Construction and Building Materials 51 (2014) 395–404

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Construction and Building Materials

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Strength and drying shrinkage of reactive MgO modified alkali-activatedslag paste

0950-0618/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.10.081

⇑ Corresponding author. Address: Geotechnical Research Office, Department ofEngineering, University of Cambridge, Trumpington Road, Cambridge CB2 1PZ,United Kingdom. Tel.: +44 7411070337; fax: +44 01223766683.

E-mail addresses: leonking1987@gmail.com, fj232@cam.ac.uk (F. Jin).

Fei Jin a,⇑, Kai Gu a,b, Abir Al-Tabbaa a

a Department of Engineering, University of Cambridge, Trumpington Road, Cambridge CB2 1PZ, United Kingdomb School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China

h i g h l i g h t s

� Reactive MgO was studied as a shrinkage-reducing agent in AAS.� The reactivity and content of MgO affected the AAS performance remarkably.� Both reactive MgO used reduced shrinkage and meanwhile increased strength of AAS.� The composition of the hydration products varied depending of MgO used.

a r t i c l e i n f o

Article history:Received 16 September 2013Received in revised form 22 October 2013Accepted 28 October 2013Available online 28 November 2013

Keywords:Reactive MgOAlkali-activated slagStrengthDrying shrinkageMicrostructure

a b s t r a c t

Conventional alkali-activated slag (AAS) cements suffer from significant drying shrinkage which hinderstheir widespread application. This paper investigates the potential of using commercial reactive MgO toreduce the drying shrinkage of AAS. Two different reactive MgOs were added at a content of 2.5–7.5 wt%of the slag, which was activated by sodium hydroxide and water–glass. The strength and the dryingshrinkage of those reactive MgO modified AAS (MAAS) pastes were measured up to 90 days. It is foundthat MgO with high reactivity accelerated the early hydration of AAS, while MgO with medium reactivityhad little effect. The drying shrinkage was significantly reduced by highly reactive MgO but it also gen-erated severe cracking under the dry condition. On the other hand, medium-reactive MgO only showedobservable shrinkage-reducing effect after one month, but the cement soundness was improved. Thehydration products, analysed by X-ray diffraction, thermogravimetric analysis and scanning electronmicroscopy techniques, showed that Mg was mainly incorporated in the hydrotalcite-like phases. It isconcluded that the curing conditions and the time of hydrotalcite-like phases formation and their quan-tity are crucial to the developed strength and shrinkage reduction properties of MAAS, which are highlydependent on the reactivity and content of reactive MgO.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Portland cement (PC) production is very energy intensive and isresponsible for 5–8% of man-made CO2 emissions [1]. As a result,various attempts to reduce the cement clinker consumption havebeen made over the years, one of which is the extensive usage ofindustrial by-products such as silica fume, fly ash and ground gran-ulated blast-furnace slag (GGBS). Although developed decades ago,the last decade or so has seen significant increase in research ef-forts into alkali activated binders, and in particularly alkali acti-vated slags (AAS) [2], as viable and sustainable alternatives to PCdue to the significantly lower costs, energy and CO2 emissions

[3,4]. In addition, it has also been reported that AAS concrete hasa higher rate of strength development, lower permeability, higherresistance to chemical attack, and a lower rate of heat release thanPC [5–7]. Nevertheless, there are also many disadvantages of AASconcrete including high autogenous and drying shrinkage [8–14],which significantly hinders their widespread usage.

There are a number of factors that determine the drying shrink-age of AAS including the type and content of the alkali activators[8,15–17], properties of the aggregate and the slag [18,19], andcuring environment [9,20–22]. In general, water–glass activatedslag has more shrinkage than sodium hydroxide activated slagand the drying shrinkage of AAS increases with increasing dosageof activators as well as slag fineness [10,17]. In addition, theshrinkage of AAS is very sensitive to the curing environment. It isreported that although at 70% RH the drying shrinkage of AAS con-crete is similar to that of PC concrete [20], and it is significantlyhigher at 33% and 50% RH [20,21]. Much has been published on

Table 1Physical properties and chemical compositions of the raw materials used in thisstudy.

Label MgOH MgOM GGBS

Chemical composition (%)MgO 97.5 93.2 8CaO 1 0.9 40Cl 0.2 – –SiO2 1 0.9 37Fe2O3 0.035 0.5 –Al2O3 – 0.22 13Mn 0.006 – –Na2O – – 0.3K2O – – 0.6SO3 0.85 – 2.5Reactivitya (s) 10.4 100.4 –Specific surface area (m2/g) 110.82 9.005 0.493

a Measured according to the acetic acid test [34].

396 F. Jin et al. / Construction and Building Materials 51 (2014) 395–404

the shrinkage mechanisms for AAS cement and concrete [16,20,23–25]. Some studies [23,24] reported that the hydration products ofAAS included a calcium silicate hydrate (C–S–H) gel with a lowerCa/Si ratio than that in PC, and a Si-rich gel, which contains a high-er uncombined water content than in C–S–H, which would evapo-rate during the drying process, resulting in substantial shrinkage[20,24]. By investigating the relationship between pore profileand the drying shrinkage, Shi [15] concluded that AAS concretesand mortars have a lower total porosity and a larger refined porestructure than the PC, which was later confirmed by Collins andSanjayan [25] who reported that AAS paste had up to 82% poresin the mesopore range, while PC had only 36.4%. They concludedthat the elimination of the moisture from mesopores (the contentof which is much higher in AAS than in PC) causes higher capillarystress and therefore greater shrinkage during drying.

In order to reduce the drying shrinkage of AAS concrete, Bakha-rev et al. [26] investigated the effect of different admixtures/addi-tives on water glass-activated slag concrete. Lower dryingshrinkage (at 50% RH) was observed for the AAS compared to PCconcrete prepared with 6% gypsum and concluded that gypsum re-duces both autogenous and drying shrinkage due to the formationof expansive phases such as ettringite (AFt). With chemical admix-tures, specimens with superplasticiser showed the highest dryingshrinkage, followed by specimens without admixtures and thenthe specimens with water-reducing admixtures, whilst the speci-mens containing air-entraining agent exhibited the lowest shrink-age. On the other hand, the compressive strength results showedthat superplasticiser admixture resulted in a 25% loss of 28 daystrength and water reducing (based on lignosulphonates) reducedthe early strength up to 14 days, while the air-entraining agent hadno negative effect [26]. They concluded that it was not desirable touse superplasticiser in AAS concretes, which is consistent withother findings that some of the conventional admixtures used forPC have detrimental effects in AAS concrete [8,27]. Palacios andPuertas [28] investigated the effect of polypropylenglycol-basedshrinkage-reducing admixture (SRA) on the drying shrinkage ofwater glass-activated slag mortars. They reported that 1% and 2%SRA reduced shrinkage by 7% and 50% respectively at 50% RH,while it reduced by 50% and 85% respectively at 99% RH, whichwas attributed to the change induced in the pore structure bythe admixture and the decrease in the surface tension of the porewater, agreeing well with the suggestions by others [15,25]. Interms of compressive strength, SRA was found not to cause sub-stantial change under 50% and 99% RH.

The use of magnesia, MgO, as a shrinkage reducing mineraladditive, dates back to the mid-1970s in the construction of theBaishan concrete arch gravity dam [29], where it proved to be amore efficient and economical measure of controlling the shrink-age of PC than conventional admixtures [30]. The volume compen-sation during the drying process was due to the chemical reactionbetween MgO and water forming brucite (Mg(OH)2), which resultsin 118% volume increase [31]. The effect of MgO in the AAS systemshas recently been investigated, either in terms of its varying natu-ral content in different slag compositions [32], or as an additive[33]. As slags are usually produced at temperatures of 1400–1600 �C [7], the MgO naturally present in slags is categorised asdead burned MgO [34]; whereas reactive grade MgO (calcined un-der 1000 �C) or hard burned MgO (calcined at �1000–1400 �C) areoften chosen for use as additives.

Ben Haha et al. [32] investigated the effect of natural MgO con-tent in different slags on the performance of AAS and revealed thatalthough the main hydration product is still C–S–H gel, MgO reactswith the slag to form hydrotalcite (Mg6Al2(OH)16CO3�4H2O)-likephases, the content of which increases as the MgO content in theslag increases. They also concluded that since these hydrotalcite-like phases are more voluminous than C–S–H, that they result in

higher strength, hence the higher the MgO content, the higherthe strength. Shrinkage performance was not studied. In the workby Shen et al. [33], the addition of 10% light-burned dolomite (con-taining >85% reactive MgO) was found to reduce the shrinkage ofwater–glass activated Slag/FA (AAFS) cement by �50%, and wasonly slightly higher than that of PC. Meanwhile, the 3 and 7 daycompressive strengths were found to increase slightly while the28 day strength remained unchanged. The shrinkage reductionwas attributed to the expansion by hydration and carbonation ofMgO [33]. However, there has been very limited work performedon the effect of reactive MgO on the mechanical and shrinkageproperties of AAS binder and the nature of their interaction. In par-ticular, detailed investigation of the evolution of the hydrationproducts is needed to elucidate the different roles, if any, that reac-tive MgO, as an additive, and any naturally present dead burnedMgO play in AAS systems.

Therefore, this paper deals with the mechanical and shrinkageproperties of AAS modified with reactive MgO. Furthermore, asthe characteristics of different reactive MgOs vary significantly,depending on their precursors and calcination history [34], it is ex-pected that their performance in the AAS systems would also vary.Thus, two different reactive MgOs, whose reactivity varies greatly,were selected to examine the effect of their characteristics on theirperformance in AAS systems. The unconfined compressive strengthand drying shrinkage of the reactive MgO modified AAS (MAAS)were investigated and the microstructure studied to elucidateany changes in the nature of the hydration products and the factorsthat affect the mechanical performance changes observed.

2. Materials and methods

The slag used was obtained from Hanson, UK. The hydraulic index for this slagwas calculated to be 1.6 according to Mantel [35], who reported that it should beP1.0 to achieve good performance. Two different reactive MgOs (from Richard Ba-ker Harrison, UK) with different characteristics were used as the additive. Based onthe characterisation in [36], MgOH is categorised as a highly reactive MgO, whichalmost completes its hydration in one day, while MgOM in grouped as a mediumreactive MgO, which continues to hydrate after one month [36]. The chemical com-positions of the slag and reactive MgOs are shown in Table 1. Sodium silicate pow-der (Fisher Scientific, wt. ratio: SiO2/Na2O = 2, technical grade) and sodiumhydroxide (NaOH) pellets (Fisher Scientific, technical grade) were used as the alkaliactivators. Liquid NaOH solution was prepared the day before mixing by dissolvingthe pellets in a predetermined amount of deionised water.

Six MAAS paste mixes were used in which the reactive MgO content varied from2.5% to 7.5% by mass of slag and the nomenclature used is shown in Table 2 where Hand M refer to MgOH and MgOM respectively and are followed by the MgO contentin the mix. All other components were kept constant including a water to solid(including the slag, reactive MgO, sodium silicate and NaOH) ratio of 0.32 to ensuregood workability. A seventh mix, denoted C, was a conventional AAS paste. The dos-age of the activator was calculated by fixing the modulus (Na2O/SiO2) equal to 1.35,and 4% Na2O of the mass of the slag. The sodium silicate powder was first mixed

Table 2Nomenclature based on MgO type and content in the paste mix.

Nomenclature GGBS (%) MgO (%) Modulus Na2O (%) W/S

C 100 0 1.35 4 0.32H2.5 100 2.5 1.35 4 0.32H5 100 5 1.35 4 0.32H7.5 100 7.5 1.35 4 0.32M2.5 100 2.5 1.35 4 0.32M5 100 5 1.35 4 0.32M7.5 100 7.5 1.35 4 0.32

F. Jin et al. / Construction and Building Materials 51 (2014) 395–404 397

with the slag and reactive MgO powder for 3 min in a bench-top mixer to achievehomogeneity to which the NaOH solution was then added. After mixing for another3 min, the mix was cast into the cubic (40 � 40 � 40 mm) or prism(40 � 40 � 160 mm) moulds in two layers and hand-vibrated to eliminate the voids.The samples were then covered with cling film to avoid moisture loss. After 24 h,the samples were demoulded carefully and the cubes were transferred into thewater tank maintained at 21 ± 2 �C, while the prisms were stored under the temper-ature of 21 ± 2 �C and relative humidity of 50 ± 5%.

The shrinkage measurement was performed using a length comparator todetermine the linear dimension variation of the specimen along the longitudinalaxis. The first reading was recorded immediately after demoulding and the mea-surement was conducted in triplicate for each of mix every day for the first twoweeks, and then at 28, 56, 90 days. The unconfined compressive strength (UCS) ofthe cubic samples, in triplicate, was determined according to [37] at the curingage of 1, 7, 28 and 90 days. Then the crushed samples were ground and stored inacetone for 3 days to arrest the hydration and then vacuum dried for P3 days fol-lowed by oven drying at 60 �C for 24 h before microstructural analysis. The sampleswere then further ground to pass through a 75micron sieve prior to X-ray diffrac-tion (XRD) tests and thermogravimetric analysis (TGA). XRD was carried out on aSiemens D500 X-ray diffractometer using a scanning range from 5 to 55 (2b), witha scanning speed of 1 s/step and resolution of 0.05�/step. TGA was conducted onPerkinElmer STA6000 equipment from 50 to 800 �C with the increasing rate of10 �C/min. Scanning electron microscope (SEM) combined with Energy dispersiveX-ray spectroscopy (EDS) was performed on the JEOL JSM 5800LV machine and4–7 points on the paste were picked for determination of its elemental composition.

Fig. 1. Unconfined compressive strength of MAAS pastes (a) MgOH series; and (b)MgOM series.

Fig. 2. Relative strength of the MAAS pastes compared to the reference AAS paste.

3. Results and discussion

3.1. Strength development

Fig. 1(a) and (b) shows the UCS development of the MAASpastes. It is clear there is significant strength development byday 7 followed by a much slower rate thereafter. In the short term(1 and 7 day), the UCS of the MgOH mixes increased with the MgOcontent. After 28 day, the 5% MgOH addition was found to producethe lowest UCS and was very close to that of the reference sample(�75 MPa). The 7.5% MgOH addition provided the highest strengthmixes of �86 MPa at 28 day. The MgOM mixes showed a differentdevelopment pattern with a reduction in UCS at 1 day. After 7 days,the 2.5% MgOM content mix had the highest strength while the 5%and 7.5% additive content mixes showed approximately the sameUCS compared to the reference.

To illustrate the effect of MgO type and content on the strengthdevelopment, the relative strength (ratio between MgO modifiedpastes to the reference sample) is shown in Fig. 2. It is apparentthat the two different MgO samples showed opposite effects onthe AAS paste after demoulding. The highly reactive MgO (MgOH)increased the UCS remarkably for 2.3, 3.7, and 4.4 times at 2.5%,5% and 7.5% of MgOH addition, respectively. Meanwhile, the MgOM

decreased the strength to 0.5, 0.34 and 0.24 times that of the refer-ence sample at 2.5%, 5% and 7.5% of MgOM addition, respectively.After 7 days, the highest increase of UCS (�31%) was achieved by7.5% addition of MgOH, while for MgOM, only 2.5% addition in-creased the UCS by�10%. The 28 and 90 day UCS data showed sim-ilar trend for both reactive MgO. The UCS of 2.5% MgOM and 7.5%MgOH were close and increased the strength most significantly(�15% at 28d and �27% at 90d). The 5% addition of MgOM andMgOH were close and showed nearly the same strength compared

to the reference sample after 28 days. Finally, the 2.5% MgOH and7.5% MgOM resulted in a 7–10% increase of UCS.

3.2. Drying shrinkage and mass loss

Fig. 3 shows the drying shrinkage of the pastes up to 90 days.The drying shrinkage of the MgOM set was close to the referencespecimen during the first two weeks. After 1 month, the shrinkageof M7.5 almost stopped, showing a final shrinkage (at 90 day)

Fig. 3. Drying shrinkage of the pastes. Fig. 5. Shrinkage rate of the pastes.

398 F. Jin et al. / Construction and Building Materials 51 (2014) 395–404

decreased by �13% than the reference specimen. In contrast, MgOH

was much more effective in reducing the drying shrinkage of theAAS paste, especially at the early age. It is observed that although2.5% MgOH only decreased the drying shrinkage slightly comparedto the reference specimen, the 5% and 7.5% MgOH mixes caused sig-nificantly shrinkage reduction by 15.4% and 26.5% respectivelyafter 3 months’ curing.

As for the mass loss, similar to the shrinkage data, MgOM onlydecreased it slightly compared to the reference sample, whileMgOH decreased it significantly by 26%, 37% and 43% for MgOH

addition from 2.5% to 7.5%, respectively (see Fig. 4). It was also ob-served that although M7.5 had approximately the same weight losswith the reference, its final shrinkage was much less. As is knownthat the weight loss is mainly controlled by the evaporation of thefree water from the paste, the reduction of the shrinkage by addingMgOM is attributed to the latent hydration of MgO (less reactive,see Table 1), which resulted in pore filling at later age. On the con-trary, MgOH hydrated very fast and could fill the pores at the veryearly stage, decreasing the weight loss significantly.

An enhanced interpretation of the shrinkage evolution duringthe initial stage can be achieved by a close look at the shrinkagerate, which corresponds to the relationship between shrinkage var-iation and the reading interval [10]. It is reported that there shouldbe two peaks in the shrinkage rate pattern, the first of which cor-responds to the evaporation of the free water from the macropores(>50 nm) and the second which is due to the evaporation of freewater from the mesopores (2–50 nm) and to the chemical shrink-age or self-desiccation. However, the first peak always occurs at

Fig. 4. Mass loss of the pastes during drying process.

the very initial stage after setting leaving the second peak observa-ble if the samples were demoulded after 1 day [10]. Fig. 5 depictsthe shrinkage rate of MAAS pastes for both MgOs and their threedosage levels. Only one peak was found corresponding to the sec-ond peak in [10] for the reference sample as well as the MgOM

modified pastes, while no peak existed for MgOH modified samples.The absence of this peak in MgOH set samples is attributed to theaccelerated slag hydration caused by the MgOH addition, whichagreed well with the UCS data. The enhance slag hydration andvoluminous hydration products has successfully filled the macrop-ores to produce mesopores, hence reduced the weight loss and dry-ing shrinkage while increased the early age strength, although, itmay also induce higher capillary force as will be discussed later.In comparison, MgOM only brought the second peak forward byone or two days, which suggested that the pores were still moreeffectively filled with more voluminous hydration products, result-ing in the reduction of macropores and simultaneous increase ofthe mesopores.

The images of the shrinkage samples at 7 and 28 day are shownin Figs. 6 and 7 respectively. It is obvious that the two reactive MgOsamples performed differently in terms of crack developmentwhich also varied depending on their content. The reference sam-ple developed cracks soon after drying, which prevailed during thecuring process (Fig. 6(a) and Fig. 7(a)). The 2.5% addition of MgOH

reduced crack propagation, with small cracks observable but filledwith hydration products (Fig. 6(b)). However, more than 5% MgOH

content increased the crack numbers significantly after 7 days(Fig. 6(c) and (d)). After 28 days, although H5 and H7.5 mixes stillshowed more severe cracking than the reference samples, it wasfound that the cracks were to some extent healed, especially forH7.5 paste (Fig. 7(d)). To compare, M2.5 showed very smooth sur-face (Fig. 6(e)), while M5 and M7.5 exhibited some cracks after7 days (Fig. 6(f) and (g)). The curing successfully healed thosecracks after 28 days, with all the initial cracks filled with hydrationproducts (Fig. 7(e), (f) and (g)). The healing property is attributed tothe formation of Ht, which has a lower density (2.0 g/cm3) com-pared to that of a tobermorite like C–S–H (2.23 g/cm3), resultingin more effective pore filling and a lower porosity [32]. The forma-tion of Ht by the reaction of reactive MgO and slag was confirmedin earlier studies by the authors [38,39]. It is concluded that theformation rate of Ht increases with the reactivity of MgO sincethe quicker dissolution of MgO results in the Mg2+ being availablein the pore water to react with the broken Al–O bond within theslag by alkali activation.

In the micromechanical model developed by [40,41], they foundthat the smaller pore size generates larger capillary tensile forcesat the meniscus (the interface between water and air), hence

Fig. 6. Pictures of the pastes after 7 days’ drying (a) C; (b) H2.5; (c) H5; (d) H7.5; (e)M2.5; (f) M5; and (g) M7.5.

Fig. 7. Pictures of the pastes after 28 days’ drying (a) C; (b) H2.5; (c) H5; (d) H7.5;(e) M2.5; (f) M5; and (g) M7.5.

F. Jin et al. / Construction and Building Materials 51 (2014) 395–404 399

causes higher drying shrinkage. In the present study, it was foundthat the reactivity and the content of reactive MgO lead to differentcrack formation of the MAAS pastes. When reactive MgO wasadded in the AAS, it changed the pore profile by filling the poreswith more voluminous Ht, resulting in higher percentage of mes-opores in the paste, which is governed by the content of reactiveMgO added. In addition, the timing of this pore filling effect is crit-ical to the soundness and drying shrinkage of the paste, which isrelated to the reactivity of MgO. Highly reactive MgO (MgOH) pro-duced more Ht in a given time than the medium reactive grade(MgOM), filling more pores and resulting in higher early strength.It was found that 2.5% of both reactive MgO did not pose any det-rimental effect on the AAS paste in terms of cracking. When morethan 5% reactive MgO was introduced, the MgOH generated moreHt in the short term, reducing the pore size, which also increasedthe capillary force during the drying process. Meanwhile, the earlyage strength was not high enough, so the cracks formed quickly atthe surface of the samples. In comparison, when more than 5% ofMgOM was added, the content of voluminous Ht gradually in-creased, which helped to fill the cracks formed initially. In addition,the strength of the paste had developed sufficiently to bear thecapillary force thus no additional cracks formed in the later age.

3.3. Hydration products and microstructure

3.3.1. XRD resultsThe XRD pattern for the reference sample is consistent with ear-

lier studies [42–44] that the major hydration products of AAS are

C–S–H gel as well as Ht (Fig. 8(a)). The broad and diffuse peak at25–35� 2b reflects the short range order of the CaO–Al2O3–MgO–SiO2 glass structure of the slag. Compared with the dead burnedMgO, reactive MgO reacted much faster with slag to produce moreHt which is indicated by the increased strongest characteristicpeak of Ht at 2b � 11.7� with the increase of the reactivity andcontent of MgO indicating that more Ht was formed. In addition,the higher content of unhydrated MgO (indicated by the higherpeak at 2b � 42.9�) was detected in pastes with MgOM consideringits lower reactivity. Increasing the curing time did not generatenew phases, but it increased the crystallinity of C–S–H as indicatedby the sharper peak at around 2h � 29.5� (Fig. 9) [45]. It should benoted that there was no brucite present in all the pastes regardlessof the MgO and its content even at 1 day. According to [46–49], theactivation of slag initially consists of a breakdown of the covalentbonds Si–O–Si and Al–O–Si [50]. With the reactive MgO modifica-tion, MgO hydrolysed on the surface and either reacts with the bro-ken Si–O or Al–O to form magnesium silicate hydrate (M–S–H) orHt, hindering the precipitation of brucite. The findings here con-firmed early studies that Mg is quickly consumed to form Ht orM–S–H in combination with silica fume or slag [39,51–53],although M–S–H is hard to be detected by XRD [54].

3.3.2. TGA resultsTG and first derivative of TG (DTG) signs are direct and fast

measurements of the weight loss and its rate of occurrence duringanalysis, by which different materials can be identified based ontheir thermal characteristics [55]. The TG/DTG curves of pastescured for 1 day and 28 days are shown in Figs. 10 and 11. From

Fig. 8. XRD patterns of the pastes cured for 1 days (a) reference and MgOH series;and (b) MgOM series.

Fig. 9. XRD patterns of the pastes cured for 28 days (a) reference and MgOH series;and (b) MgOM series.

400 F. Jin et al. / Construction and Building Materials 51 (2014) 395–404

the DTG curves, several peaks can be observed. For temperaturesup to 250 �C, the weight loss was attributed to the dehydrationof C–S–H and M–S–H [53,56,57]. The temperature range of 250–500 �C denotes the decomposition of Ht, since brucite was foundto be totally consumed by the reaction with slag from the XRDanalysis. The small peak at around 520–570 �C was attributed tothe loss of coordinated water in M–S–H [58], which is consistentwith a previous study [53]. The temperature range of 600–800 �Cis the decomposition range of various carbonate-containing phasesincluding Ht [59], magnesium carbonate [60], and calcium carbon-ate [61], originating from the raw material and the carbonation dueto exposure to the air. Generally, all the weight losses increasedwith the increase of MgO content and the curing time, which isattributed to the higher hydration degree thus more hydrationproducts formed. In addition, with the same amount of reactiveMgO addition, MgOH gave higher values for all of the three vari-ables than MgOM.

The total weight loss (indicating chemically bound water con-tent) and the bound water content in C–S–H are often used as ameasurement of the hydration extent of blended cements withslags [10,62,63]. The calculated weight losses from TG data weresummarised in Table 3, where the total weight loss was denotedas Dm. It can be seen that MgOH addition increased the hydrationdegree significantly at 1 day while MgOM barely changed it, whichindicated that MgOH accelerated the hydration remarkably whileMgOM had little effect. Therefore, the increase of the early strengthby MgOH addition was attributed to the acceleration of slag hydra-tion while the decrease by MgOM addition was ascribed to the dilu-tion effect (see Fig. 3). At 28 day, both MgO increased the hydrationdegree with MgOH exhibiting a more profound influence.

3.3.3. SEM resultsFig. 12 shows typical SEM images of the MAAS pastes after

demoulding at 1 day. The early hydration product was mainlyreticulated C–S–H gel covering the slag particles for the referencesample, H2.5 and MgOM mixes. The porous structure observedindicated their low strength. In contrast, higher than 5% MgOH

addition (H5 and H7.5) resulted in a significant change to themicrostructure at 1 day (Fig. 12(c) and (d)), appearing to be denseC–S–H gels, which generated high chemically bound water con-tent, as shown above (Table 3). The gels have occupied the spacesthat were initially filled with water and generated a more compactmicrostructure, which further led to increased strength as pre-sented above. Clearly, sufficient addition of MgOH accelerated theearly age hydration of the slag and caused a denser structure ofthese two mixes than the other mixes. It is consistent with theTGA data that MgOH increased the early hydration significantlywhile MgOM exhibited almost no effect. The mechanism for theacceleration of slag hydration by addition of highly reactive MgOcould be attributed to the fast heat release during the dissolutionprocess of MgO. Martin [64] found that the heat of hydration fromthe highly reactive MgO paste was >25 times higher than that of aless reactive MgO. It was measured that the highly reactive MgOpaste’s temperature increased to �100 �C in 30 min. After 28 days,all mixes formed dense microstructure consisting primarily ofC–S–H gels. In addition, small cracks were visible for all the mixes.Fibrous Ht was detected in H2.5 blend growing on the C–S–H gel(Fig. 13(b)). Surprisingly, cubic crystals resembling hydrogarnet(C3AH6) were observed in M2.5 mix (Figs. 12(e) and 13(e)), whichis only reported in Glukhovsky et al.’s [65] early work onalkali-activated slag paste. It was claimed that the formation of

Fig. 10. TG/DTG for the pastes cured after 1 day. Fig. 11. TG/DTG for the pastes cured after 28 day.

Table 3Weight loss calculated from TGA.

Blend Weight loss (wt%)

1 day 28 day

C–S–H/M–S–H Dm C–S–H/M–S–H Dm

C 5.25 10.48 6.57 13.17H2.5 6.33 15.03 7.78 15.97H7.5 7.02 16.78 8.18 17.05M2.5 5.28 10.35 7.24 13.89M7.5 5.70 12.31 7.08 14.62

F. Jin et al. / Construction and Building Materials 51 (2014) 395–404 401

hydrogarnet proceeds very slowly at room temperature and is notexpected to be present in significant quantities at early age [66].The presence of this phase in M2.5 paste and the role that reactiveMgO plays in its formation is unclear and requires furtherinvestigation.

3.3.4. EDS resultsAccording to [42,43,67], the C–S–H produced in AAS is mainly

constituted of imperfect structures of tobermorite layers joint toisolated layers of calcium magnesium aluminate hydrate((C,M)4AH13) intimately mixed in their structures, with interlayercations of H2O and OH�. In addition, hydrotalcite-like phases havebeen reported to form in many studies [32,67,68]. To determinethe elemental compositions of the hydration products, EDS wereperformed on 4–7 points selected on the gels in each sample curedfor 28 days at a magnification of 2500. Fig. 14 plots the Mg/Ca vs.Al/Ca ratios of the gels. Extrapolating the straight line to Mg/Ca = 0 gives a positive number, indicating that the C–S–H gel alsocontains a portion of aluminium either in solid solution withinthe C–S–H structure or in an AFm form finely intermixed with it[68]. It was also found that Mg/Al of the Ht was highly dependenton the initial slag composition and for the reference sample thiswas found to be 1.65 and Al substitution was 0.26 (Table 3). BenHaha et al. [69] concluded that in AAS pastes, the Mg/Al ratio inHt decreased as the Al content (in slag) increased. When the initialMg/Al was 0.6, the Mg/Al of the hydration products was found tobe 1.38 and the intercept was 0.27. Note that the initial Mg/Al is

�0.78 in the slag used here (Table 1). Therefore, the slightly higherMg/Al and slightly lower Al substitution in this study can be attrib-uted to the higher initial Mg/Al of the slag used.

In an earlier study [39], the authors found that in reactive MgO–GGBS blends, the Mg/Al ratio in the formed Ht increased with theincrease of reactive MgO content up to 10% in the mix. For theMAAS pastes, the Mg/Al ratio and the Al substitution in C–S–Hwas summarised in Table 4. It was found that the addition of reac-tive MgO in AAS changed the composition of the gels significantly,while the effect was highly dependent on the content and the typeof the reactive MgO. For the highly reactive MgO (i.e., MgOH), Mg/Al and Al substitution increased slightly when its content was low-er than 5%, which agreed with an earlier study [39] that low con-tent of reactive MgO did not change the Mg/Al too much.

Fig. 12. SEM images of the pastes cured for 1 day (a) C; (b) H2.5; (c) H5; (d) H7.5;(e) M2.5; (f) M5; and (g) M7.5.

Fig. 13. SEM images of the pastes cured for 28 days (a) C; (b) H2.5; (c) H5; (d) H7.5;(e) M2.5; (f) M5; and (g) M7.5.

402 F. Jin et al. / Construction and Building Materials 51 (2014) 395–404

However, 7.5% MgOH addition caused a significant increase of Mg/Al and Al substitution. The increase of Mg/Al is consistent with[39,51] that high content of reactive MgO generates Ht with Mg/Al over 2. Meanwhile, the increase of Al substitution can be attrib-uted to the accelerated hydration of slag by adding this type ofhighly reactive MgO, leading to more broken Al-O bonds, whichwas supported by TGA and SEM results. On the other hand, themedium reactive MgO (i.e., MgOM) only slightly increased theMg/Al up to 1.35 when MgO content was 7.5%, which seemed tocorrespond to 5% of MgOH addition, considering its lower reactivityand thus less availability of Mg compared to MgOH at the same cur-ing time. In the meantime, the Al substitution decreased with the

Fig. 14. Mg/Ca vs. Al/Ca for the pastes cured for 28 days.

Table 4Calculated parameters from EDS results in Fig. 14.

Mg/Al Al substitution R2

C 1.65 0.26 0.92H2.5 1.08 0.18 0.90H5 1.33 0.19 0.49H7.5 2.12 0.23 0.91M2.5 1.24 0.31 0.96M5 1.28 0.20 0.95M7.5 1.35 0.19 0.72

increase of MgOM content, which is attributed to the increased Alcontent in Ht.

4. Conclusions

Based on the results and discussion presented above, the fol-lowing conclusions can be drawn:

F. Jin et al. / Construction and Building Materials 51 (2014) 395–404 403

(1) The major hydration products of MAAS are C–S–H andhydrotalcite-like phases (Ht), which are similar to the con-ventional AAS control mix, without reactive MgO as an addi-tive, although with more Ht formed by the reaction betweenreactive MgO and slag.

(2) The mechanical and shrinkage properties of MAAS are highlydependent on the curing condition and the reactivity of theMgO. MgO with high reactivity accelerated the early hydra-tion of AAS, while MgO with medium reactivity had littleeffect.

(3) In the water curing condition, highly reactive MgO producedHt quickly, filling the macropores in a short time and hencethe strength increased with the increase of MgO content. Incomparison, medium reactive MgO hydrated much slowerresulting in a gradual increase of formed Ht content, whichwas found to decrease the strength with the increase ofMgO content at later age since too much voluminous Ht cre-ated more cracks in the matrix, violating the soundness ofthe paste.

(4) In the air curing condition (RH = 50%), due to the accelerat-ing effect of highly reactive MgO, hence higher earlystrength and quicker change of macropores to mesopores,both the mass loss and shrinkage were reduced significantly.However, it was also found that a higher than 5% content ofhighly reactive MgO showed more cracks which is attributedto higher capillary force developing in the refined pores. Incomparison, medium reactive MgO only decreased shrink-age after the one month, but it was observed that the initialcracks were gradually healed by the hydration products,which can be ascribed to the slower formation of volumi-nous Ht.

(5) The Ht was found to intermix with C–S–H gels. The elemen-tal composition of the gels depends on the MgO type andcontent as an additive. Mg/Al ratio increased with the addi-tion of the reactive MgO. Al uptake in C–S–H increased withthe increased content of the highly reactive MgO, while itdecreased with the increased content of the medium reac-tive MgO.

Acknowledgements

The authors are grateful to Cambridge Overseas Trust (COT) andChina Scholarship Council (CSC) for their financial help of the PhDstudentship for the first author.

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