Introducing an effective curing method for mortar ... · Introducing an effective curing method for mortar containing high volume cementitious materials ... blast furnace slag (GGBFS)

Construction and Building Materials 107 (2016) 365–377

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Introducing an effective curing method for mortar containing highvolume cementitious materials

http://dx.doi.org/10.1016/j.conbuildmat.2015.12.1000950-0618/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail addresses: [email protected] (E. Aprianti), [email protected],

[email protected] (P. Shafigh).

Evi Aprianti, Payam Shafigh ⇑, Rodiah Zawawi, Zahiruddin Fitri Abu HassanDepartment of Building Surveying, Faculty of Built Environment, University of Malaya, 50603 Kuala Lumpur, Malaysia

h i g h l i g h t s

� The characteristics of wastes that are used as cementitious material were investigated.� The combination of OPC–GGBFS is better than OPC–fly ash in all ages.� The PRF mix is the best combination for ternary blended cement.� HAC condition has good potential to be used as an effective curing method for mortar.� Local waste like POFA and RHA have good potential to be used as alternative cementitious material in cement based materials.

a r t i c l e i n f o

Article history:Received 23 June 2015Received in revised form 16 November 2015Accepted 15 December 2015Available online 23 January 2016

Keywords:CementMortarCuringSustainabilityWaste materialPozzolan

a b s t r a c t

This study reported an on-going research project of producing sustainable construction material for pre-cast industry. This paper studies the physical and chemical properties of materials, compressive strength,flexural strength, water absorption and porosity of multi blended cements under different curing meth-ods. Fly ash (FA), ground granulated blast furnace slag (GGBFS) and rice husk ash (RHA) were used toreplace with 50% ordinary Portland cement by mass. Specimens were cured in water (WC), air underroom temperature (AC), the combination of hot-water at 60 �C for 24 h followed by curing in water(HWC), and air (HAC). The results showed that HAC could be an effective curing method with higher com-pressive and flexural strengths, lower water absorption and porosity for blended cement mortars.Mortars containing GGBFS in binder had higher enhancement on compressive strength under early hotwater curing. While, at 24 h hot water curing mortar containing OPC–RHA–FA binder showed better qual-ity in properties compared to the other binders.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, the cement industry is direct to be environmentally-friendly. It is due to a magnitude number of mortar and concreteused in construction all over the world. In 2013 (Fig. 1), 4 bil-lion tons of cement produced, as estimated concrete productionexceed 4000 million tons annually [1]. A growing number of con-crete productions per year will lead to increase manufacturedcement significantly. Since, one of major contributor to CO2 emis-sion is cement production, it affected to climate change and globalwarming. This environmental problem will most likely beincreased due to exponential demand of Portland cement. By2050, demand is expected to rise by 200% from 2010 levels, reach-ing 6000 million tons/year [39]. To eliminate the effect of OPC

production on climate changes, the use of Portland cement andnon-renewable materials should be reduced. In recent years,blended cement with pozzolanic or supplementary cementitiousmaterials is widely used in cement and concrete construction byreplacing part of cement [2,3,11]. The main reasons for using thiskind of alternative materials are environmental, economic, or tech-nical benefits. Mineral admixtures such as fly ash (FA), rice huskash (RHA) and silica fume (SF) are silica-based pozzolanic materialsand renewable so they can partially replace by Portland cement[4]. The kind of alternative material that is used often dependson the availability and on the field of application [9]. Howeverthe common alternative materials used include GGBFS and FA.The utilization of mineral admixtures improved the compressivestrength, pore structure, and permeability of the mortars and con-cretes with time [4,5]. This is because the total porosity decreasewith increasing hydration time [4–7].

CHINA 58.6%

EUROPE 0.2%

CEMBUREAU 5.8%

CIS 2.6%

OCEANIA 0.3%

AMERICA (excl. USA)

4.9%

USA 1.9%

AFRICA 4.8%

ASIA (excl. CHINA, JAPAN,

INDIA) 12.3%

INDIA 7.1%

JAPAN 1.5%

WORLD CEMENT PRODUCTION 2013, BY REGION AND MAIN COUNTRIES

Fig. 1. The cement production around the world in 2013 [1].

366 E. Aprianti et al. / Construction and Building Materials 107 (2016) 365–377

There are many reports [2,4,7,11,12,17] that investigated theuse of waste and by-product materials as pozzolan in cementreplacement. The use of pozzolan as supplementary cementitiousmaterials has been found to provide a visible enhancement onthe mechanical properties of mortar. Furthermore, the mitigationdamage of mortar became particular concern for durability ofcement-based materials. Aldea et al. [2] mentioned two technolog-ical developments which can improve the ability of the material tomaintain ecological processes in the future. They are the incorpo-ration of several artificial waste materials into the concrete mortarand the use of a superplasticizer (SP) into mix design. Bagel [4] andBoubitsas [7] supported the previous statement by conducting astudy about the effect of binary and ternary blended cement onmortar. For instance, the high level portions of slag and silica fumeused in the binding system cause the mortars reached relativelysatisfactory level of compressive strength and contributed to thesignificantly denser pore structure [4]. In 2001, Boubitsas [7] eval-uated the effect of binary blended cement mortar containing 45%(by mass) GGBFS and 55% OPC with presence of 1% superplasti-cizer. The results obtained possess the highest improvement ofmechanical properties, hydration kinetics and microstructure ofhardened mortar. Several reports also explained the technicaldetails for low and moderate level replacement of mortar [8,11–13]. The percentage of low and moderate for cement replacementlevels are 5–40% by mass, respectively [11]. In addition, Agarwal[12] has reported that 10% by mass replacement of pozzolan mor-tar, the compressive strength obtained was 18 MPa in 7 days and25 MPa in 28 days. Karim et al., [13] investigated a set of mortarspecimen which was made with 40% of natural pozzolan as cementreplacement. The compressive strength result for 90 days reach upto 39 MPa under constant 40 �C temperature for 6 h hot water cur-ing. On the other hand, high volume replacement levels of cemen-titious material have been an interesting topic for research and alsoindustry. There is a little information focusing on the use ofhigh volume cementitious materials as a cement replacement[8–10,14]. Varga et al., [8] evaluated properties of mortar contain-

ing high volume of type C fly ash under standard curing. Testresults indicated that the use of 40% (by mass) type C fly ash inmortar increased at the early age day compressive, but reducedthe modulus of elasticity. However, all these strength propertiesand abrasion resistance showed continuous and significantimprovement at the ages of 190 and 365 days, which was mostprobably due to the pozzolanic reaction of FA at later ages. It wasconcluded that class C fly ash can be suitably used up to 50% levelof cement replacement in mortar for use in precast elements andreinforced concrete construction. Herera et al. [9], studied concretewere produced with mass substitution of cement by fly ash up to75%. They concluded that using this level of fly ash was not effec-tive to gain the strength of concrete. Then, Sajedi and Razak [14]make an experiment using high volume replacement levels up to60% of slag, with constant w/c ratio of 0.33 and under water andair curing conditions. They used chemical activation and found thatthe maximum strength could be achieved about 63 MPa at 56 agedays for 50% replacement level. Based on many investigations [8–14] on the effect of using cementitious materials in mortar, it wasfound that the effect of curing method and the volume level ofcement replacement significantly influence the strength anddurability.

GGBFS, FA, and RHA are a latent hydraulic binder. It must beactivated to react and provide the desirable mechanical propertiesusing several methods. One of these activation methods is the ther-mal method. The heat curing on cementitious systems and heattreatment of mortar have become a regulated practice in the pre-cast industry [15]. Presently, the most developing countries havedeveloped specifications for the regulation of heat curing for pre-cast concrete. Previous investigations [16–19,24] showed that hotwater curing method improves strength at the early ages. How-ever, at a later age, a loss of ultimate strength may be occurredin specimens. This is due to the important numbers of formedhydrates have no time to arrange suitably and this causes a lossof ultimate strength. This behavior has been called the crossovereffect [18]. For ordinary Portland cement (OPC), it appears thatthe ultimate strength decreases with curing temperature linearly[16].

The main objective of this paper is to study the effect of differ-ent curing conditions on mechanical properties of mortars contain-ing cementitious materials. In this experimental work, four mixescontaining high volume cementitious materials as cement replace-ment have been used and one mix as a control. Mortar specimenswere cured under four curing conditions after demoulding to findthe effective curing condition for mortar containing high volumecementitious materials.

2. Experiment

2.1. Properties of materials

2.1.1. CementThe cement used in all mixes was ordinary Portland cement (OPC). The specific

gravity of cement was about 3.14. Based on particle size analysis (PSA) tests(Fig. 2a), the specific surface area (SSA) by BET method for OPC was determinedto be 2667.24 m2/kg. The chemical compositions of OPC have been determined by‘‘X-ray fluorescence spectrometry (XRF)” testing method. The compositions ofOPC are given in Table 1.

2.1.2. GGBFSThe specific gravity of GGBFS is approximately 2.87, with its bulk density vary-

ing in the range of 1180–1250 kg/m3. The color of GGBFS is whitish (off-white).Based on PSA tests, the SSA for GGBFS has been determined at 3197.2 m2/kg(Fig. 2b). It can be seen that SSA GGBFS was 1.90 times of OPC, which means thatparticles of GGBFS are 90% finer than those of OPC. The chemical compositions ofGGBFS are given in Table 1. As with all cementing materials, the reactivity of theGGBFS is determined by its SSA. In general, increased fineness results in betterstrength development, however, in practice; fineness is limited by economics, per-formance considerations and factors such as setting time and shrinkage [12,20,22].

Fig. 2. Brunauer–Emmett–Teller (BET) result for specific surface area (SSA) (a) OPC, (b) GGBFS, (c) fly ash and (d) RHA.

Table 1Chemical composition of OPC, GGBFS, FA and RHA (% by mass).

Chemical composition OPC GGBFS FA RHA

SiO2 20.14 36.01 39.86 93.25CaO 60.82 40.54 12.72 0.41Al2O3 3.89 13.17 17.10 0.62MgO 3.10 5.42 6.79 0.42Fe2O3 3.35 0.42 14.98 0.91P2O5 0.064 0.011 0.20 0.86MnO 0.14 0.18 0.18 0.09K2O 0.24 0.34 1.03 2.29TiO2 0.16 0.57 0.89 0.12SO3 2.25 1.77 0.58 0.10SrO 0.02 0.05 0.06 0.02LOI* 2.33 0.59 0.70 3.42

* LOI = Loss on Ignition.

E. Aprianti et al. / Construction and Building Materials 107 (2016) 365–377 367

2.1.3. Fly ashFly ash is the most common source material for geopolymer because it is avail-

able in abundance throughout the world. It also contains amorphous alumina silica.The specific gravity of the fly ash used in the study is approximately 2.28, with itsbulk density of 994 kg/m3. The color of fly ash was whitish gray. Based on PSA tests,the SSA for fly ash has been determined at 2858.6 m2/kg by BET method. The BETresult was shown in Fig. 2c.

2.1.4. RHARice husk was obtained fromMuhairi Resources Sdn. Bhd. Enterprises, Selangor,

Malaysia. The raw rice husk was burnt in the manual incinerator. The combustionprocess was performed using a simple furnace designed and built at the University.The temperature of incineration is up to 700 �C and the burning duration was for24 h. It should be emphasized that the silica is in amorphous form and silica-crystals grew with time of incineration. The combustion environment affects theSSA of RHA. Hence, the time, temperature and environment must be consideredin the processing of rice husks to produce ash with maximum reactivity. After burn-

ing, RHA was grind for 16,000 cycling. The specific gravity of RHA was approxi-mately 2.11. The SSA of RHA is determined at 7667.8 m2/kg (Fig. 2d). The color ofrice husk ash was gray.

2.1.5. AggregatesThe fine aggregate used in the mixes was mining sands with specific gravity and

fineness modulus (BS812: clause 21) of 2.65 and 2.72, respectively. The maximumgrain size of sand was 4.75 mm.

2.1.6. SuperplasticizerIn order to have appropriate consistency with low water to binder (W/B) ratio,

superplasticizer (SP) was required. The specific gravity of SP used was approxi-mately 1.195. It was dark brown in color, with a pH in the range of 6.0–9.0. The con-sumed content of SP in the mortar depends on the replacement level ofcementitious material. For a flow of 140 ± 10 mm, the SP used was 0.5–1% of totalbinder.

2.1.7. WaterThe water used in all mixes was water in pipeline of the lab. It was assumed

that the specific gravity of the used water was about 1.

2.1.8. Scan electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR)and X-ray diffractometry (XRD) for cementitious materials in powder form2.1.8.1. Scan electron microscopy (SEM). The scanning electron microscope (SEM)uses a focused beam of high-energy electrons to generate a variety of signals atthe surface of solid specimens. The signals that derive from electron-sample inter-action several information about the sample including external morphology (tex-ture), chemical composition, and crystalline structure and orientation of materialsmaking up the sample [20]. The SEM is also capable of performing analyses ofselected point locations on the sample [27].

SEM micrographs of OPC and SCMs samples are shown in Fig. 3. The samplesused are in powder form. They are OPC, GGBFS, FA and RHA. It can be seen thatOPC (Fig. 3a) and GGBFS (Fig. 3c) have a similar shape resembling an irregularprism. Furthermore, Fig. 3e shows that all FA grains are in the spherules shape.Due to the spherules shapes of fly ash, the use of this SCM will increase the work-ability of mortar/concrete. While SEM of RHA (Fig. 3g), display prism elements withmany porous on it. Due to these porosities in RHA and consequently higher specific

Fig. 3. SEM result for (a and b) OPC; (c and d) GGBS; (e and f) FA; (g and h) RHA.


surface area, it can be used as SCM in low or moderate volume. The use of high vol-ume RHA causes mixture be dried. However, when it is used by other type of SCMs,the workability of mixture could be good or satisfactory. Therefore, RHA combinetogether with FA were used in this study.

2.1.8.2. Fourier transform infrared spectroscopy (FT-IR). FTIR is a technique which isused to obtain an infrared spectrum of absorption, emission, photo-conductivityor Raman scattering of a solid, liquid or gas. FTIR can identify unknown materials

and determine the quality or consistency of a sample. It also can determine theamount of components in a mixture. The infrared spectra are recorded using PerkinElmer model RX-1 FTIR spectrometer for the powdered samples which are made toa pellet by mixing with KBr. FTIR spectra of the hydrated OPC, GGBFS, Fly ash andRHA are shown in Fig. 4.

FT-IR analyzes for RHA illustrated the bands observe at about 3440 cm�1. Itshows the chemically or physically absorbed water. Peaks in the range of 750–900 cm�1 are the characteristic of O–Si–O bond as described by Colthup spectra

Fig. 3 (continued)

Fig. 4. FTIR spectra result for OPC, GGBFS, FA, RHA.


structure correlation charts for infrared frequencies [32]. These peaks confirm thepresence of silicate in the samples. Bands observed in the range of 900–1100 cm�1 can are attributed due to Si–O–Ca bonds [33]. The peaks in the rangeof 1600–2500 cm�1 show the presence of unreacted calcium oxide in the samples.This is a finger print evidence for the higher degree of silicate polymerization of theprecipitated CSH. It indicates the completion of hydration in this stage and wellcoincide with the compressive strength results [32]. Therefore, it can be concludedfrom this figure that the pozzolanic reaction is higher in rice husk ash cement whichaccelerates the rate of reaction and enhances the strength.

In Fig. 4 also described spectrometry of OPC. High intensity peaks can be seen inthe range of 750–900 cm�1 which are the characteristic of O–Si–O bond. Thesepeaks confirm the presence of silicate in the samples. It is indicative of early fasterdissolution of gypsum and other alkali sulfates and ettringite formation. Waterstretching band at 3640 cm�1 has grown in intensity with a shift (3688 cm�1).The shift of water stretching band may be due to conversion of ettringite to mono-sulphate [20,28,31]. Low intensity peaks of GGBFS can be seen in the same range.The FTIR of GGBFS showed little or no shift in the characteristic peaks clearlydepicting that the interactions between the components are physical in natureand prevented the leakage of GGBFS from the building materials during phase tran-sition of the mortars.

In Fig. 4 for fly ash, no peak has been observed in the same range indicatingabsence of calcium oxide. Disappearance of peaks represents the disappearance ofcalcium oxide or coating of silica on calcium silicates. It is noted that the relativeintensities of the bands at around 1000, 970 and 800 cm�1 are much higher in flyash with higher reactivity. FTIR spectroscopy in combination with particle size anal-ysis provides a fast approach to predict the reactivity of fly ash, from the perspectiveof alumino-silicate glass chemistry.

In all four spectra, the peaks of calcium silicate are different indicating presenceof various types of calcium silicates but exact type could not identified only by FTIR.

2.1.8.3. X-ray diffraction (XRD). X-rays are electromagnetic waves, similar to light,but with a much shorter wave length (k = 0.2–200 Å) [31]. Diffraction is a physicalphenomenon that consists in electromagnetic waves avoiding of obstacles when

obstacles have a size that compares to the wavelength [32]. So, X-ray diffraction(XRD) is a rapid analytical technique primarily used for phase identification of acrystalline material and can provide information on unit cell dimensions. The sam-ples analyzed are OPC, GGBFS, FA and RHA in powder form.

As seen in Fig. 5, XRD results appeared in the number of peaks. The peaksappeared corresponding to the crystallinity of materials. If the intensity of somepeaks is high it indicates the abundance concentration of those crystalline planes[29,30]. The narrow sharper peaks mean larger crystallite size or more crystallinity.The XRD pattern for GGBFS in Fig. 5b showed a diffuse band due to its amorphousand poor crystalline composition. If FA (Fig. 5c) absorbed water, the mixture will bedry while it increases workability. It is due to FA having higher amorphous struc-ture than OPC (Fig. 5a). As displayed in Fig. 5, OPC has more peaks than FA whichwas dominated by the crystal structure. RHA absolutely amorphous due to thewider sharper peaks was shown in Fig. 5d. It should be noted that micromolecularphase and reactivity level of OPC, GGBFS, FA, and RHA classified as crystal andamorphous. Crystal has a regular and repeatedly structure of atom while amor-phous has an irregular structure [29]. Amorphous structure can easily soluble inwater due to weak bonds between molecules (flexible). Karim et al. [37], reportedthat generally, crystal structure has a strong reaction between compounds ratherthan the amorphous structure viewed from their bonding structure. However inthe most cases, reactivity is primarily due to the subatomic properties of the com-pound [35,37–39]. Therefore, OPC and GGBFS will activated if be in crystal form,while reactivity of FA and RHA when be in amorphous form as shown in Fig. 5. Gen-erally, pozzolanic material contains crystalline and amorphous. It can be filler if notreact with calcium hydroxide (CaOH) [38]. Crystalline has a coarse grain called crys-tal whereas amorphous in a finer granule likely similar to flour form. So, when reactwith water, materials in amorphous form is better than crystalline due to theirfineness.

2.2. Mix proportions and mixing procedure

Table 2 represents the mix proportions for different mortars. In all mixes w/cratio is 0.32. PC contains 100% ordinary Portland cement (OPC). While in the restmixes, 50% of OPC was substituted by one or two types of cementitious materials.PG, PF, PGF and PRF represent mortar containing 50% GGBFS; 50% FA; 25% GGBFSwith 25% FA and 25% RHA with 25% FA, respectively. For each individual test, thespecimens used are 3 samples. The total specimens used in this study are 116 cubespecimens and 60 prism specimens.

At first, sand were put in as a mixture and mixed for 5 min. After that thecement and supplementary cementitious material (SCMs) were added and mixingwas done for 5–8 min. Later on, the calculated water was poured into the mixand the mixing continued for 2 min. Then, the superplasticizer was added and mix-ing continued for 2–3 min. Afterward, the flow test was performed and the speci-mens were molded. The molds were filled with fresh mortar and compacted withvibration table.

2.3. Test for fresh mortar

In order to have appropriate consistency for each mortar, a flow table testaccording to ASTM C230/C230M-08 [23] was performed. The range of flow amountswere 140 ± 10 mm. First, some mortar was put in the truncated brass cone in two

Fig. 5. XRD results for (a) OPC, (b) GGBFS, (c) fly ash and (d) RHA.

Table 2Mix proportion of mortars (kg/m3).

Mix code Binder Water w/c SP* (%) Sand Flow

Cement GGBFS FA RHA

PC 550.0 0 0 0 213.40 0.32 0.60 1537.6 140 ± 10 mmPG 275.0 275.0 0 0 213.40 0.32 0.55 1537.6PF 275.0 275.0 0 0 213.40 0.32 0.50 1537.6PGF 275.0 137.5 137.5 0 213.40 0.32 0.60 1537.6PRF 275.0 0 137.5 137.5 213.40 0.32 1.00 1537.6

* SP = superplasticizer.


layer sand each layer was compacted 10 ± 5 times by a steel rod of 16 mm diametertill it solid. The cone was then lifted and the mortar was collapsed on the flow table.Following that, both the flow table and mortar were jolted 15 times in a period of60 s. The jolting of the table allowed the mortar to spread out and the maximumspread to the two edges of the table was recorded. The average of both recordswas calculated as flow in millimeter.

2.4. Curing conditions

The specimens were demolded 24 h after casting. After demoulding, the speci-mens were divided to five groups to consider four conditions on properties of mor-tars containing high volume cementitious materials. Some specimens directly curedin normal water (WC) with 23 ± 3 �C temperature and also cured in air (AC) underroom temperature of 27 ± 4 �C with 70 ± 10% relative humidity. The rest of speci-mens were heated in water at 60 �C for 24 h first, and then they were divided intotwo groups to be cured in air (HAC) under room temperature as well as water cur-ing (HWC) until the test day. In addition, to investigate early hot water curing on

the early strength of mortar, 3 cube specimens were heated for 2½ h and also5 h. In this condition, the compressive strength at 1 day age conducted. The detailsof curing conditions and testing ages were provided in the Table 3.

Hardened mortars can reach their maximum strength within several hoursthrough elevated curing temperature. However, the ultimate strength of hardenedmortars and concretes has been shown to decrease with curing temperature[15,21]. It was found that by increasing the curing temperature up to 60 �C andthe heating time to 24 h causes a continuous increase in compressive strength[12–19]. Therefore, this temperature was selected in this study.

2.5. Test for hardened mortar

2.5.1. Compressive and flexural strengthAll of the specimens within four conditions were cured until they were used for

compressive strength tests at 3, 7, 14, 28, 56 and 90 days age. The compressivestrength measurements were carried out using an ELE testing machine press witha capacity of 2000 kN and a loading rate of 0.k kN/s. Compressive tests were done

Table 3The details of curing conditions and testing ages.

No. Type of curingconditions

Description The detail of testing ages

1 Water curing (WC) After demolded, the specimens directly put in water(23 ± 3 �C) for curing. It also called normal curing

(a) For WC and AC, The details of testing ages are 1 day, 3 days, 7 days, 14 days,28 days, 56 days, and 90 days

(b) Particularly for the specimens which are using thermal process, for 1 daydivided by 3 part as follows:1. After demoulding directly do the compressive test for 3 samples2. The total of 3 specimens were put in a hot water under 60 �C for 2½ h

and then, tested3. The total of 3 specimens were also put in a hot water for 5 h then tested

So, the ages for HAC and HWC are 1 day (3 tests), 3 days, 7 days, 14 days,28 days, 56 days and 90 days

2 Air curing (AC) After demolded, the specimens directly put in anopen place under room temperature (27 ± 4 �C with70 ± 10% relative humidity) for curing. It also calledair curing

3 Hot water–watercuring (HWC)

After demolded, the specimens directly put in a hotwater (60 �C for 24 h) for curing. It also calledthermal process. Then, the specimens directly put inwater (23 ± 3�C) for curing. It also called watercuringHWC is the combination of thermal process andnormal curing

4 Hot water–air curing(HAC)

After demolded, the specimens directly put in a hotwater (60�C for 24 h) for curing. It also calledthermal process. Then, the specimens directly put inan open place under room temperature (27 ± 4�Cwith 70 ± 10% relative humidity) for curing. It alsocalled air curingHAC is the combination of thermal process and aircuring (under room temperature)


according to BS EN 12390-3-09 [25]. While, for flexural strength test conducted at 7and 28 age days two curing conditions of HWC and HAC. For flexural strength test,the specimens were put the specimen above 2 equal pedestals then saddled with aload evenly located in the middle span, as well as the addition of the load graduallyuntil it reaches the fracture and obtain the maximum load value (Pmaks) was shownat Fig. 6.

2.5.2. Water absorptionWater absorption test conducted at 56 days age. Water absorption was deter-

mined using a mortar cube of 50 mm � 50 mm � 50 mm. In this regard, cube spec-imens were dried in oven for 24 ± 2 h until constant weight. Then they wereweighted in air immediately (Wa) after removing from oven and also after immer-sion periods of 30 min and 72 h. Thus, water absorption of the specimen was calcu-lated as 100 � (Wa �Wd)/Wd [37].

2.5.3. PorosityThe porosity test for hardened mortar conducted when the mortar reach

56 days age. Porosity tests carried out on cubical samples with the size of50 � 50 � 50 mm. The purpose of this test is to determine the percentage of poresof the concrete/mortar to the volume of solid concrete/mortar. As it is known thatthe compressive strength of mortar/concrete is influenced by the porosity [28–30].More porous present in the mortar specimen, the lower compressive strength pro-duced [31]. The porosity test set up was shown in Fig. 7. The steps of the test are asfollows:

1. Samples of each condition at 56 days removed from the curing tubs andaerated.

Fig. 6. The illustrated test for flexural strength of mortar.

2. Prepare the specimens and then put in the oven at 100 ± 5�C for 24 h.3. The specimens were removed from the oven and aerated at room temperature

(25 �C) and then weighed. The weight of the mortar obtained was oven dry con-dition (C).

4. The specimens were put in a dessicator to vacuum process using a vacuumpump as mentioned in Fig. 7. The vacuum process of the specimen as long as24 ± 2 h. After that, the specimen flowing with water until all sides of the spec-imens completely submerged in water. Immersion process of the specimensalso in vacuum conditions and performed for 24 h. Then, weighed and obtainedthe weight in water (A).

5. The specimens were removed from the water and wipe the surface to get a SSDcondition then the samples were weighed. The weight of the mortar conditionsof SSD obtained after immersion (B).

From the above test results, the porosity value was calculated based on the fol-lowing formula (Eq. (1))

Porosity ¼ B� CB� A

� �� 100% ð1Þ

where, A is the weight of the specimens in water, B is the weight of the specimens inSSD condition and C is the weight of the specimens in oven dry condition.

3. Result and discussion

3.1. Compressive strength

The compressive strength results were divided by four sections,as follows.

3.1.1. The compressive strength in normal and under early hot-watercuring conditions

The results obtained in the study for compressive strengths,based on heating time, are given in Table 4. The data inventoryin this table examined two subjects. The compressive strengthsobtained for different short time heat durations and also providedthe result for 1 day test immediately after demoulding. Asexpected, substitution of OPC by cementitious materials up to50% significantly reduced the 1-day compressive strength. Thelowest reduction was observed in PG mix by a reduction about38%. While, the great reduction was observed in mixes containingFA. The average reduction of these mixes was about 54%. It showsthat contribution of FA (from 25% to 50%) in high volume substitu-tion level can significantly reduce the compressive strength ofmortar. However, test results of early heating curing show thatthe reduction on the compressive strength can be compensated

Fig. 7. Porosity test using SMART-CL vacuum dessicator with moisture trap.

Table 4The 1-day compressive strength test results in normal and early hot-water curingcondition.

Type ofmixes

1 day without heatingprocess (MPa)

2.5 h in hotwater (MPa)

5 h in hot water(MPa)

PC 32.53 41.10 41.60PG 20.17 30.05 34.67PF 15.57 17.10 19.50PGF 13.44 16.90 21.60PRF 16.40 16.50 18.94

0

10

20

30

40

50

60

70

1 day 3 days 7 days 14 days

Com

pres

sive

stre

ngth

(MPa

)

Age (days)

(a) Water Curing

PC

PG

PF

PGF

PRF

0

10

20

30

40

50

60

70

1 day 3 days 7 days 14 days

Com

pres

sive

stre

ngth

(MPa

)

Age (days)

(b) Air curing

PC

PG

PF

PGF

PRF

Fig. 8. The compressive strength of mortar without heating process at early ages forWC (a) and AC (b) conditions.


by using early hot water curing. Hot water curing for 2.5 h signifi-cantly improved the compressive strength of mortar containing50% GGBFS. The 1-day compressive strength of the mix containingGGBFS (PG mix) after 2.5 h hot water curing was almost the sameas control mortar under normal water curing.

Among two mortars containing ternary blended cement (PGFand PRF), the enhancement for 1-day compressive strength wasfor mortar containing 25% GGBFS. Increasing the duration of hotwater curing from 2.5 h to 5 h did not have any effect on the com-pressive strength gain. While, for all mortars containing binaryblended cement, compressive strength improved of about 14–28%. This improvement was more significant for mixes containing25% or 50% GGBFS. The results indicated that early hot water cur-ing is an effective method in order to gain 1-day strength forcement based materials. In general, it can be specified that earlyhot water curing is more effective to enhance the compressivestrength if the sum of CaO content of the binder is more thanSiO2 content.

3.1.2. The compressive strength result at early age for WC and ACwithout heating process

Fig. 8 shows the compressive strengths development of all mor-tars containing 50% of pozzolan. Based on the results, it can be seengenerally that at 3, 7 and 14 days strengths, for specimens cured inthe water (WC) the compressive strength is greater than the spec-imens cured under room temperature (AC). This reality has provenfor binary blended cement. Conversely, the aforementioned state-

ment is reversed for ternary blended cement containing RHA atthe same ages. The strength of ternary blended cement mortar con-taining RHA and cured under room temperature (AC) was highercompared to the strength of specimens cured in water. It may bedue to two reasons: first, as observed in the Fig. 3 that RHA is a por-ous material. This porosity can observe the free water at the timeof casting. This absorbed water may have a significant role forinternal curing. This role of RHA was already reported by otherresearchers [40–42]. They confirmed that RHA particles are finestand porous appeared to be most effective in mitigating process.


RHA particles obviously best prepared to absorb a certain amountof water into its pores. Then, in the lack of available water, theabsorbed water can be released to maintain hydration of cement(induced). The pores structure of RHA which is induced calledinternal curing of mortar. Second, it was shown in the FTIR andXRD test that RHA has high reactivity potential in the form of silicaamorphous. In addition, it has high specific surface area. Therefore,mortar containing a combination of RHA and FA gain betterstrength under AC condition compared to WC condition.

At 3 days age, compared to PC, the results revealed that PGF andPRF showed the lower strength in the decrease under WC about15% and 33%, respectively, while in AC condition about 9% and4%, respectively. The greater incremental strength was observedin mixes containing GGBFS. While, the highest strength loss wasfound in PF mixes by a total reduction about 23% and 26% forWC and AC, respectively. This fact shows that the type of binderplays a major role in strength improvement of the mortars in dif-ferent curing conditions.

3.1.3. The effect of hot-water curing condition on the compressivestrength at early ages

Variations of compressive strength for the specimens cured inHWC and HAC conditions are shown in Fig. 9. From this figure, itcan be seen that in the most cases, the strengths of mortars curedin HAC are higher than those of mortars cured in HWC in all earlyages. The results revealed that with the use of heating process, thecompressive strength of PC mortar under room temperature (HAC)increased by on average 14% compared to PC mortar cured in water(HWC). In addition, when 50% GGBFS is used in mortar (PG), thestrength increased by on average 12% compared to PC mortarunder HAC condition at all early ages. Reversely, when PG mortarscured in HWC condition obtained lower strength by 24% and 18%compared to PC mortars for 3 days and 7 days, respectively. Itcould generally be said that whenever PG mortars are heated, itis prefer to cure under HAC condition than cured in water (HWC).

0 10 20 30 40 50 60 70 80

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Fig. 9. The compressive strength result of mortar under heating process at earlyages in HWC (a) and HAC (b) conditions.

Meanwhile, the strength rate of PF and PGF compared to PC atall early ages, for both in HWC and HAC conditions, decreased by24% and 10%, respectively. It was reported [34,38] that the appear-ances of the strength was slowed in the early ages and continu-ously for a long curing period by adding FA for more than 30%. Itis due to the chemical reaction in mortar, in terms of hydrationprocess. The hydration process result indicates that chemical com-position such as SiO2 and CaO (in the vicinity of the interface) isone of the major factors for the increase in bond strength [37].However, Chindaprasirt and Rukzon [28] reported that FA mightbecome effective for strength increase by addition of other poz-zolanic materials which participate with some reactions to gainthe strength. Therefore, as mentioned from XRD results (Sec-tion 2.1.8.3), the other cementitious material which has a highreactivity similar to FA in silica amorphous form is RHA. Other casefor mortar containing RHA with FA (PRF) compared to PC at 3 days,the strength decreased by 4% and 6% in HWC and HAC condition,respectively. However, this strength is increased by 6% and 16%at 7 and 14 days age, respectively for HWC condition. Surprisingly,PRF mortar obtained the equivalent compressive strength at14 days as much as 65 MPa in HAC condition. This results showthat there is continuity in the hydration progress of PRF mortarfrom 7 days to 14 days, while the whole latent potential of PRFmortar released during early ages due to the heat effect. It can besaid that by using combination of RHA with FA as partial substitutecement, several benefits provided such as reduced materials costsdue to cement savings [36] and environmentally friendly related tothe utilization of waste materials as well as reduced CO2 emissionsfrom cement exploitation. As conclusion, it seems that whenevermortars cured under room temperature after heating (HAC), thestrengths increase with the curing duration. Then, the fact showsthat a proper combination of cementitious materials in binderplays a main role in the strength improvement of mortars.

3.1.4. The compressive strength of mortars for all conditions at laterages

The compressive strength of five group mortars in different cur-ing conditions continuously investigated at 28, 56 and 90 days age.The difference between curing conditions with and without heat-ing process described clearly using column comparison as shownin Fig. 10. It can be seen generally that HAC condition showedthe highest strength results compared with other three conditionsat later ages. It was cleared that under WC condition, PC mortargave lower strength than PG mortar but shows higher strengththan PF, PGF and PRF mortars. It is related to the presence of crys-talline structure from GGBFS which can improve strengths at laterages [17]. Generally, from the results obtained for AC condition, itwas observed that the strength of PRF mortars were higher thanthose of PC, PG, PF and PGF mortars in all later ages. The resultsrevealed that compared to PRF mortar, PC mortar and PG mortarunder AC condition showed strength loss about on average 4%and 8%, respectively. Otherwise, PC mortar shows higher strengthwhen compared to PGF mortar with incremental on average about23% at all later ages. It should be noted that for the specimenscured at room temperature, the maximum relative humidity was85% and air temperature of 27 ± 3 �C whereas for the specimenscured in water is 100% with temperature by on average 23 �C. Highrelative humidity and air temperature may be the reasons forstrength gain for some mortars under HAC condition and also forthe most mortars under HAC condition.

In two curing conditions with heating process, it is seen that thestrength results of mortars cured in HWC is lower than HAC. Other-wise, for PGF mortar, the strengths were improved at all ages inwater curing condition with or without heating process. Asreported before [17], the OPC and GGBFS make mortar becomemore sensitive to air curing condition (AC). This fact can be attrib-

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syad09syad65syad82

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Fig. 10. The compressive strength result of mortars at later ages for WC (a), AC (b), HWC (c) and HAC (d) conditions.

Table 5The flexural strength results of mortar under heating process at 7 and 28 days age inHWC and HAC conditions.

Flexural strength (MPa)

7 days 28 days

HAC HWC HAC HWC

PC 4.6 4.0 12.7 12.0PG 5.2 4.5 12.2 11.0PF 11.2 8.5 14.1 12.5PGF 11.6 9.6 15.1 13.4PRF 13.4 10.6 17.8 12.7


uted to high consistency of GGBFS nearby water. Whereas PRFmortar gain better strength at later ages if curing under room tem-perature than cured in water with or without heating process. Thisshows that the physical and chemical properties of RHA and FAcombine with OPC more reactive when cured under room temper-ature. However, this also affected from high volume replacementlevel of cementitious materials. The encouraging results obtainedwere associated to the synergy between the ashes used [16]. Karimet al., [37] reported that generally, reactivity is favoured byincreasing fineness of the pozzolanic materials. The finest particlestend to concentrate near the interface between aggregate andcement matrix. The finest particle size leading to reduced porosityand enhanced internal bonding capacity of mortar at the sametime. Chemical and physical properties such as fineness, active alu-mina and glass content are the main factors determining the poz-zolanic activity and strength contribution of FA and RHA [34–36].Reactivity of RHA is attributed to its high content of amorphous sil-ica, and to its very large surface area governed by the porous struc-ture of the particles as shown in XRD and SEM results(Section 2.1.8). RHA have the highest SiO2 content but when mixwith OPC in high volume, it will dry. At the stage of hydration pro-cess, the fly ash performed faster than GGBFS. It is due to the fine-ness particle and containing silica amorphous. So that, RHAcombine with fly ash and produced the better flow and good work-ability. The combination of OPC, RHA and FA to be composite bin-der exhibit as the best mix compare to all combinations. It is due tothe chemical and physical properties of these cementitiousmaterials.

Overall, the strength comparison of five group mortars at thelater ages showed that PRF mortars gave the highest strengthscured under HAC condition. The lowest strengths are related toPF mortars almost in all curing conditions and PG mortar has med-ium strengths and improve steadily in all curing conditions.According to the results obtained in the study, it can be said thatthermal activation with air cured (HAC) is one of the effectivemethods for the activation of OPC–cementitious materials.

Based on curing conditions criteria, the strength of the speci-mens cured in air at room temperature after heat process (HAC)is the highest. The second level of strength is attributed to thespecimens cured in water after the heating process (HWC). Thethird level is for those cured in water without the use of heating(WC) and the last is attributed to the specimens cured in air underroom temperature without the use of heat process (AC). This resultalso shows that the thermal technique like HAC is a feasible and


efficient method for the activation of ordinary Portland cementwith high volume cementitious materials in mortars and concreteswithout the use of water to cure the specimens after heatingprocess.

3.2. Flexural strength

Table 5 shows the effect of two curing conditions with heatingprocess (HWC and HAC) on the flexural strength of mortars con-taining 50% cementitious materials. As shown in this table, flexuralstrength results under HAC condition are higher than HWC condi-tion. At 7 days age, flexural strength of mortar under HAC condi-tion, compared to the control mix (PC), increases by 13%, 143%,152% and 190% when GGBFS, FA, GGBFS with FA and RHA withFA are used, respectively. Moreover in HWC condition at 7 daysage, compared to PC mortar, the flexural strength also increasedby 12%, 112%, 140% and 165% for PG, PF, PGF and PRF mortars,respectively. Generally under HAC condition, the flexural strengthfrom ternary blended cement mortar was better than binaryblended cement mortar. Particularly, mortar containing RHA andFA (PRF) obtained the highest flexural strength. Furthermore, fromfive group mortars investigated, the average incremental strengthobtained under HAC increase by 10%, 14%, 23%, 22% and 34% whencompared to HWC condition, for PC, PG, PF, PGF and PRF, respec-tively. As a conclusion, the highest flexural strength for cementi-tious mortar could be only achieved under HAC condition byusing combination of OPC and RHA with FA. The increase in flexu-ral strength could be due to the improvement of the bond betweenthe cement and fine aggregate in the presence of RHA with FAwhich are finer than OPC [28,35,37]. On the other hand, RHA andFA have a high reactivity in the form of silica amorphous as shownin SEM, XRD and FTIR tests (Figs. 3–5). The silica amorphousreleased due to the large specific surface area. Amorphous materialproduced high flexibility due to the rubbery domain structure.Hydration process of pozzolan materials particularly in amorphousform reached high strength under thermal conductivity when reactwith calcium hydroxide [37]. Therefore, compared to the other fourgroups of mortar containing cementitious materials, the rate offlexural strength gained by PRF under HAC was better.

3.3. Water absorption

In order to evaluate the effect of curing conditions on the waterabsorption of mortar containing high volume cementitious mate-rial, a bar graph represent the results of initial (30 min) and final

Fig. 11. Water absorption results of mortar

(72 h) water absorptions at 56 days age are depicted in Fig. 11. Ithas been reported that mortar with high quality usually has waterabsorption lower than 5% [26,39]. In addition, it was specified thatcement based materials can be classified as poor, average and goodfor initial water absorption values of 5% and above, 3–5% and 0–3%,respectively [43]. Sajedi and Razak [17] already reported thatcementitious material–cement mixtures displayed lower waterabsorption than the OPC control mix. Therefore, It can be seen fromFig. 11 that in the most curing conditions PC mortar absorbedwater more than mortar containing cementitious materials at thebeginning of the sorption. In general, mortars containing SCMshave lower water absorption in all curing conditions compared tocontrol mortar (PC). According to the criterias [43], all mortars con-taining high volume SCMs can be considered as good quality. Ascan be seen from Fig. 11 that during the initial period of waterabsorption, WC condition show the highest result about 2.5% forPC mortar whereas the lowest result about 1.5% for PGF mortar.The water absorption of mortars in HAC condition is lower thanmortars cured under WC and AC conditions for initial absorption.

Therefore, HAC method is the effective technique to produce adenser microstructure. It is due to the heating process is one ofthe methods for increases the degree of hydration which causedinterlocking between cement and pozzolan then released thedense mortar [28,30,31]. It means that the incorporating of fine-ness cementitious material and HAC condition produced the densemicrostructure of mortar which results better compressivestrength and lower water absorption.

3.4. Porosity

The porosity is one of the factors that affect the strength of themortar. The number of pores contained in the mortar will greatlyaffect the density of the mortar itself. The porosity test resultsare shown in Fig. 12. It can be seen that PC mortar both in waterand air curing conditions without heating process have a porosityabout 20% and 22%, respectively. Furthermore, mortar containing50% of fly ash (PF) tend to obtained porosity more than 20% in allcuring conditions. Malami and Kaloidas [29] reported that the fac-tors such as the curing conditions, age and the degree of hydrationsignificantly affected the total porosity of mortars, particularly inthe case of OPC–cementitious mortars. Khan [30] recommendedthat the particle size of cementitious materials should be finer thanOPC to achieve the better performance.

The lowest porosity was attributed to PRF under HAC about11%. While, the highest porosity results were observed in mortar

s in four curing conditions at 56 days.

0

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sity

(%)

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Fig. 12. The relationship between type of mixes and percentage (%) porosity ofmortar.


containing FA by an average about 23%. It is due to the use of highvolume replacement level of FA in mortar, so the interfacial zonebetween cement and aggregate increased and become thick. Thethickness of interfacial zone will cause low dense mortar. Pandeyand Sharma [31] reported that a good compressive strength hasthe high dense mortar. In addition, other researchers also reported[28–31] that ternary blended cement produces mortars withhigher strengths using heating process. Furthermore, the porosityof mortar containing two cementitious materials like RHA and FAwas decreased due to the filler effect from the fineness particleof RHA and FA used. The particle size of materials influenced tothe filler effect of mortar which is reduce the volume of void spacebetween cement and aggregate. Test results show that the mortarscontaining any type of cementitious materials have lower porosityand consequently the strength improved when specimens cured inHAC condition. It means that HAC condition is the effective way toreactivate pozzolan in hydration process and significant reductionon the total of porosity.

4. Conclusion

In this study, the effect of curing condition on some propertiesof a mortar containing high volume cementitious materials such asGGBFS, FA and RHA were investigated. 50% of OPC was substitutedby one or two types of these pozzolanic materials. The specimenswere cured in 4 conditions of WC, AC, HWC and HAC afterdemoulding. For these conditions, specimens were tested for1 day immediately after demoulding and also after short timehot water curing (with a constant temperature of 60 �C) of 2.5 hand 5 h. In addition, the cube specimens were tested from 3 to90 days age for all curing conditions. The flexural strength test con-ducted at 7 and 28 days in two curing conditions of HAC and HWC.Furthermore, the water absorption and porosity test were con-ducted at 56 days age of cube specimens. Based on the experimen-tal work, the following conclusions can be drawn:

1. The substitution of OPC by cementitious materials such as FA,GGBFS and RHA up to 50% significantly reduced the 1-day com-pressive strength in normal condition. Whereas, the short timeearly hot water curing can be specified as an effective method inorder to gain 1-day strength for cement based materials. Thisheating method is more effective when GGBFS is used in themixture.

2. Generally, the compressive strength of OPC–cementitiousmaterials cured in WC is greater than cured in AC condition atearly ages. The greater incremental strength was observed inmortar containing GGBFS (PG), both in WC and AC conditions.PG and PC mortars showed similarity effect in order to gainthe strength at early ages when cured in WC condition.

3. The compressive strength of OPC–cementitious materials curedin HAC are higher than those of mortar cured in HWC in all earlyages.

4. Mortar containing cementitious materials cured under HACobtained the highest compressive strength results comparedto the other three curing conditions at the later ages. It wasprove that the thermal technique (heat) like HAC is a feasibleand efficient method for the activation of ordinary Portlandcement with high volume cementitious materials in mortarsand concretes. Therefore, it will reduce the exploitation of waterusage and save energy as well.

5. From the results at the later ages, it is found that PRF mortarsgave the highest compressive strength cured under HAC condi-tion. While, the lowest strength are related to PF mortars in allcuring conditions. Therefore, the fact shows that the type andcombination of binder plays a major role in strength improve-ment of the mortars in different curing conditions.

6. The highest flexural strength is attributed to PRF mortar as18 MPa at 28 days under HAC condition.

7. Thermal activation then following air curing (HAC) is the effec-tive way to produce a denser microstructure of mortar contain-ing cementitious materials and consequently to achieve thehigher compressive strength with lower water absorption.

5. Recommendation

For future work, it is recommended to do an experiment aboutthe effect of HAC on carbonation process of concrete.

Acknowledgement

The authors would like to thank the University of Malaya forfully funding the work described in the publication through theInstitute of Research and Monitoring or Institut Pengurusan danPemantauan Penyelidikan (IPPP) with registration No. PG046-2014A.

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Introducing an effective curing method for mortar ... · Introducing an effective curing method for mortar containing high volume cementitious materials ... blast furnace slag (GGBFS)