Russia and China have suggested that,maximum compressive strength values were obtained in AAS con-cretes activated with water glass solution (liquid sodium silicate)[20,21]. Later, Bakharev et al. [6] reported that sodium silicate pro-vides the best activation for slag pastes and mortars cured at 60 �C.Sodium silicate solution of Ms = 0.75 and 4% Na was recommendedfor use in AAS concrete based on workability and compressivestrength properties. Nevertheless, slags have a variable compositiondepending on the raw materials and the industrial process; hence,each slag responses differently to activation [22]. It is necessary todetermine the most suitable Ms value and Na concentration valuesfor each case. This research has been implemented by this method-ology with a poor quality slag (with a low hydration modulus value).Thus, determination of the optimum Ms value and Na concentrationof activator solutions for the utilization of this type of slags carriesgreater importance. Another originality of this study is the determi-nation of the optimum values of activator solutions for autoclavingand atmospheric steam curing cases. This research has proved thepossibility of high strength AAS mortars with low drying shrinkageproperties by using poor quality slag with very low alkali contents.

2. Materials and experimentation

Ground granulated blast furnace slag (GGBFS) has been procuredfrom Eregli steel plant, Turkey. Specific gravity and specific surface

S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383 375

(Blaine) values of GGBFS were 2.88 and 410 m2/kg, respectively.GGBFS contained, 90% particles finer than 45 lm. It was neutralwith the basicity coefficient [Kb = (CaO + MgO)/(SiO2 + Al2O3)] of0.81. The hydration modulus [HM = (CaO + MgO + Al2O3)/SiO2] ofslag was 1.33. Portland cement (CEM I 42.5R) with Blaine finenessof 369 m2/kg has been used as the reference binder. The chemicalcomposition of Portland cement (PC) and GGBFS have been pre-sented in Table 1. As shown in Table 1, SiO2 and Al2O3 contents ofGGBFS are significantly higher than PC while CaO content of PC isconsiderably higher than GGBFS.

Alkali activated slag (AAS) mortars were produced by theactivation of GGBFS with technical grade sodium hydroxide andwater glass (liquid sodium silicate). Sodium silicate had a chemicalcomposition of SiO2 = 27% and Na2O = 8% and the silicate modulus(Ms) was 3.38. Sodium hydroxide and sodium silicate were mixedin different proportions, providing the Ms in the range of 0.4–1.6.Furthermore, solutions with Ms = 0 were produced by using NaOH.Four levels of Na2O concentration in the mixtures with slag, 2%, 4%,6%, and 8% were investigated. Activator solutions were prepared1 day before the casting of mortars.

Twenty alkali-activated mortar mixtures and a control PC mor-tar mixture were prepared. Crushed limestone sand (0–4 mm) wasused as aggregate in all mortar specimens. Aggregate to binder(cement or GGBFS) ratio of 2.75 and water to binder ratio of 0.44were kept constant for all mixtures. All test batches were mixedby using an electrically driven mechanical mixer conforming tothe requirements of ASTM C305 [23]. Initially, binder and aggre-gate were mixed in a dry state for a minute and then the activatorsolution was gradually added while mixing continued for about 3minutes. Fresh mixtures were cast into steel molds. The specimenswere kept in a humidity cabinet (20 �C temperature and 90% rela-tive humidity) before heat-treatment. One group of specimens waskept in the humidity cabinet for 5 h before steam curing at 100 �Cfor 8 h. After steam curing, the specimens were demoulded. Othergroup of the specimens was kept in the humidity cabinet for 24 h.After demoulding, the specimens were autoclaved at 210 �C andunder 2.0 MPa pressure for 8 h. The heat-treatment cycles areshown in Fig. 1.

Three prismatic specimens (40 � 40 � 160 mm) from each mix-ture were subjected to flexural strength test according to ASTMC348 [24]. The specimens were loaded from their mid-span andthe clear distance between simple supports was 120 mm. The com-pressive strength tests were performed following the flexural testson two broken pieces left from flexural test according to ASTMC349 [25]. Shrinkage values were measured on mortar bars(25 � 25 � 285 mm) according to ASTM C596 [26]. The first read-ing of length was taken at the end of the heat treatment. Thenthe prisms were kept in laboratory conditions (20 �C temperatureand about 55% relative humidity). The length change of specimenswas recorded periodically up to 6 months.

3. Results

The effectiveness of Ms and Na2O values of the solutions on theactivation of GGBFS in respect to the mechanical properties, dryingshrinkage, and microstructures of heat cured AAS mortars are dis-cussed in the following sections.

Table 1Chemical compositions (%) of GGBFS and Portland cement.

SiO2 Fe2O3 Al2O3 CaO MgO Na2O K2O SO3 LOI

GGBFS 40.20 1.68 11.66 35.90 5.88 0.30 1.47 0.90 0.88PC 19.10 3.96 4.40 61.85 2.05 0.27 0.70 3.72 1.82

3.1. Mechanical properties

Compressive and flexural strength of steam-cured AAS mortarsare given in Figs. 2 and 3, respectively. The strengths of PC mortarsare marked by dash lines in all figures. It should be noted that 6%and 8% Na2O incorporated Ms = 0.4 and 0.8 mixtures were foundinsufficient due to their workability and setting properties [27].Thus, the mechanical properties of these mixtures in both heat cur-ing methods were found lower than expected.

Compressive strength values in the range of 15–90 MPa wereachieved by steam curing (Fig. 2), although slag used in this studyhas a low Hydration modulus (HM). HM of slag is a quality criterionfor blast furnace slag and should exceed 1.4 according to Japanesestandard to ensure sound hydration property [4]. Compressivestrength of AAS mortars increased significantly with the increasingvalues of Ms and Na2O. Increase in strength is more pronounced athigh Na2O contents and high Ms. For example, increase in Ms from0 to 1.6 results in a 193% increase in strength at 2% Na2O contentwhile this increase reaches to 340% at 8% Na2O content. Similarly,for Ms = 0 case, increase of Na2O content from 2% to 8% results in a40% increase in strength. However, this ratio is 111% for Ms = 1.6.Another result derived from these values is that, the effect of Ms

value on the compressive strength of AAS seems to be much moresignificant than Na2O content.

As can be seen from Fig. 3, flexural strength of mortars activatedby water glass generally showed a maxima at 4% Na2O while theflexural strength of NaOH activated mortars (Ms = 0) increased con-tinuously up to 8% Na2O content. Nevertheless, increase in flexuralstrength values of water glass activated GGBFS mortars with theincrement of Ms and Na2O is not impressive as compressivestrength case.

As seen in Figs. 2 and 3, compressive and flexural strength val-ues of steam-cured AAS mortars are higher than PC mortars whenthe Na2O and Ms values are higher than 4% and 0.4, respectively.

The variation of compressive and flexural strength values ofautoclave cured AAS mortars with respect to their Na2O contentare presented in Figs. 4 and 5, respectively. As shown in Figs. 4and 5, the strength of autoclaved PC mortar is significantly lowerthan the steam cured one. Under the conditions of high temperatureand pressure, the chemistry of hydration has been substantiallyaltered. C–S–H forms, however it is converted to a crystallinestructure a–calcium silicate hydrate (a–C2SH) which cause anincrease in porosity and reduction in strength [28–31]. This resultindicates the need of extra silica source, such as silica fume, flyash, etc., in order to obtain high strength values with PC.

2% Na2O incorporated AAS mortars at high Ms values crackedseverely during autoclaving; hence the mechanical properties ofthese mortars were lower than expected. For Ms = 0 and 0.4 series,highest compressive strength values were obtained at 2% Na2O.Compressive strength of AAS mortars increased with the increaseof Ms. However, for autoclave curing increase in compressivestrength with Na2O content and Ms value is lower than the steamcuring.

The effectiveness of autoclave curing compared to steam curingin respect to compressive strength and flexural strength are givenin Figs. 6 and 7, respectively. As shown in these Figures, at low Ms

ratios (Ms = 0.4 and especially Ms = 0) autoclave curing is moreeffective than the steam curing. However, for higher Ms values,approximately similar strength values were obtained for both cur-ing methods. The effectiveness of autoclaving is more significant incase of compressive strength compared to flexural strength.

3.2. Drying shrinkage

Drying shrinkage measurements on the specimens under stan-dard curing conditions (without heat treatment) showed that AAS

Fig. 1. Heat treatment cycles.

0

10

20

30

40

50

60

70

80

90

100

8642

Com

pres

sive

str

engt

h, M

Pa

Ms=0 Ms=0.4 Ms=0.8 Ms=1.2 Ms=1.6 PC

Na2O,%

Fig. 2. Compressive strength of steam cured AAS mortars and PC mortar.

0

2

4

6

8

10

12

14

8642

Flex

ural

str

engt

h, M

Pa

Ms=0 Ms=0.4 Ms=0.8 Ms=1.2 Ms=1.6 PC

Na 2O,%

Fig. 3. Flexural strength of steam cured AAS mortars and PC mortar.

376 S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383

0

10

20

30

40

50

60

70

80

90

8642

Na 2O,%

Com

pres

sive

str

engt

h, M

Pa

Ms=0 Ms=0.4 Ms=0.8 Ms=1.2 Ms=1.6 PC

Fig. 4. Compressive strength of autoclave cured AAS mortars and PC mortar.

0

2

4

6

8

10

12

14

16

8642

Na2O,%

Flex

ural

str

engt

h, M

Pa

Ms=0 Ms=0.4 Ms=0.8 Ms=1.2 Ms=1.6 PC

Fig. 5. Flexural strength of autoclave cured AAS mortars and PC mortar.

Fig. 6. The effectiveness of autoclave curing compared to steam curing forcompressive strength.

Fig. 7. The effectiveness of autoclave curing compared to steam curing for flexuralstrength.

S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383 377

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120 140 160 180

Time, days

Dry

ing

shri

nkag

e (x

10 -6

)

PC-Standard PC-Steam PC-Autoclave

AAS-Standard AAS-Steam AAS-Autoclave

Fig. 8. The effect of heat treatment on the drying shrinkage of AAS mortar (6% Na2O and Ms = 1.2) and PC mortar.

Fig. 9. SEM images of 2% Na2O (Ms = 0) mortars, (a) steam cured and (b) autoclave cured (left sides SE, right sides BSE images).

378 S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383

mortar with Ms = 0 has a similar drying shrinkage behavior with PCmortars. However, higher Ms values resulted in significantly higherdrying shrinkage values compared to PC mortars [27]. Thus, exper-iments aimed at determining the effect of heat treatment on dryingshrinkage were realized on AAS mortars with Ms = 1.2.

Drying shrinkages of 6% Na2O incorporated AAS mortars withMs = 1.2 and PC mortars versus time under different curing

methods are presented in Fig. 8 up to 6 months. Drying shrinkagecurves for standard water cured mortars are also given in Fig. 8 forcomparison. As shown in Fig. 8, drying shrinkage values of AASmortar are higher than PC mortar at all curing conditions. Never-theless, drying shrinkage values of both mixtures reduced withthe heat curing. However, autoclave curing is more effective indecreasing drying shrinkage compared to steam curing.

Fig. 10. SEM images of steam cured 6% Na2O incorporated AAS mortars, (a) Ms = 0, (b) Ms = 0.4 and (c) Ms = 1.2 (left sides SE, right sides BSE images).

S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383 379

As seen in Fig. 8, drying shrinkage values of AAS mortars arenearly 8 and 4.5 times of PC mortar under standard and steam cur-ing, respectively. However, in the case of autoclaving, drying shrink-age levels of PC and AAS mortars are approximately equal to eachother. Drying shrinkage of PC mortar reduced about 34% by steamcuring and 80% by autoclave curing. These values are 63% and 96%for 6% Na2O incorporated Ms = 1.2 AAS mortar. In other words, heatcuring is more effective in decreasing of drying shrinkage of AASmortars compared to PC mortars.

3.3. Microstructure investigations

Microstructure investigations on mortars were carried out bySEM investigations and mercury intrusion porosimetry (MIP).These investigations were realized on 6% Na2O incorporatedmortar samples with Ms values of 0, 0.4 and 1.2. Also, SEM investi-

gations were carried out on 2% Na2O incorporated Ms = 0 mortardue to its superior mechanical performance.

SEM investigations were applied on fractured surfaces by sec-ondary mode (SE) and on polished sections by backscattered mode(BSE). The general microstructural features of AAS matrix weredetermined by using backscattered electron (BSE) imaging. In SEM(BSE) images, light gray areas show unhydrated slag grains, darkgray areas represent limestone aggregates, and black areas indicatepores and cracks. SEM (BSE) images show existence of considerableamount of unhydrated slag particles. Fig. 9 presents SEM (SE andBSE) images of the steam and autoclave cured Ms = 0 mortars with2% Na2O content. As shown in Fig. 9, autoclave cured AAS mortarhas a well-packed structure, while steam cured one has a porousstructure. This might be the possible reason of higher mechanicalproperties of autoclave cured Ms = 0 mortars compared to steamcured ones.

Unhydrated GGBFS particle

Partially hydrated GGBFS particle

Fully hydrated GGBFS particles

microcrackAggregate

Unhydrated part of GGBFS particle

Hydrated part of GGBFS particle

Fig. 11. SEM (BSE) images of steam cured 6% Na2O (Ms = 0.4) mortar.

Fig. 12. SEM images of autoclave cured 6% Na2O incorporated AAS mortars, (a) Ms = 0, (b) Ms = 0.4 and (c) Ms = 1.2 (left sides SE, right sides BSE images).

380 S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383

Fig. 13. SEM (SE) image of formations in the spherical pores for autoclave curedmortars.

S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383 381

SEM (SE and BSE) images of steam cured 6% Na2O incorporatedAAS mortars are given in Fig. 10. SEM (SE) images display a porousstructure formation for Ms = 0 mortar. However, this porous struc-ture converts to a massive and well-packed structure with theincreasing values of Ms. SEM (BSE) images show that the crackintensity in the matrix phase increases with the higher Ms valuesdue to the tensions created by the shrinkage. This behavior wasalso reported by Puertas et al. [32]. At relatively high magnificationrates (2000�), in contrast to Ms = 0 mortar, the rings of reactionproducts around the slag grains were detected for Ms = 0.4 and1.2 mortars (Fig. 11). As shown in Fig. 11, some of the slag grainswere fully hydrated, and the others were partially hydrated. Themain reaction product for all series is calcium silicate hydrate(decrease of Ca/Si ratio from 2.5 to 1.2 by increasing Ms). The bind-ing property of C–S–H structure improves with a decrease in theCa/Si ratio [33]. EDS analyses revealed that C–S–H structure con-sists of Na element between 6.4% and 3.2%.

SEM (SE and BSE) images of autoclave cured 6% Na2O incorpo-rated mortars are given in Fig. 12. SEM (SE) images indicate awell-packed structure in case of autoclave curing compared tosteam curing, especially for Ms = 0 and Ms = 0.4 series. SEM (BSE)images showed that hydrated parts of the slag grains substantiallyincreased in case of autoclave curing, especially for Ms = 0. The ringsof reaction products around the slag grains observed in all mixtures(Fig. 12), at magnification rate of 500�. The main reaction product ofautoclaved mortars is also calcium silicate hydrate with a lowerCa/Si ratio compared to steam curing (decrease of Ca/Si ratio from1.7 to 1.0 by increasing Ms). The EDS analyses also revealed theexistence of Na element in C–S–H structure between 6.3% and 2.8%.

Matrix

(a)

Aggregate

Fig. 14. SEM (SE) images of transition zone of 6% Na2O incorporated

SEM (SE) microphotographs of all autoclave cured AAS showedthat the spherical pores are filled with needle-like formations. Anexample to these formations is presented in Fig. 13 for 6% Na2O incor-porated Ms = 1.2 mortar. Energy dispersive spectroscopy (EDS) analy-ses showed that Ca/Si, S/Ca and Al/Ca ratios of this type of formationsare between 1.08–1.46, 0.009–0.018 and 0.14–0.20, respectively.These crystals may be classified as Xonotlite. Similar fibrous forma-tions were also detected in air voids by Shi et al. [34] for autoclavedalkali-activated blast furnace slag pastes.

SEM (SE) investigations on the aggregate-matrix transition zoneshowed the existence of micro-cracks. Their intensity increaseswith the increasing Ms values in case of steam curing. However,similar crack formations in aggregate-matrix transition zone werenot observed in autoclave cured samples. The transition zoneimages of steam and autoclave cured mortars are presented inFig. 14, in case of 6% Na2O incorporated Ms = 1.2 mortars.

The pore size distributions of 6% Na2O incorporated AAS mor-tars according to curing conditions are given in Figs. 15 and 16for Ms = 0 and Ms = 1.2, respectively. As shown in Figs. 15 and 16,Ms = 0 mortar has a coarser pore size distribution, and containsmore macropores and mesopores compared to Ms = 1.2 mortar.For AAS mortars with Ms = 0, steam curing caused to coarser poresize distribution above 25 nm, while autoclaving resulted in a finerpore size distribution. It is well known that pores that have diam-eter higher than 100 nm is responsible for the lower compressivestrength of the binder [35]. This explains the higher compressivestrength for autoclaving compared to the steam curing. However,pore size distributions of Ms = 1.2 AAS mortars did not change sig-nificantly under all curing conditions. These mortars have also sim-ilar strength values.

4. Discussions

The main reaction product of steam and autoclave cured AAS isNa-substituted C–S–H with low Ca/Si ratios. Malolepszy [36] pos-tulated the formation of a solid solution of Na2O–CaO–SiO2–H2O(NCSH), since Na+ ions in alkali-activated cement have a very lowsolubility in water. Ca/Si ratios of C–S–H in autoclaved AAS sam-ples are lower than the steam cured ones. This ratio also decreaseswith the increasing Ms values. Lower Ca/Si ratios of C–S–H contrib-ute to the higher strength of autoclaved samples as well as mortarsthat have high Ms values.

Significantly higher compressive strength values were recordedfor the autoclave cured AAS mortars compared to the steam curedones at low Ms ratios up to 0.4. The possible reasons of this mightbe due to; the formation of a finer pore size distribution, lowerCa/Si ratio of C–S–H, denser and compact structure of matrix,

Matrix

(b)

Aggregate

Ms = 1.2 AAS mortar (a) steam cured and (b) autoclave cured.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

110100100010000100000

Pore diameter (nm)

Cum

ulat

ive

volu

me

intr

uded

(cm

3 /g)

AC

SC

STDC

Mesopores Macropores

Fig. 15. Pore size distribution of 6% Na2O incorporated Ms = 0 AAS mortars under different curing conditions (STDC: Standard curing, SC: Steam curing, AC: Autoclave curing).

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

110100100010000100000

Pore diameter (nm)

Cum

ulat

ive

volu

me

intr

uded

(cm

3/g

)

AC

SC

STDC

Mesopores Macropores

Fig. 16. Pore size distribution of 6% Na2O incorporated Ms = 1.2 AAS mortars under different curing conditions (STDC: Standard curing, SC: Steam curing, AC: Autoclavecuring).

382 S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383

increase of hydrated parts of slag grains, and stronger aggregate-matrix interface.

Higher compressive strengths were obtained by increasing Ms,especially in the case of steam curing. This might be due to; thetransformation of porous and poorly packed structure to a denserand compact structure by increasing of Ms. Also, Ca/Si ratio ofC–S–H decreased with the higher values of Ms, and the porositydecreased with the increasing Ms values. These alterations leadto formation of a more durable matrix.

The mechanical properties of NaOH activated slag binders werereported significantly lower than the sodium silicate activated slagbinders by various researchers [6,7,37]. Nevertheless, test resultsshowed that compressive strength about 70 MPa can be obtainedby NaOH activator solution (Ms = 0) in case of autoclave curing.Besides, Na2O concentration of this mixture was also considerablylow (2%). Moreover, this mixture did not contain any extra silicasource which is necessary to obtain high strength for PC basedbinders in case of autoclaving. This means that, SiO2 in GGBFS takesan active part in autoclave reactions for AAS mortars, and thus theneed of extra silica source disposes.

In case of high Ms ratios (P0.8) and low Na2O concentration of 2%,specimens cracked considerably during autoclaving due to insuffi-cient strength of the specimens before the process. A 4% Na contentby weight of slag was suggested as the lowest level that is necessaryfor the activation of slag to provide sufficient activation for harden-ing during the first 24 h of hydration [6]. Fernandez-Jimenez et al. [7]recommended minimum activator concentration as 3% of Na2O byslag weight due to the retardation of the activation process.However, when the pre-treatment period of autoclaving prolongedto 36 h, a compressive strength value of 76.4 MPa was obtained for2% Na2O incorporated Ms = 1.2 mortar [27].

It is well known that, flexural strength is more sensitive tocracks than the compressive strength. Flexural strength of AASmortars did not improve with the increasing values of Ms as wellas compressive strength due to increasing of microcracks in thematrix phase and matrix-aggregate interface with the increasingvalues of Ms.

Alkali-activated slag concrete, especially activated by sodiumsilicate, exhibits higher drying shrinkage values than the ordinaryPortland cement concrete under standard water curing conditions

S. Aydın, B. Baradan / Materials and Design 35 (2012) 374–383 383

[6,13,37–39]. However, heat treatment is very beneficial indecreasing the drying shrinkage. Autoclaving seems to be a moreeffective method than steam curing in this manner. Reducedshrinkage by heat treatment may be attributed to the lower watercontent of C–S–H formed during heat treatment [40].

5. Conclusions

– High performance AAS binders can be produced by NaOH acti-vation under autoclave curing. Compressive strength valuesabout 70 MPa can be achieved using only 2% Na2O (by weightof slag) without any extra silica source other than slag.

– In respect to the mechanical properties, autoclaving is a moreappropriate curing method for the activator solution that haslow Na2O concentrations and low Ms ratios. Microstructureinvestigations showed that the higher mechanical propertiesof Ms = 0 mixture for autoclaving compared to the steam curingmay be attributed to; the higher ratio of hydrated parts of theslag grains, reduced Ca/Si ratio of C–S–H, reduced pore size dis-tribution of the matrix, and the formation of a stronger aggre-gate-matrix interface.

– Steam curing can be preferred for the activators that have highMs values due to the some advantages of this method such as,lower energy consumption, application easiness, etc.

– High drying shrinkage of AAS mortars activated by sodium sili-cate can be significantly reduced by heat treatment especiallywith autoclaving.

– As a general conclusion, high performance AAS mortars may bea promising alternative binding material for the precast con-crete industry compared to conventional cement based binders,where GGBFS sources are available.

Acknowledgements

This study is a part of a project supported by the ScientificResearch Council of Dokuz Eylul University (2009.KB.FEN009). Theauthors gratefully acknowledge for their financial support. In addi-tion, the authors thank to KARÇ_IMSA for supplying the materials.

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