Construction and Building Materials 27 (2012) 398–403

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Construction and Building Materials

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Blends of limestone powder and fly-ash enhance the responseof self-compacting mortars

Syed Ali Rizwan a,⇑, Thomas A. Bier b

a Department of Structural Engineering, NUST Institute of Civil Engineering (NICE), National University of Sciences & Technology (NUST), Islamabad, Pakistanb Chair of Construction Materials Technology, IKGB, TU Freiberg, Germany

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

Article history:Received 26 October 2010Received in revised form 6 July 2011Accepted 18 July 2011Available online 31 August 2011

Keywords:Limestone powderFly ashBlended secondary raw materialsFlowStrengthMicrostructureVolume stabilityRelative water absorptionSelf-consolidating mortars

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.07.030

⇑ Corresponding author. Tel.: +92 3344255188; faxE-mail address: syedalirizwan@hotmail.com (S.A.

Because concrete and mortars both contain aggregate phase, it may be expected that the trend of resultsobtained from mortar formulations should be similar to those of concrete or vice versa. Many researchershave used either limestone powder (LSP) or fly-ash (FA) in self-compacting concrete systems. Howeverthe present study shows that using either of these secondary raw materials (SRMs) in self-compactingmortar systems does not produce an optimized response. However mixing LSP and FA in suitable propor-tions and then using this blended SRM enhances the response of self-compacting mortar systems as bothFA and LSP seem to complement the properties of each other especially the negative ones. The formula-tions contain two base SRMs (limestone powder and fly-ash) and in others these two basic SRMs havebeen replaced at 20% of their mass by some other SRMs or their combinations. Parameters like flow,strength, microstructure, relative water absorption and early volume stability of self-compacting mortarsystems have been studied. The results indicate that self-compacting mortars using blended secondaryraw material of limestone powder and fly-ash had an improved overall response than those using eitherof them.

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1. Introduction

Mineral admixtures, known as secondary raw materials (SRMs),and chemical admixtures are essential components of both self-consolidating concrete and self-consolidating mortar systemscharacterized by conflicting requirements of simultaneous highdeformation (low yield stress) and high segregation resistance (ade-quate viscosity). High deformation is achieved by using efficientsuper plasticizer (SP) and high segregation resistance is achievedby using either low water–powder ratio (w/p) or a moderate w/pwith viscosity enhancing agent (VEA). The term powder, is com-monly known to material engineers, and means all fine material ofaverage particle size (D50) less than 125 lm [1]. Material engineersand scientists try to use suitable SRMs imaginatively in their formu-lations so that properties like water demand of system, cement con-tent and shrinkage, early heat generation, improvements in flow andplacement and micro-structural refinements could be optimized toobtain enhanced strength and durability [2]. Limestone powder andfly ash have often been used successfully, independent of each other,as secondary raw materials in self-consolidated concrete by manyresearchers [3–15]. However the results of present study show thatindependent use of either limestone powder or fly ash in self-consol-

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idating mortar systems may not give optimized formulation re-sponse in both fresh and hardened states. For example, limestonepowder when used separately in self-consolidating mortars requireshigh super-plasticizer content and produces high early shrinkage.On the other hand fly ash when used independently in such systemsincreases the retardation; though it improves ease of placementsand reduces early heat generation and hence reduced shrinkage.Three series of self-compacting mortars were investigated. The firstseries of such systems used LSP and FA separately. In the second ser-ies, mutual blends of these two SRMs were obtained by replacing20% mass of either of SRM with the other. The third series of self-compacting mortar systems consisted of SRMs obtained with 20%mass replacements of both limestone powder and fly ash with eithersilica fume or amorphous rice husk ash in respective formulations(see Fig. 1). This replacement level was selected because a smalleramount of secondary raw material results in optimum efficiencyand gives a large increase in strength while the use of large amounthas a smaller effect [16]. Blended secondary raw materials used inthis work resulted in self-consolidating mortars with binary and ter-nary binder systems having complex hydration mechanisms whichare obviously not yet fully understood. It has already been shown bythe authors that particle size, shape, morphology and internal poros-ity of secondary raw materials all have a very significant effect on theoverall response of self-compacting systems [17,18]. Moreover, ithas also been indicated that using mutual blends of LSP and FA

Fig. 1. SP content required for the target flow of SCM systems using various SRMs.

Table 1Physical and chemical analysis of the powders (percentage).

Oxides FA LSP RHA SF CEM 1 42.5 R

SiO2 51.44 7.07 90.0+ 95 18.92P2O5 1.89 – – – –Fe2O3 5.55 0.88 0.32 0.05 2.27MgO 2.51 1.13 0.37 0.40 1.72CaO 4.03 48.57 0.60 0.25 63.18TiO2 0.99 0.10 – – –Al2O3 26.13 2.53 <0.01 0.20 5.09Na2O 1.23 0.47 0.14 0.10 1.48K2O 2.63 0.68 2.3 1.20 1.01LOI 2.71 38.32 4–6 – 1.35SO3 – 0.28 0.14 – 3.48Particle size D50 (lm) 26.59 7.176 6.8 8.66 18.5BET Area (m2/g) 1.65 4.99 28.92 20.45 0.81Density, (g/cc) 2.32 2.75 2.26 2.35 3.17

Note: – = not measured, FA = Fly-ash, LSP = Limestone powder, RHA = amorphousrice-husk-ash, SF = Silica fume, powder type, CEM 1 42.5 R = OPC.

Fig. 2. Flow times of SCM systems using FA, LSP and their 20% by massreplacements with other SRMs.

S.A. Rizwan, T.A. Bier / Construction and Building Materials 27 (2012) 398–403 399

improves the overall response of self-consolidating mortar systems.This response is even further improved when either limestone pow-der or fly-ash contain 20% mass of either silica fume or amorphousrice husk ash [19]. This work provides the data and assistance tomaterial engineers and scientists to try mutual blends of limestonepowder and fly ash and those with other SRMs in typical applicationsof self-consolidating mortar systems. Hydration reaction of self-consolidating concrete gets accelerated and modified in the pres-ence of limestone powder due to improved nucleation possibilitieswhile the presence of fly ash makes interference in cement hydra-tion and changes reaction kinetics in the sense that fly ashes tendto lengthen the dormant period [20].

2. Experimental – Materials

Limestone powder used in this study had 92.3% CaCO3 content. CEM I 42.5 R(ASTM Type 1 OPC) and naturally occurring local sand of 0–2 mm size with finenessmodulus (FM) of 2.39 was used. The specific surface of powders was measuredusing Brunauer–Emmet–Teller (BET) gas adsorption. A polycarboxylate ester(PCE) based liquid super plasticizer (SP) with 30% solids and a pH of 5.8 and a den-sity of 1.04–1.08 g/cm3 was used to produce flow target of 31 ± 1 cm measured byHagerman’s mini-slump cone of dimensions 6 � 7 � 10 cm3. Selected mix propor-tions of SCM systems were 1:1:2 by mass (cement:SRM:sand), water–cement ratio(w/c) of 0.40 and water powder ratio (w/p) of 0.2 were used. The physical and chem-ical powder properties are given in Table 1. The treatment about SRM particle char-acterization and mixing regime employed can be found in literatures [2,17].

3. Results

The results of these self-compacting mortar systems have beenreported below in terms of flow, strength, microstructure, relative

water absorption and early volume stability in the following lines.It will be shown that the use of suitable mutual blends of limestonepowder and fly ash improves the overall response of resulting self-consolidating mortar systems.

3.1. Flow of self-consolidating mortar systems

The flow target of various self-consolidating mortar systemsincorporating limestone powder, fly ash and their above describedblends was constant at 31 ± 1 cm and was achieved at differentsuper plasticizer contents for various types of SCM formulations.It was earlier proposed to measure T 25 cm spread time for self-consolidating mortars and pastes on the analogy of measuring T50 cm time for self-consolidating concrete using Abram’s cone[2] because of similar ratios (2.5) of specified spread diameters tothe base diameters of respective measuring cones. Therefore theinferences obtained from T 50 cm time of self-consolidating con-crete may be the same as those obtained from the proposed T25 cm time of self-consolidating mortars. Self-consolidating mor-tar systems are generally characterized by high flow greater than26 cm [21] measured by Hagerman’s mini-slump cone of6 � 7 � 10 cm3 dimensions and high segregation resistance. Theflow of self-consolidating mortar systems heavily depends uponparticle characteristics of the powders (binders in most cases) interms of size, shape, surface morphology and internal porosityand the detailed data is given in literatures [17,19]. T 25 cm timemay give useful information about the rheological propertiesincluding yield stress and viscosity of self-consolidating mortarsystems while elaborating the role of different secondary rawmaterials. Fig. 1 shows the super-plasticizer content required forthe target flow of self-consolidating mortar systems using variousSRMs while Fig. 2 shows the flow times of such systems.

3.2. Strength of self-consolidating mortar systems

4 � 4 � 16 cm3 specimens of self-consolidating mortar sys-tems containing FA, LSP, their mutual blends and with otherSRMs were cast, cured and tested in saturated surface dry (SSD)condition as per DIN 196-1. The curing regime consisted of stor-ing the specimens in 90% + relative humidity chamber during thefirst 24 h and thereafter water curing was observed till the testage. Flexural strength of a formulation at typical age is the aver-age of three 4 � 4 � 16 cm3 specimens while compressivestrength test is done on broken halves in flexure and strengthat typical age is average of six specimens. Fig. 3 shows the

Fig. 3. SCM strengths using FA, LSP and their other blended SRMs at 28 days.

Fig. 4. Relative water absorption of SCM systems using various SRMs at differentages.

Table 2Maximum MIP pore sizes of SCM [systems using different SRMs.

SCM system Max. pore size (nm) from partial MIP diagram at the age of(days)

1 7 28

0.8LSP + 0.2RHA 32.39 22.95 4.970.8LSP + 0.2SF 29.81 6.17 3.850.8LSP + 0.2FA 37.5 28.02 24.970.8FA + 0.2LSP 28.0 6.88 6.19LSP Not determined Not determined 25.37FA Not determined Not determined 19.31

400 S.A. Rizwan, T.A. Bier / Construction and Building Materials 27 (2012) 398–403

strength response of the self-consolidating mortar systems whichis clearly attributable to the different types of secondary rawmaterials used. Limestone powder is relatively inert and usuallydoes not take part in chemical reactions and therefore offers asimple strength quantification tool for self-consolidating mortarsystems by considering its formulations as base line in compari-son to those having pure and blended pozzolanic secondary rawmaterials [1,2,17,22].

3.3. Relative water absorption of self-consolidating mortar systems

The relative water absorption has been calculated in terms ofper cent mass difference of specimens in saturated surface dry(SSD) condition at any age with reference to the amount of waterpresent in the specimen at the age of 24 h (after initial curing). Itmay be pointed out here that the relative water absorption ofself-consolidating mortar systems may depend upon several fac-tors including the maximum pore size, the type and characteristicsof secondary raw materials and type of both internal and externalconnectivities of such systems. Fig. 4 shows the relative waterabsorption of self-consolidating mortar systems using differentsecondary raw materials.

3.4. Microstructure of self-consolidating mortar systems

In order to study the microstructure of self-consolidating mor-tar systems, the sample preparation was done in laboratory afterstopping hydration at different ages by heating the samples in oven

at 105 �C for 24 h. Mercury Intrusion Porosimetry (MIP) was donewith the help of Autoscan 33 Porosimeter using contact angle of140� for determining maximum pore sizes. The maximum poresizes of various formulations of SCM systems are given in Table 2.In general finer the microstructure of any formulation, greater wasthe strength of formulations of self-consolidating mortar systemscontaining various secondary raw materials. Higher strengths wereobtained for self-consolidating mortar systems using secondaryraw materials of higher pozzolanic activity. In an earlier study itwas shown that self-consolidating mortar systems using limestonepowder, fly ash, 80% fly ash and 20% rice-husk ash and 80% fly ashand 20% silica fume gave maximum MIP average pore sizes at28 days in the descending order respectively [2] because the de-gree of pozzolanic activity increased in ascending order. The pres-ent study also confirmed that blending LSP with other SRMsincreased the strength of SCM systems.

3.5. Early volume stability of self-consolidating mortar systems

It has already been shown by the researchers that both volu-metric and linear measurements of shrinkage yield almost thesame results [23]. Early volume changes during the first 24 h arevery important (mostly chemical shrinkage in this case) for self-compacting concrete systems because of higher paste content,low mixing water and use of mineral admixtures. In this study amodified version of German classical ‘‘Schwindrinne’’ meaningshrinkage channel apparatus measuring 4 � 6 � 25 cm3 was usedfor measuring the total linear early shrinkage of various self-con-solidating mortar systems using different secondary raw materialsat 20 ± 1 �C with relative humidity of 31 ± 5%. It is the amount oftotal shrinkage which is of concern to construction engineers andtherefore international codes of practice also specify the total per-missible shrinkage of concrete [17]. The measurement sensitivityof the apparatus was 1.2 lm/m. The linear early shrinkage mea-surements of SCM specimens using different SRMs were made inuncovered and covered conditions to simulate the possible com-monly used field conditions.

4. Discussion

In the following lines, the discussion on results on variousparameters studied in this research will be made sequentially.

4.1. Flow of SCM formulations

It has been suggested in literature that the two basic Binghams’rheological parameters (yield stress and viscosity) can be deter-mined from the simple mini-slump flow test of cement paste ifthe time of total spreads of various formulations was known[24]. It has been proven that SRM particle characterization is themain parameter affecting the flow characteristics of SCM systemsfor a given mix proportions and using different secondary rawmaterials [17]. Self-consolidating mortar systems using fly ashneeded least super plasticizer (SP) content for the flow target due

S.A. Rizwan, T.A. Bier / Construction and Building Materials 27 (2012) 398–403 401

to its spherical internally hollow particle shape and glassy surfacemorphology. Limestone powder based SCM systems required thehighest super plasticizer content for similar flow levels due to theirirregular, rough and patchy particle characteristics resulting in in-creased internal friction during flow.

Replacing 20% mass of limestone powder or fly ash either byrice-husk ash or silica fume increased the super plasticizer contentrequired to meet the target flow of resulting self-consolidatingmortar systems. It happens due to highly irregular, abrasive andinternally porous rice husk ash particles [2,17], which also ab-sorb/adsorb some water resulting in the availability of reducedeffective water in the mix and higher internal resistance to flow[17]. Silica fume also adsorbs significant water and gives high flowtimes for constant target spreads. Limestone powder particles areirregular and therefore require the highest super plasticizer con-tent to meet the flow target [17]. It was noted that blended second-ary raw material consisting of 80% limestone powder and 20% silicafume used in self-consolidating mortars gave higher T 25 cm timethan system using 80% limestone powder and 20% rice-husk ashblended secondary raw material. It is due to the fact that SF parti-cles absorb significant water and it was confirmed when very highmercury intrusion was observed during mercury intrusion porosi-metry (MIP) on mentioned secondary raw materials [17]. It meansthat numerous surface-pores at the external surface of silica fumeparticles are present [2,19] which adsorb significant water as wellas super plasticizer from the mix solution. Limestone powder hasirregular and broken surface which further increases internal resis-tance to flow. Irregular particles of rice husk ash taken as 20%replacement of limestone powder resulted in lesser T 25 cm timeof self-consolidating mortars but higher funnel time. T 25 cm coneflow time and V-funnel time should be seen separately as in thefirst case mostly the internal friction has to be over come duringunconfined flow of self-consolidating mortars (after lifting of cone)while in the funnel flow both the internal and external frictions(between self-consolidating mortar and gradually reducing funnelsection) are present which increases the overall friction. In case offunnel time, the secondary raw material particle shape thereforebecomes very important parameter for the flow. This is the reasonthat with the inclusion of 20% replacement of either LSP or fly ashby rice husk ash, the funnel times of corresponding self-consolidat-ing formulations are greatly increased. In summary the flow of self-consolidating mortar systems depends heavily on powder particlesize, shape, surface morphology and its internal porosity in addi-tion to other influencing factors like mixing regime, sequence ofadmixtures addition, and water/super plasticizer contents.

-3000.00

-2500.00

-2000.00

-1500.00

-1000.00

-500.00

0.00

500.00

Shri

nkag

e (M

icro

ns/m

)

Tim

LSP and Other 20%

LSP-Uncovered

0.8LSP+0.2RHA-Uncovered

0.8LSP+0.2SF-Uncovered

0.8LSP+0.2FA-Uncovered

0.00 8.00

Fig. 5a. SCM systems using LSP and its 20%

4.2. Comparing flexural and compressive strength of SCM formulations

Replacing 20% fly ash by mass with limestone powder reducesthe strength of corresponding self-consolidating mortar while the20% mass replacement of fly ash by pozzolanic secondary rawmaterials including silica fume and amorphous rice husk ash in-creases the strength of corresponding self-consolidating mortarsystems. It is due to pore refinement effect given in Table 2,[2,17]. The margin of strength increase depends upon the degreeof pozzolanic activity of secondary raw material used. Moreoverreplacing 20% limestone powder by mass with pozzolanic second-ary raw materials also increases the strength of corresponding self-consolidating mortars.

Comparing self-consolidating mortar systems having eitherlimestone powder or fly ash as secondary raw materials and their20% by mass replacements with rice-husk ash and silica fume, the28 days strength of fly ash based self-consolidating systems wasmore than those of limestone powder based systems because ofboth filler as well as pozzolanic action. Both rice-husk ash and sil-ica fume when incorporated in fly ash as its 20% by mass replace-ment, give almost comparable strengths with silica fume onegiving the highest strength. The strength of self-consolidating mor-tar systems depends on the maximum MIP pore sizes and on thetype and degree of pozzolanic activity of secondary raw materials.

4.3. Microstructure of SCM formulations

The microstructure of self-consolidating mortar systems wasstudied by using mercury intrusion Porosimetry (MIP). The maxi-mum pore sizes of various formulations are given in Table 2. Animproved and finer microstructure (small and discontinuouspores) results in higher strength and in enhanced durability [2].Fly ash and its blends produced higher strength self-consolidatingmortars and hence lower maximum pore sizes than correspondingones using limestone powder and its blends. It was due to higherpozzolanic activity and hence pore refinement effect of fly-ashbased systems. It should be kept in mind that fly ash based systemsusually show enhanced pozzolanic activity around and after28 days of age.

4.4. Early volume stability of SCM formulations

From Figs. 5a and 5b, it is obvious that only self-consolidatingmortar systems containing limestone powder gave highest shrink-age in both exposure conditions and with all cements as shown in

e (Hours)

REPLACEMENT SRMs

LSP-Covered

0.8LSP+0.2RHA-Covered

0.8LSP+0.2SF-Covered

0.8LSP+0.2FA-Covered

16.00 24.00

by mass substitute SRMs with CEM I.

-3000.00

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0.00

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icro

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Time (Hours)

FA and Other 20% REPLACEMENT SRMs

FA-Uncovered FA-Covered0.8FA+0.2RHA-uncovered 0.8FA+0.2RHA-Covered0.8FA+0.2 SF-Uncovered 0.8FA+0.2 SF-Covered0.8FA+0.2LSP-Uncovered 0.8FA+0.2LSP-Covered

0.00 8.00 16.00 24.00

Fig. 5b. SCM systems using FA and its 20% by mass substitute SRMs with CEM I.

402 S.A. Rizwan, T.A. Bier / Construction and Building Materials 27 (2012) 398–403

earlier work [19] also. Fly ash based self-consolidated mortar sys-tem gives reduced uncovered early shrinkage with delayed hard-ening (as seen by the delayed onset of shrinkage demonstratedby steep slope of the curve) possibly due to the presence of carbon[25] but shows small expansion (+126 lm) at the end of 24 h incovered condition. From these diagrams it is obvious that replacing20% by mass limestone powder by rice-husk ash, also reduces theshrinkage. However, fly ash and rice-husk ash in 80:20 ratio give agood secondary raw material blend for self-consolidating cementi-tious systems in terms of volume stability with retarded hardeningin covered conditions. It ends up in +31 lm expansion of the corre-sponding self-consolidating mortar at the end of 24 h. BlendedSRM consisting of both ashes (80% Fly ash and 20% rice husk ashdelays the hardening of self-consolidating mortar further due to in-creased carbon content in ashes. It is also obvious that fly ash andlimestone powder in 80:20 ratio can also be a good choice of ablended SRM to be used in self-consolidating mortars in coveredconditions. It ends up in +152 lm expansion at the end of 24 hmeasurement. Silica fume and rice-husk ash as limestone powderreplacements are good blended secondary raw materials and givereasonably closer values of 24 h linear shrinkage of correspondingself-consolidating mortar systems in both the exposure conditionsinvestigated (Fig. 5a). The difference is that rice-husk ash is muchmore reactive than fly ash especially in the short term. Replacinglimestone powder with rice husk ash increases hardening time ofself-consolidating mortars and the rate of hardening can be seenin the shrinkage curves by an experienced material scientist. Highearly shrinkage of limestone powder is attributed to faster hydra-tion due to nucleation, internal porosity, speedy uptake and con-sumption of water [20]. In Fig. 5a blended SRM consisting of 80%limestone powder and 20% fly ash in self-consolidating mortar sys-tem gives lowest shrinkage in both exposure conditions. In Fig. 5b,blended SRM consisting of 80% fly ash and 20% limestone powderalso seems to be good again from early volume stability of self-consolidating mortars. This blended SRM also shows otherimprovements in other properties of resulting self-consolidatingmortar systems.

4.5. Relative water absorption of SCM formulations

The blended SRM with 20% silica fume in 80% fly ash reducesthe relative water absorption of resulting self-consolidating mortarsystem while systems with 20% replaced fly ash with rice-husk ash

increases it but it does not necessarily mean a bigger average porediameter or lower strength.

It appears that the relative water absorption of a cement basedsystem is not simply a function of maximum pore size but it alsodepends upon the type of porosity and its connectivity to the sur-face of the sample. Secondary raw materials with internal porositywhen used in self-consolidating mortar systems are likely to resultin greater relative water absorption but not necessarily lowerstrengths. It may be possible that such systems have some kindof surface connectivity of the sample but within the sample itself,such connectivity may be discontinuous. This is the reason thatself-consolidating mortar system using blended SRM of 80% flyash and 20% rice husk ash gave higher water absorption than thatusing 80% fly ash and 20% silica fume.

4.6. Concluding remarks

Many researchers have used either limestone powder or fly-ashas a secondary raw material in self-consolidating concrete systemswithout exploring some other SRMs or their mutual blends. Theidea was to test different SRMs and their suitable blends in self-consolidating mortars in order to obtain an enhanced response ofsuch systems. There is every likelihood that the same SRMs wouldalso produce a similar trend of response in self-consolidating con-crete. It has been known that for a constant total SRM content,increasing limestone powder content in a blended SRM reducesthe hardening times and strength of such systems. For fly-ashbased self-consolidating systems the reverse is true. Howeverwhen a suitably blended SRM containing both fly-ash and lime-stone powder is used in self-consolidating mortars, the overall re-sponse of such systems is definitely enhanced than thoseemploying either of the two SRMs.

Acknowledgements

The authors are thankful to Mr. Karl Kiser, Plant Manager, Agri-lectric International Technologies, Lake Charles, LA, USA for provid-ing the amorphous rice-husk ash used in this investigation. We aregrateful to Mr. Javed Bashir Malik, Associate/Structural group lea-der, Carter & Burgess, Houston Texas, USA for bearing the expensesof the ash transportation to Germany.

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