Using Isothermal Calorimetry to Assess the WaterAbsorbed by Fine LWA during MixingJavier Castro1; Igor De la Varga2; and Jason Weiss, M.ASCE3

Abstract: Fine lightweight aggregate (LWA) is being increasingly used for the manufacture of internally cured concrete. Internally curedconcrete can be viewed as a concrete that contains fluid (water) in the pores of the LWA (or other porous inclusions) that can be released to thepaste after setting. Most research performed on internally cured concrete assumes the LWA has been prewetted for some time before mixing(generally lab studies use 24-h soaking, whereas this value varies in practice). The research described in this paper investigated the potentialfor using aggregate that begins the mixing process from a different moisture state (i.e., not 24-h prewetting). Specifically, this paper considersoven-dry aggregate as a worst-case scenario to determine how much water would be absorbed by the LWA during the mixing and placementprocesses. To determine the amount of water absorbed by the LWA, isothermal calorimetry was used. Two different mixing scenarios wereinvestigated, showing that 56–71% of the 24-h water absorption could be achieved using the worst-case scenario of oven-dry aggregates.Although the use of oven-dry aggregate would be extremely rare for a field application (except for possible application to bagged products),this approach could be extended to include fine LWAwith other initial moisture conditions. DOI: 10.1061/(ASCE)MT.1943-5533.0000496.© 2012 American Society of Civil Engineers.

CE Database subject headings: Aggregates; Curing; Cement; Hydration; Mixing.

Author keywords: Lightweight aggregate; Internal curing; LWA absorption capacity; Calorimetry; Cement hydration.

Introduction

Internal curing (IC) typically refers to the use of prewetted light-weight aggregate (LWA) (or other water-filled inclusions, such assuper-absorbent polymers or cellulose fibers) that can release waterat the correct time to provide curing water throughout the crosssection of concrete. The idea that LWA can provide moisture tothe cement as it hydrates in concrete has been known for more than5 decades (Klieger 1957). Bloem (1965) reported that “highabsorption lightweight aggregate (LWA) may have the beneficialeffect of supplying curing water internally.” Philleo (1991) dis-cussed the potential for improved strength and durability as a resultof the beneficial release of water from LWA. Holm et al. (2004)reported anecdotal evidence of reduced plastic shrinkage crackingin LWA mixtures during high-rise construction. Although these ob-servations have been made on the benefits of LWA for improvedcuring, it is only recently that the use of LWA has been specificallydesigned to improve the curing of concrete (Bentz and Snyder1999; Jensen and Hansen 2001; RILEM 2007; Lopez et al.2009; Henkensiefken et al. 2009; Castro et al. 2012). This

development is owing primarily to issues associated with increasedobservances of cracking in higher-strength, lower water-to-cementratio (w∕c) concrete (Weiss 1997; RILEM 2003).

The mixture proportioning approach for IC concrete uses amethodology proposed by Bentz et al. (2005). This methodologyestimates the volume of IC water by relating it to volume of thechemical shrinkage using Eq. (1)

MLWA ¼ Cf × CS × αmax

S × ϕLWAð1Þ

where MLWA = mass (kg∕m3) of LWA (in a dry state) that needs tobe prewetted to provide water to fill in the voids created by chemi-cal shrinkage; Cf = cement content (kg∕m3) of the mixture; CS =chemical shrinkage of the cement (mL of water∕g of cement),which is considered 0:064 mL∕g of cement; αmax = expectedmaximum degree of hydration, which owing to space limita-tion is considered to be ðw∕cÞ∕0:36 for w∕c less than 0.36 and1 for w∕c higher than 0.36; ΦLWA = absorption capacity(kg of water∕k go dry LWA) of the LWA (taken normally asthe 24-h absorption value); and S = expected degree of saturationof the LWA, expressed as a function of the taken absorption value(ΦLWA). In the case of a mortar prepared with a w∕c ¼ 0:30, asdescribed in this paper, the maximum degree of hydration willbe 0.83 (0:30∕0:36).

It is well known that the absorption of water by the LWA takesplace over time (Landgren 1964; Holm et al. 2004). Castro et al.(2011) reported a test to determine the time-dependent waterabsorption and S values from the most widely used LWAs in NorthAmerica. Although this research proposed a generalized equationto describe the rate of absorption, it was shown that LWA absorbs65–80% of the 24-h absorption during the first 5 min of beingsoaked in water. It also has been reported that the water absorptioncan continue taking place over weeks, months, and even years at amuch lower rate (Landgren 1964; Holm et al. 2004). In cementi-tious mixtures, the LWA may be able to continue absorbing water

1Pontificia Universidad Católica de Chile, School of Engineering,Casilla 306, Correo 22, Santiago, Chile (corresponding author). E-mail:jecastro@ing.puc.cl

2Graduate Research Assistant, School of Civil Engineering, PurdueUniv., School of Civil Engineering, 550 Stadium Mall, West Lafayette,IN 47907-2051. E-mail: idelavar@purdue.edu

3Professor and Director of Pankow Materials Laboratory, AssociateDirector of the Center for Advanced Cement Based Materials, PurdueUniv., School of Civil Engineering, 550 Stadium Mall, West Lafayette,IN 47907-2051. E-mail: wjweiss@purdue.edu

Note. This manuscript was submitted on June 3, 2011; approved onFebruary 1, 2012; published online on February 3, 2012. Discussion periodopen until January 1, 2013; separate discussions must be submitted for in-dividual papers. This paper is part of the Journal of Materials in CivilEngineering, Vol. 24, No. 8, August 1, 2012. ©ASCE, ISSN 0899-1561/2012/8-996–1005/$25.00.

996 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / AUGUST 2012

from the mixture before set (Trtik et al. 2011), but as the LWAis added to the batch in a prewetted condition, this additionalabsorption is normally not considered to be significant for designpurposes. However, the concept of the aggregate absorbing waterbefore set will become relevant if the LWAwas added to the mixerfrom a dry condition.

As mentioned, internally cured concretes are generally designedto be cast using prewetted LWA. In discussions with local industryexperts regarding the implementation of IC, some concern wasraised by some concrete producers regarding the potential chal-lenges of batching of wet, fine LWA. This question was the resultof a potential concern that overwetted fine LWA may be difficult tohandle in a plant because it may “arch” and “stick” in the aggregatebins. Similar concerns have been expressed by some producerswith handling overwetted LWA during freezing conditions, whichcan occur in some winter operations. Although some producershave reported no problems, some found that increasing the volumeof the charge in the aggregate bins can help reduce the arching(the increased mass helps to overcome arching), and others mayprefer to use the LWA in a less wet state. If the aggregate wereused in a drier state, some have raised questions whether thismay be able to provide the expected benefits of IC. A recent study(Golias et al. 2012) measured the properties of IC concrete usingLWA with different amounts of water (i.e., different S values).When the mixture proportioning was adjusted to account for theactual amount of water absorbed, IC was found to be effective evenif oven-dry aggregates were used. This work seeks to provide animproved understanding of the water that may be absorbed by theLWA during mixing. This paper discusses a worst-case scenario inwhich the LWA that is added to the batch is from a dry condition.

The impetus for using isothermal calorimetry to determine theabsorption of an IC agent comes from the work of Johansen et al.(2009). Johansen et al. (2009) used isothermal calorimetry to studythe water absorption of pulp fibers by taking advantage of thedifferent heat signature associated with different w∕c systems.Johansen et al. (2009) used the rate of heat evolution for pulp fibers,whereas this paper uses an approach on the basis of the total heatrelease at a specific time owing to the potential effect that chemicaladmixtures may have on altering the rate of reaction at early ages inthe mixtures.

This paper describes a research program that used isothermalcalorimetry to estimate the amount of water that is absorbed bydry LWA during mixing and placement. Although this approachis used for oven-dry aggregate in this paper, this approach canbe extended to other initial moisture conditions for the aggregate.These findings can improve how internally cured concrete mixturesare proportioned, batched, and placed as this technology grows.

Experimental Program

Table 1 shows the testing matrix that was used in this investigation.Five series of mixtures were considered. The first series consistedof plain cement pastes with different w∕c. The second seriesconsiders a single w∕c; however, the dosage of a water-reducing(WR) admixture was varied. The third series consisted of plainmortars with different w∕c. The fourth and fifth series are internallycured mortar mixtures. The fourth series considers mixtures inwhich the LWAwas allowed to absorb water for 24 h before mixing(referred to as prewetted). The fifth series of mixtures contains aconstant volume of oven-dry LWA, but different water absorptionsduring mixing are assumed.

Constituent Materials

This study used ASTM C150 (2011) Type I ordinary portland ce-ment (OPC), with a Blaine fineness of 370 m2∕kg and an estimatedBogue phase composition of 56% tricalcium silicate (C3S), 16%dicalcium silicate (C2S), 12% tricalcium aluminate (C3A), 7%tetracalcium aluminoferrite (C4AF) and a sodium oxide (Na2O)equivalent of 0.68% by mass. The sand used was natural river sand,with a fineness modulus of 2.71, an apparent specific gravityof 2.58, and water absorption of 1.0% by mass. Portions of thenormal-weight sand were replaced with expanded shale (Buildex,Marquette,KS), with a fineness modulus of 3.10 and a specificgravity of 1.45. The 24-h absorption of the LWA was determinedto be 17.5% according to the paper towel method [New YorkDept. of Transportation (NYDOT) 2008; Castro et al. 2011]. Apolycarboxylate-based high-range WR admixture (Glenium3030, BASF, Cleveland) was added at variable dosage by massof cement to maintain the same (mini) slump in all mortars. Slumpwas measured following the procedure of Kantro (1980). Mixtureproportions are shown in Table 1. Sieve analysis of the normal-weight aggregate and LWA is shown in Table 2.

Mixing, Casting, and Sample Preparation

The mixing procedure used for the cement paste and plain mortars(without LWA) was in accordance with ASTM C305 (ASTM 2006).For the internally cured mortars prepared with prewetted LWA (de-fined as LWAmortars), the LWAwas oven dried, air cooled, and thensubmerged in water for 24� 1 h before mixing. The volume of waterused to submerge the LWA included both mixing water and the waterthe LWAwould absorb in 24 h. The excess water (water not absorbedin 24 h) was then decanted and used as the mixing water. For theinternally cured mortars prepared with dry LWA (defined as dryLWAmortars), the LWAwas oven dried and air cooled before mixing.Then two different procedures were used for mixing. In the first mix-ing procedure (Mixing Procedure 1), all of the dry materials werepremixed in the mixer, then the water was added, in which casethe water is simultaneously being absorbed by the aggregate and re-acting with the cement. In the second mixing procedure (Mixing Pro-cedure 2), the dry LWAwas allowed to soak in the water in the mixerfor 5 min before being put in contact with the normal-weight sand andthe cement. For all mixtures, the normal-weight sand was oven driedand cooled for 24 h before mixing. The water and cement were con-ditioned for 24 h at room temperature before mixing.

Testing Protocols

Surface Dry Absorption

As previously mentioned, it has been reported that water absorptionby the LWA can continue taking place over weeks, months, andeven years at a much lower rate (Landgren 1964; Holm et al.2004); Holm et al. (2004) stated that because the prewetting ofLWA will fill only a fraction of the LWA pores, “the use of theASTM expression ‘saturated surface dry (SSD)’ is inappropriatefor LWA, theoretically inaccurate and analytically misleading.”The authors fully agree with this statement and will use the term“surface dry” (SD) throughout this paper.

To determine the SD condition for prewetted LWA, the papertowel method was used. The paper towel method uses a test pro-cedure from the NYDOT (2009). The paper towel method involvesimmersing the aggregate in water [two conditions were used, eithersubmerged in water for 24 h or under vacuum for 1 h at 98.2 kPa(737 mmHg), followed by keeping the aggregate under waterfor the following 23 h], after which time the water is decantedand the surface of the aggregate is patted dry. After prewetting,

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the aggregate was spread out, and the paper towel is laid across thesurface of the aggregates. This process of prewetting the aggregateand using the paper towel to determine whether surface moisture isstill present is repeated at different moisture contents (preferablenear the SD condition). Once it appears that the paper towel isno longer picking up moisture from the aggregate (as determinedby visual inspection for a change in color from the paper towel) it isassumed that an SD condition has been reached, and the aggregatemoisture can be determined.

Time-Dependent Absorption

To evaluate the absorption of water as a function of time, a volu-metric flask test was conducted (Castro et al. 2011). In this test, theLWA (entire gradation shown in Table 2) is prepared by placing it inan oven at 105� 2°C for 24� 1 h and is then cooled for 24 h. A100� 10 g of sample is placed in a 250-ml volumetric flask.Deionized water is added to approximately 80% of capacity ofthe flask. The flask is then manually agitated for 2–3 min to min-imize entrapped air bubbles between the aggregate particles. Afteragitation, the fine particles are allowed to settle to see the level ofthe fluid on the flask. Additional water is added to bring the waterlevel in the flask to its calibrated capacity. The total mass of theflask, aggregate sample, and water is recorded 5 min after the waterwas first in contact with the aggregate. As the aggregate continuesabsorbing water over time, the level of the fluid will decrease.Additional water is then added to the flask to keep the fluid at itscalibrated capacity at regular intervals, recording the total mass andthe actual time of measurement. Each time before water is added,the flask is agitated for approximately 30 s to eliminate new airbubbles.

The procedure described in the previous paragraph does not cap-ture the water absorption during the first 5 min after water is placedin contact with the aggregate. However, by assuming the absorption

Table 1. Mixture Proportion of Mixtures Used in This Research

Mixture designation w∕cCement(kg∕m3)

Water(kg∕m3)

Normal-weightaggregate, saturatedsurface dry (kg∕m3)

Lightweightaggregate, dry

(kg∕m3)

Water inLightweightaggregate(kg∕m3)

Water reducer(g∕100 g ofcement.)

Paste 0.30 0.30 1,615.6 484.7 — — — —Paste 0.33 0.33 1,540.8 508.5 — — — —Paste 0.36 0.36 1,472.5 530.1 — — — —Paste 0.39 0.39 1,410.1 549.9 — — — —Paste 0.42 0.42 1,352.7 568.1 — — — —Paste 0.45 0.45 1,299.9 584.9 — — — —Paste 0.50 0.50 1,220.3 610.2 — — — —Plain mortar 0.30–0.0% 0.30 727.0 218.1 1,429.7 — — —Plain mortar 0.30–0.6% 0.30 727.0 218.1 1,429.7 — — 0.6

Plain mortar 0.30–1.2% 0.30 727.0 218.1 1,429.7 — — 1.2

Plain mortar 0.30 0.30 727.0 218.1 1,429.7 — — 0.6

Plain mortar 0.33 0.33 693.3 228.8 1,429.7 — — 0.4

Plain mortar 0.36 0.36 662.6 238.6 1,429.7 — — 0.2

Plain mortar 0.39 0.39 634.5 247.5 1,429.7 — — —Plain mortar 0.42 0.42 608.7 255.7 1,429.7 — — —Plain mortar 0.45 0.45 584.9 263.2 1,429.7 — — —Plain mortar 0.48 0.48 562.9 270.2 1,429.7 — — —Plain mortar 0.51 0.51 542.5 276.7 1,429.7 — — —Lightweight aggregate mortar 0.27 0.27 764.2 206.3 1,053.1 209.6 36.7 0.8

Lightweight aggregate mortar 0.30 0.30 727.0 218.1 1,031.5 221.6 38.8 0.6

Lightweight aggregate mortar 0.33 0.33 693.3 228.8 1,012.0 232.4 40.7 0.4

Lightweight aggregate mortar 0.36 0.36 662.6 238.6 994.2 242.3 42.4 0.2

Lightweight aggregate mortar 0.39 0.39 634.5 247.5 1,012.7 232.1 40.6 —Lightweight aggregate mortar 0.42 0.42 608.7 255.7 1,029.7 222.6 39.0 —Lightweight aggregate mortar 0.45 0.45 584.9 263.2 1,045.3 213.9 37.4 —Dry lightweight aggregate 50% 0.30 727.0 218.1 1,031.5 221.6 19.4 0.6

Dry lightweight aggregate 55% 0.30 727.0 218.1 1,031.5 221.6 21.34 0.6

Dry lightweight aggregate 60% 0.30 727.0 218.1 1,031.5 221.6 23.28 0.6

Dry lightweight aggregate 65% 0.30 727.0 218.1 1,031.5 221.6 25.22 0.6

Dry lightweight aggregate 70% 0.30 727.0 218.1 1,031.5 221.6 27.16 0.6

Dry lightweight aggregate 80% 0.30 727.0 218.1 1,031.5 221.6 31.04 0.6

Table 2. Gradation of Lightweight (LWA) and Normal-Weight (NWA)Aggregates Used in this Research

LWA

NWA

Sieve number Percentage passing

4 98.1% 99.7%

8 84.8% 93.0%

16 56.4% 75.6%

30 34.0% 49.8%

50 20.4% 19.3%

100 11.9% 2.7%

Fineness modulus 2.94 2.60

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value at 24 h is the same as the 24-h absorption obtained bythe paper towel method, the difference between both values canbe used to back-calculate the absorption that occurs during thefirst 5 min of this test.

Lightweight Aggregate Desorption

The loss of water from the LWA pores during drying can bedescribed through a desorption isotherm [a plot of normalized massloss as a function of relative humidity (RH) at a constant temper-ature]. A dynamic vapor desorption analyzer was used to enable thesorption behavior to be evaluated under carefully controlledtemperature and humidity conditions. Additional information aboutthe procedure can be found in Castro et al. (2010, 2011).

For the dynamic vapor desorption tests, aggregate particles thatpassed the #8 sieve and retained on the #16 sieve were used. TheLWA particles were prepared by placing them in an oven at 105�2°C for 24� 1 h, cooled for 24 h, and placed in deionized water forthe desired time before testing, which was 6 h, 24 h, and 9 days inthis study (the same aggregate particles were used in each of thethree tests). Once the prewetting of the aggregate was complete, theLWAwas removed from water, “patted to SD” using absorbent tow-els, and placed in a tared 180-mL quartz pan. The pan containingthe sample was then suspended from the balance (�0:001-mg ac-curacy) and placed in the RH chamber to equilibrate at 23:0�0:05°C and 98� 0:1% RH for up to 48 h or until the samplehad achieved a stable mass (mass change < 0:001%∕15min).After the sample equilibrated, the RH in the chamber was changedin 1% RH steps to 80% RH, allowing the sample to equilibrate atthe new humidity (mass change < 0:001%∕15min or 48 h ofexposure, unless noted otherwise). After equilibrating at 80%RH, the samples were dried to 0% RH at 23� 0:05°C for up to48 h or until the sample had achieved a stable mass.

Although the desorption analysis was performed on one sizeaggregate in this study, it has been shown that the desorption ofthis aggregate corresponds well with a full gradation of aggregates,as those shown in Table 2 (Pour-Ghaz et al. 2011).

Isothermal Calorimetry

The IC supplies additional water that can increase hydration of thecement (Henkensiefken et al. 2009), and densify the microstructureat the surface of the LWA (Elsharief et al. 2005; Peled et al. 2010).To measure the potential increase in hydration, an isothermalcalorimeter (TAM Air by AS Instruments) was used as previouslyreported by Pane and Hansen (2005), Poole et al. (2007), Johansenet al. (2009), and Lura et al. (2010). Approximately 10 g of exter-nally mixed mortar were placed into each vial, with two replicatesfor each mixture. The cumulative heat of hydration was measuredduring the first 7 days after mixing. Detailed instructions about theuse of the isothermal calorimeter can be found in the equipmentoperator’s manual (TA Instruments 2008).

Experimental Results

Aggregate Absorption and Desorption

Fig. 1 shows the normalized water absorption of the LWA (normal-ized to the 24-h absorption) as a function of time. According toFig. 1, the LWA absorbs nearly 63% of the 24-h absorption inthe first 5 min of being soaked under water. The y-axis for Fig. 1can be thought of as the saturation coefficient that is described inEq. (1) (Castro et al. 2011). It is also shown that the 24-h absorptionof water is substantially less than the absorption determined fromvacuum saturation (98.2 kPa).

Fig. 2 shows the normalized desorption response for the LWAthat was presoaked for three different lengths of time: 6 h, 24 h, and9 days. From Fig. 2 it can be observed that a substantial portion ofthe moisture is lost at high relative humidities, indicating thatthe pore size of the aggregate is large enough that the water canbe easily given to the paste as needed during hydration. Furthermore, extension of the presoaking period produces a differencein the amount of water that is held in the pores of the LWA(absorption) at high relative humidities, but it does not appear toappreciably change the amount of water that is held in the poreat relative humidities lower than 98% (desorption).

Isothermal Calorimetry

Fig. 3 shows the measured heat of hydration during the first 7 daysfor plain paste mixtures with seven different values of w∕c (0.30,0.33, 0.36, 0.39, 0.42, 0.45, and 0.50). Fig. 3(a) shows the rate ofheat release during the first 18 h, whereas Fig. 3(b) shows thecumulative heat release. Before setting (i.e., before approximately4 h, depending on the w∕c owing to particle spacing), the rate ofheat release is very similar. Even during the first 18 h, the hydrationrates are not dramatically different (although the 0.30 mixtureappears to be slightly influenced by the admixture used, and the0.50 mixture may be slightly retarded). This is consistent withresults reported by Bentz et al. (2009) and Bentz (2010). At latertimes, however, the rate of heat release (and cumulative heat)begins to diverge, and a higher cumulative heat release is observedfor higher w∕c pastes. This increased heat can be interpreted as anincrease in the degree of hydration, which is in agreement withmeasurements on the basis of nonevaporable water reported byTaplin (1959), although Taplin’s measurements were made at later

Fig. 1. Time-dependent water absorption of lightweight aggregate

Fig. 2. Moisture desorption of lightweight aggregate presoaked fordifferent periods of time

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ages. Similar behavior has been observed by Schindler andFolliard (2005).

Fig. 4 shows the heat of hydration for a series of mortar mixtures(w∕c ¼ 0:30) that were intentionally prepared with differentamounts of WR admixture in dosages expressed as percentageof the cement mass. Differences in the dosage of WR can alter(delay or accelerate) the reaction of the cement. At early ages,the measured heat of hydration is dependent on the dose of chemi-cal admixtures; however, by an age of 72 h, all of the mixtures have

a similar heat release (heat of hydration). This is important becauseit indicates that measures of the total heat of hydration at a later age(e.g., > 48 h) may be less prone to variations in the rate of heatrelease owing to the use of chemical additives. By reducing thepotential influence of chemical admixtures on the value used todetermine absorption, the later age value may (for some cases inwhich the admixture dosage varies) provide information that is amore direct representation of the w∕c when using calorimetry todetect the amount of water absorbed by LWA.

Fig. 5 shows that when a hydrating system receives additionalwater from curing (either internal or external), it generatesadditional heat, which can be interpreted as additional cementhydration. Fig. 5 also shows the measured heat of hydration fora plain cement paste and a plain mortar mixture, both prepared witha w∕c of 0.30 and without additives. The mixtures show a verysimilar behavior. These data enabled confidence that gooddispersion was obtained in the mixing procedure.

Fig. 5 also shows the heat of hydration for a plain mortar andan internally cured mortar (LWA mortar) mixture, both preparedwith a w∕c of 0.30 and without WR additive. At early ages, thehydration is similar. However, the mixture with LWA demonstratesslightly more hydration after approximately 12 h, which can beinterpreted as an increase in the degree of hydration. This isconsistent with the findings of Geiker (1983) and Lura et al.(2010).

Fig. 5 shows the measured heat of hydration for a cement pasteand a cement paste with additional curing water on the top. Bothmixtures were prepared with a w∕c of 0.30 and without WR addi-tive. At early ages, the hydration is similar. However, the mixturewith water on the top demonstrates greater hydration after approx-imately 24 h, which can be interpreted as an increase in the degreeof hydration.

In Fig. 5, the second y-axis shows the cumulative heat of hydra-tion normalized by the theoretical maximum heat of hydration. Thismaximum theoretical value (476 J∕g of cement) was calculatedconsidering the heat released by the hydration of the cementcompounds (measured in the cement), as reported by Benstedand Barnes (2002). The values used for each of the primarycement compounds were C3S ¼ 490 J∕g, C2S ¼ 225 J∕g,C3A ¼ 1160 J∕g, and C4AF ¼ 375 J∕g.

Figs. 6 and 7 show the measured heat of hydration for sealedmortars made using normal-weight aggregate and sealed mortarsmade with IC. The results show that initially (before approximately18 h), all the mixtures demonstrate a similar rate of heat release,irrespective of the w∕c (again recalling that mixtures prepared with

Fig. 3. Effect of water-to-cement ratio on hydration of cement paste,expressed as (a) rate of hydration; (b) cumulative heat of hydration

Fig. 4. Effect of addition of water reducer on heat of hydration ofcement

Fig. 5. Hydration of cement on cement paste and mortar and effect ofinternal and external curing on cement hydration

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lower w∕c contain a higher dosage of WR, which may slightlyretard the cement hydration). It can be observed that after the first18–24 h, a difference begins to be noticed in the cumulative heatrelease between the different w∕c mixtures. This is similar to whatis shown in the paste specimens (Fig. 3). Mixtures with a higherw∕c show a greater heat release. This is consistent with lower w∕cmixtures having a reduction in their hydration owing to a lack ofwater and a lack of space for hydration products to form, which isalso consistent with the information shown in Figs. 3 and 5. Spec-imens with IC show increased heat release owing the additionalmixing water, as shown in Fig. 5, which is consistent with the re-sults presented by Lura et al. (2010).

Discussion of Initial Results

Observations and Hypothesis

A few observations can be drawn on the basis of the data providedin the previous section. First, although the cumulative heat releasecurves are approximately similar, irrespective of w∕c at early ages(i.e., up to approximately 18–24 h), they begin to diverge at a laterage with higher w∕c, exhibiting a higher heat release. This isconsistent with results provided by Bentz et al. (2009) and withmeasurements on the basis of nonevaporable water provided byTaplin (1959) and Mills (1966). This is likely the result of addi-tional space in the microstructure of the higher w∕c materials(i.e., more space between particles) and the increased access tofluid for hydration (i.e., a higher internal RH). Second, the systemsthat contain additional water (either through supplying water at thesurface of the sample or through IC) show a higher cumulative heatrelease when compared with mixtures of the same w∕c mixture atthe same age. This implies that the presence of additional waterhelps to promote additional hydration. This has also been noticedby Geiker (1983), Bentz and Snyder (1999), and Lura et al. (2010).Third, when chemical admixtures are used, the rate of heat releasecan be altered; however, the cumulative heat of hydration is similarat later ages (i.e., 72 h in Fig. 4).

This paper intends to estimate how much water is absorbed byan oven-dry LWA when it is added during the mixing process.On the basis of the information shown in Fig. 1, it can be assumedthat the water absorbed by the LWA in the mixing process wouldbe less than the amount of water absorbed if the LWA is stored inwater for 24 h.

Variations in Water-to-Cement Ratio and Paste Volumeon the Basis of Lightweight Aggregate AbsorptionVariations

Fig. 8 shows volumetric proportions for cases in which differentassumptions are made regarding the extent of water absorbedby the LWA and the amount of water added to the mixture.Fig. 8(a) shows an ideal proportioning condition in which it isassumed that the w∕c in the paste is maintained at 0.30 and theLWA with 17.5% absorption. Although Fig. 8(a) shows an idealcase, Figs. 8(b) and 8(c) show less-than-ideal cases.

Fig. 8(b) shows that the aggregate absorbed less water than wasassumed in the design (assume an extreme in which where no waterwas absorbed by the LWA, S ¼ 0). As a result, more water wouldremain in the paste, and the w∕c of the mixture would be higherthan the design of 0.30. In addition to the higher w∕c, the volume ofthe paste would also be higher than the designed system (i.e., themixture would overyield). Conversely, Fig. 8(c) assumes thatthe aggregate absorbs more water than what is assumed in thedesign. It can be assumed that in this case the value of S is similarlyextreme, with a value of 1.3. As a result, the volume of the paste(underyielding) would be less than the designed mixture, and thew∕c would be less than 0.30.

Fig. 9 shows how the w∕c would vary using the relative fluidabsorption of the aggregate at 24 h (S) on the basis of the aggregateproperties, chemical shrinkage, and w∕c used in this paper. It isshown that the w∕c would vary between 0.28–0.35 dependingon the amount of water absorbed by the aggregate.

Heat of Hydration: Influence of Water-to-Cement Ratioand Additional Water Supply

The cumulative heat release at 24 and 72 h is shown in Fig. 10 forplain and internally cured mortar mixtures. Although the heat re-lease at 24 h represents an approximately 37% degree of hydration,

0 3 6 9 12 15 18Time (h)(a)

(b)

Plain Mortar 0.51Plain Mortar 0.48Plain Mortar 0.45Plain Mortar 0.42Plain Mortar 0.39Plain Mortar 0.36Plain Mortar 0.33Plain Mortar 0.30

0

1

2

3

4

5

6

Rat

eo

fH

ydr a

tio

n(1

0-3 W

/ g

of

cem

ent)

0 12 24 36 48 60 72

Time (h)

Plain Mortar 0.51Plain Mortar 0.48Plain Mortar 0.45Plain Mortar 0.42Plain Mortar 0.39Plain Mortar 0.36Plain Mortar 0.33Plain Mortar 0.30

0

50

100

150

200

250

300

Cu

mu

lati

veH

eat

of

Hyd

rat i

on

(J /

g o

f ce

men

t)

0.0

0.1

0.2

0.3

0.4

0.5 Deg

reeo

fH

y dr ati o

n(H

eat/Max. T

heo

retical Heat)

Fig. 6. Effect of water-to-cement ratio on hydration of plain mortarsystems, expressed as (a) rate of hydration; (b) cumulative heat ofhydration

Fig. 7. Effect of water-to-cement ratio on hydration of internal curedmortar systems

JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / AUGUST 2012 / 1001

it is similar for all of the mixtures, irrespective of the w∕c or if IC isused. The mixtures with a higher w∕c have a greater amount of totalheat release at 72 h. Although the cumulative heat of hydrationincreases with increasing w∕c, a plateau begins to develop forhigher values of w∕c. This plateau begins to shift to the right atlater ages (Taplin 1959). In addition to w∕c, the heat of hydrationis related to the amount of IC water for the samples at later ages.

Heat of Hydration: Role of Water Volume

Although it may be tempting to assume initially that the waterabsorbed during mixing would immediately enable complete ICto occur, it must be remembered that a sufficient volume of ICwater must be supplied, and the water must be well distributedthroughout the paste. This can be shown quantitatively in Fig. 11that when no water is available for IC, the cumulative heat is the

Fig. 8. Effect of lightweight aggregate absorption on volume proportion of internally cured mortar systems

Fig. 9. Effect of lightweight aggregate absorption on effective water-to-cement ratio

0.24 0.27 0.30 0.33 0.36 0.39 0.42 0.45 0.48 0.51 0.54

water to cement ratio

LWA Mortar at 72 hPlain Mortar at 72 hLWA Mortar at 24 hPlain Mortar at 24 h

100

150

200

250

300

350C

um

ula

tive

Hea

to

fH

ydra

tio

n(J

/ g o

f ce

men

t)

0.20

0.30

0.40

0.50

0.60

Deg

ree of H

ydratio

n(H

eat/ Maxim

um

Th

eoretical H

eat)

Fig. 10. Plot of cumulative heat release at 24 and 72 h as a function ofwater-to-cement ratio

0 25 50 75 100 125 150 175Volume of Internal Curing Water / g of Cement (%)

Plain Mortar, without LWAIC Mortar, 24h soak LWADry LWA Mixtures, assuming 55% absorption

230

240

250

260

270

Hea

to

fH

ydra

tio

n(J

/ g

of

cem

ent)

Fig. 11. Plot of cumulative heat release at 72 h as a function of water-to-cement ratio

1002 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / AUGUST 2012

lowest. If IC water is available, the measured cumulative heat riseswith increasing IC water. This increase occurs until the volume ofIC water and spatial distribution of this water are sufficient, atwhich point the heat of hydration appears to reach a plateau at256 J∕g for the cement used in these tests.

Determining Water Absorbed by Dry LightweightAggregate during Mixing

Test Procedure

A test procedure was developed to determine how much water wasabsorbed by the oven-dry LWA during mixing. This test procedureconsists of preparing mixtures with aggregate in two differentmoisture conditions. The first aggregate moisture condition(24 h prewetted LWA) was used in a reference mixture. The secondaggregate moisture condition was used in mixtures prepared withoven-dry LWA. The oven-dry LWA mixtures are designed assum-ing different volumes of water would be absorbed during themixing process (i.e., incrementally from 50%–80% of the 24-habsorption).

The test procedure compares the heat release in the referencemixture (the 24-h prewetted aggregate) with the heat release bythe samples made with the oven-dry LWA. To understand the pos-sible results that can be obtained in comparing the heat release fromthe mixtures mentioned previously, it is important to consider threepotential outcomes (similar to those shown in Fig. 8). First, if theoven-dry LWA absorbs exactly the amount of water that has beenassumed to be absorbed, the w∕c will be the same as the referencecase, IC will be provided, and the cumulative heat release will bethe same. The second case assumes that the oven-dry LWA absorbsless water than that assumed to be absorbed. In the second case, thew∕c of the actual mixture with the oven-dry LWA will be higherthan the reference mixture, and the cumulative heat released, forthe mixture with the oven-dry LWA will be higher than the refer-ence case. The third case assumes that the oven-dry LWA absorbsmore water than what was assumed. In the third case, w∕c in the dryLWA mixture will be lower than the reference case, and the cumu-lative heat release will be lower than the reference case. Thisimplies that if the heat release from the reference mixture is com-pared with the heat release from a series of mixtures made usingthe oven-dry LWA, there will be one case in which the heat releaseis the same, which would correspond to the correct aggregateabsorption and w∕c.

The assumption that the additional heat resulting from IC in theoven-dry LWA mixtures is the same as the one provided in theprewetted mixture is valid if the demand for IC water is lower thanthe water stored in the LWA pores. This happens if the degree ofhydration of the cement (water demand) is lower than the waterabsorbed for the dry LWA (water stored). For the cement usedin this research, at 72 h the degree of hydration of the internallycured mortar with a w∕c of 0.30 is 49% (Fig. 7). Then the dryLWA absorption calculation will be valid if the absorption is higherthan 49% of the 24-h LWA absorption. This is consistent with theresults shown in Fig. 12.

Mixing Procedures for Dry Lightweight Aggregate

The approach described in the previous section was used to evalu-ate the water absorption of dry LWA using two different mixingprocedures. Mixing Procedure 1 consisted of placing all of thedry materials in the mixer, premixing the materials, and then addingthe water to the mixture. In Mixing Procedure 2, the dry LWAwasallowed to soak in the mixing water for 5 min before it was placed

in contact with the other components of the mixture (i.e., normal-weight sand and cement).

Fig. 12 shows the heat of hydration of both procedures as a func-tion of the assumed LWA absorption (normalized by the heatreleased at 72 h for the reference mixture containing the 24-h pre-soaked LWA). Fig. 12 shows that the heat released by the dryLWA mixtures at 72 h follow linear relationships with theassumed LWA absorption. The actual aggregate absorption canbe determined when the heat release for the mixtures with thedry aggregate equals the heat release for the reference mixture(24-h presoaked LWA). This occurs when the normalized valueof heat release equals 1. Using this approach, the LWA absorptionwas determined to be 56 and 71% of the 24-h absorption for MixingProcedures # 1 and 2, respectively. The absorption obtained usingMixing Procedure 2 is higher, likely because of the time thatthe LWA had to absorb water while soaking before the cementwas added to the mixture.

Summary and Conclusions

This paper described a series of tests that evaluate the heat of hy-dration of cement paste and mortars using an isothermal calorim-eter. Cement paste prepared with different w∕c were analyzed.Results show that although the cumulative heat release curvesare similar, irrespective of the w∕c before and slightly after set, theybegin to diverge at a later age (approximately 18–24 h in thispaper), with higher w∕c exhibiting a higher heat release. This isowing to the additional space for hydration and the additionalamount of fluid available for hydration. Tests performed on mortarspecimens containing different amounts of a polycarboxylate-basedWR show that these additions can alter the rate of reaction of thecement at early ages; however, this effect seems to significantlydiminish by 72 h. By comparing the heat of hydration of mixturesprepared with dry LWAwith that of a mixture prepared with a 24-hpresoaked LWA, it is possible to determine the amount of water thatis absorbed during the mixing process by the dry LWA. The amountof absorbed water depends strongly on the mixing procedure. Forthe two mixing procedures used, it was found that the oven-dryLWA will absorb between 56% (for immediate mixing) and 71%(when the water was in contact with the dry LWA for 5 min beforemixing) of the 24-h absorption value. This is in general agreementwith the normalized absorption of the LWA at 5 min, which was

Fig. 12. Determination of the dry LWA absorption during mixing

JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / AUGUST 2012 / 1003

found to be 63%. This paper shows that aggregates with differentinitial moisture conditions can be used for IC. Mixtures that useLWA that is more dry will tend to require more LWA than mixtureswith a prewetted LWA. As such, prewetting is still recommended,except for very specific circumstances.

Disclaimer

Certain commercial products are identified in this paper to specifythe materials used and procedures employed. In no case does suchidentification imply endorsement or recommendation by PurdueUniversity, nor does it indicate that the products are necessarilythe best available for the purpose.

Acknowledgments

This work was supported in part by the Expanded Shale, Clay andSlate Institute (ESCSI) and by the Joint Transportation ResearchProgram administered by the Indiana Department of Transportationand Purdue University (Project SPR 3211), and the authors grate-fully acknowledge that support. The contents of this paper reflectthe views of the authors, who are responsible for the facts and theaccuracy of the data presented and do not necessarily reflect theofficial views or policies of the Indiana Department of Transporta-tion or ESCSI, nor do the contents constitute a standard, specifica-tion, or regulation. The experiments reported in this paper wereconducted in the Pankow Materials Laboratories at Purdue Univer-sity. The authors acknowledge the support that has made thislaboratory and its operation possible.

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