Predicting the degree of reaction of supplementary cementitious materials in cementitious pastes using a pozzolanic test

Construction and Building Materials 204 (2019) 621–630

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Predicting the degree of reaction of supplementary cementitious materials in cementitious pastes using a pozzolanic test Sivakumar Ramanathan a, Hoon Moon b, Michael Croly a, Chul-Woo Chung b, Prannoy Suraneni a,⇑ a b

Department of Civil, Architectural, and Environmental Engineering, University of Miami, Coral Gables, FL 33146, USA Department of Architectural Engineering, Pukyong National University, Busan 48513, Republic of Korea

h i g h l i g h t s
Pozzolanic test was used to develop two measures for SCM degree of reaction.
Degree of reaction increases as temperature increases but does not depend on w/cm.
Values generally agree with those from literature.
Preliminary results are promising.

a r t i c l e

i n f o

Article history: Received 25 October 2018 Received in revised form 16 January 2019 Accepted 27 January 2019

Keywords: Pozzolanicity Supplementary cementitious materials Isothermal calorimetry Thermogravimetric analysis

a b s t r a c t A pozzolanic test that can correctly identify pozzolanic materials is key in the search for alternative supplementary cementitious material (SCMs). Determination of the degree of reaction of these alternative SCMs in cementitious pastes is also important, as the reactivity of certain SCMs can be quite low. Since typical methods to determine SCM degree of reaction can be complex and laborious, the current study explores whether parameters obtained from a newly developed pozzolanic test can potentially be used to develop a simple method for determining the degree of reaction of SCMs. In the pozzolanic test, (cumulative) heat release and calcium hydroxide consumption of SCMs from a mixture of SCM and calcium hydroxide (water-to-solids ratio 0.9, pH 13.5, and testing temperature of 50 °C) are measured. Corresponding values of cumulative heat release and calcium hydroxide consumption of SCMs in a cementitious paste are also measured at two different water-cementitious materials (w/cm) ratios. The ratio between the values of cumulative heat release and the calcium hydroxide consumption in the cementitious paste and the pozzolanic test are considered to be measures of degree of reaction. Four different SCMs – class F fly ash, ground granulated blast furnace slag, metakaolin, and undensified silica fume were tested in this study. The effect of temperature and w/cm on degree of reaction were assessed. Degree of reaction values obtained from this method were compared with those from obtained from a portlandite consumption method suggested in literature and from typical values suggested in literature. Good correlation (R2 = 0.87) is obtained for the degree of reaction values determined here using the calcium hydroxide ratio and the portlandite consumption method. The degree of reaction values increase as the temperature increases but do not strongly depend on w/cm. The degree of reaction values are in general agreement with the range of values obtained from literature, however, this range is rather large, and the values depend strongly on the method used to determine degree of reaction. These preliminary results are promising and suggest that this method may potentially be used to provide information about pozzolanicity and degree of reaction of various SCMs. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Supplementary cementitious materials (SCMs) are commonly used as a partial replacement for cement in concrete as they offer ⇑ Corresponding author. E-mail address: [email protected] (P. Suraneni). https://doi.org/10.1016/j.conbuildmat.2019.01.173 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

benefits such as reductions in CO2 emissions, reductions in cost, and increase in concrete durability [1–4]. Due to the extensive use of SCMs, shortfalls for the supply of conventional SCMs such as fly ash become increasingly common [5,6]. This necessitates the identification and characterization of pozzolanic activity for alternative SCMs for their successful utilization [1,4,6,7]. However, the standard test method to identify the pozzolanic activity of a

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material, the strength activity index test (ASTM C311 [8]), has a clear drawback – it can misidentify a finely ground but unreactive material as a pozzolanic material due to the filler effect (an increase in the cement degree of hydration due to the physical presence of fine material) and particle packing effects [9–16]. This can cause potential issues in assessing long-term durability of concrete because such a finely ground unreactive material may not improve durability in a manner that is similar to conventional SCMs. Therefore, the development of a test procedure that can correctly identify pozzolanic material and quantify their reactivity are crucial components for the successful use of alternative SCMs in civil engineering applications. Several pozzolanic tests have been developed and are used by researchers. These include the Chapelle test [17–20], the Frattini test [21–23], the saturated lime test [20,21], electrical conductivity measurement [24], etc. Several issues with the aforementioned tests have been noted [17,20,21,23]. A promising pozzolanic test method seems to be the use of isothermal calorimetry to monitor the heat release of SCM-calcium hydroxide blends at 40 °C and pH 13.5 (note that heat release and cumulative heat release are used interchangeably through the text) [22]. Both the heat release and bound water from such blends have been correlated with strength gain in cementitious pastes made with the corresponding SCMs [22]. A modification of this test method has been suggested using somewhat different conditions (measurement at 50 °C, a different liquid-to-solid ratio, and no added sulfate or carbonate [23,25]). The purpose of the modified test was to measure heat release and calcium hydroxide consumption of the SCM-calcium hydroxide blends and use these parameters to ‘‘classify” SCMs. Classifications of ‘‘inert”, ‘‘pozzolanic”, ‘‘more hydraulic”, and ‘‘highly pozzolanic” were suggested [23]. It is important to differentiate the pozzolanicity of SCMs from the degree of reaction of SCMs when they are used in cementitious systems. The pozzolanicity (the response in a pozzolanic test, such as the heat release or calcium hydroxide content) may be considered to be a degree of reaction in idealized conditions [23]. Determining SCM degree of reaction in cementitious pastes can be challenging due to the filler effect of the SCMs and because of low degree of reaction values. Studies have been conducted by RILEM TC 238-SCM [10,26] and by other researchers to quantify the degree of reaction of different SCMs using selective dissolution [27–31], portlandite consumption [23,32–35], scanning electron microscopy with energy dispersive spectroscopy and image analysis (SEM-IA) [28–30,36–38], chemical shrinkage [28], quantitative X-ray diffraction (QXRD) [28,32,36,39], nuclear magnetic resonance (NMR) [10], and other methods [40–42]. A comprehensive review [10] suggests that different sets of test methods need to be used for successful quantification of degree of reaction for different SCMs. For example, for slags, the combination of image analysis, NMR, and calorimetry appears to be suitable whereas for fly ashes with low iron content, NMR and image analysis appears to be suitable. One conclusion from the group of studies is that the results obtained by means of various methods are not very precise, and the accuracy was around ±5% when compared to the mean values from inter-laboratory studies [26]. The combined utilization of these methods is complex, expensive, and laborious; therefore, it would be valuable to find a relatively simple but accurate test method that can be used to determine SCM degree of reaction. This work explores whether the degree of reaction of an SCM can be determined from the pozzolanicity of the same SCM. Pane and Hansen [43] combined the use of isothermal calorimetry and thermogravimetric analysis (TGA) to determine degree of hydration and degree of reaction in cement pastes with SCMs at different temperatures. The current study uses the same techniques to determine the degree of reaction of SCMs in cementitious pastes. This is done by comparing the heat release and calcium

hydroxide consumption in cementitious pastes with the corresponding values from a pozzolanic test, which has not been attempted before to the best knowledge of the authors. Such a test may provide a relatively simple means to determine SCM degree of reaction and a means to compare pozzolanic test response with degree of reaction. Two measures of degree of reaction are obtained from this test using isothermal calorimetry and TGA and utilizing these methods may potentially result in easier ways to determine degree of reaction, at least for some researchers. The ultimate objective of such efforts is to link the pozzolanic test response to long-term concrete properties and durability, potentially through the determination of degree of reaction. A portion of the dataset reported here is used in another publication, which focuses on effects on slag on temperature rise and heat release of cementitious materials at low water-to-cementitious materials (w/cm) ratio [45]. The pozzolanic test reported here was developed earlier [23]; pozzolanic test results reported here are for materials not studied before [23]. However, the previous studies [23,45] did not consider degree of reaction, which is the focus of this study. The current study is novel as it quantifies the effect of temperature and w/cm on SCM degree of reaction using methods based on the pozzolanic test. In addition, it clearly quantifies the effect of temperature and w/c on heat release and calcium hydroxide consumption for a range of SCMs. 2. Materials and methods A Class F fly ash (FA), ground granulated blast furnace slag (SL), metakaolin (MK), and undensified silica fume (SF) were studied. The chemical compositions of these SCMs obtained by using a calibrated X-ray fluorescence device are shown in Table 1. Control cement pastes were prepared by mixing cement with distilled water at w/cm values of 0.35 and 0.50. Pastes with SCMs were prepared by replacing 20% of the cement with SCM by mass. While such replacement levels are higher than commonly used for silica fume and metakaolin, the same value was used for all SCMs to enable a better comparison of the SCMs. 2.1. Pozzolanic test on SCMs A previously published method [23] was used to determine the pozzolanicity of the SCMs. This method was chosen due to its relative simplicity and its ability to produce two differing metrics of pozzolanicity, which are the heat release and calcium hydroxide consumption. Reagent grade calcium hydroxide and SCMs were mixed at a mass ratio 3:1 with 0.5 M KOH (liquids to solid ratio of 0.9). Hand mixing was done for four minutes using a spatula as the quantity of paste mixed is small. The role of additional sulfates and carbonates in the pozzolanic test was complex and is being explored by the authors; in this study no additional sulfates and carbonates were added, and the test procedure was identical to that in literature [23]. Immediately after mixing, approximately 6–7 g of sample was transferred into a glass calorimeter ampoule and placed into an isothermal calorimeter (TAM Air, TA instruments) maintained at 50 ± 0.05 °C. The heat flow data from the calorimeter was collected for a period of 240 h after signal stabilization. Heat flow and heat release normalized to SCM mass were determined. After 240 h, the samples were removed from the calorimeter and thermogravimetric analysis (TGA) was performed on approximately 30–40 mg of the sample. For these samples, SCM reaction was not stopped, since hydration arresting procedures are generally not trivial and complicate calcium hydroxide quantification. The temperature in the TGA (TGA 55, TA instruments) was ramped at 10 °C/min from 23 °C to 600 °C in an inert nitrogen atmosphere and the amount of calcium hydroxide was measured using the tangent method [46]. Some tests were carried out till 1000 °C and confirmed that the extent of carbonation in these samples was minimal. The final and initial calcium hydroxide amounts were compared to determine the calcium hydroxide consumption normalized to SCM mass. TGA was performed on the samples within 12 h of removal of the ampoules from the isothermal calorimeter. Replicate testing showed less than 5% difference in heat release and calcium hydroxide amounts. 2.2. Cementitious pastes In order to determine the SCM degree of reaction in cementitious pastes, 40 g of pastes were hand mixed for four minutes using a spatula. Hand mixing was adapted as the sample size was small and to be consistent with the mixing regime used in the pozzolanic test. After mixing, approximately 6–7 g of sample was placed in a glass ampoule and lowered into the isothermal calorimeter and heat release data was collected for 7 days. Isothermal calorimetry was carried out at two different

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S. Ramanathan et al. / Construction and Building Materials 204 (2019) 621–630 Table 1 Chemical composition of materials obtained by XRF (expressed as % mass). Oxide

OPC

FA

SF

MK

SL

CaO SiO2 Al2O3 Fe2O3 MgO SO3 Na2O K2O

63.29 19.88 5.15 3.13 3.47 2.47 0.28 0.91

7.94 55.41 25.46 8.33 1.77 0.45 0.70 1.41

3.43 91.62 0.51 0.03 0.25 0.02 0.34 0.37

1.46 57.71 36.70 2.44 0.48 0.25 – 0.65

39.92 32.71 15.12 0.53 6.50 3.37 0.38 0.58

temperatures: 23 ± 0.01 °C (normal conditions) and 50 ± 0.05 °C (accelerated conditions). After 7 days, the sample was removed from the calorimeter and TGA was performed on approximately 30–40 mg of the sample without stopping the hydration. Similar to the pozzolanic test, hydration of the cement paste containing SCMs was not stopped, and TGA was performed on the samples within 12 h of removal of the ampoules from the isothermal calorimeter. The temperature in the TGA was ramped at 10 °C/min from 23 °C to 1000 °C in an inert nitrogen atmosphere and the amount of calcium hydroxide was measured using the tangent method [46]. Replicate testing showed less than 5% difference in heat release and calcium hydroxide amounts.

2.3. Calculation of the SCM degree of reaction The SCM degree of reaction in the cementitious pastes was calculated by calculating the ratio of the response of the cementitious paste to that of the pozzolanic system as shown in Eq. (1).

a¼

N cem N poz

ð1Þ

where a is the degree of reaction, Ncem is the heat release (J/g SCM) or calcium hydroxide consumption (g/100 g SCM) in the cementitious pastes at 7 days, and Npoz is the heat release (J/g SCM) or calcium hydroxide consumption (g/100 g SCM) in the pozzolanic test at 10 days. The first measure of the degree of reaction is the heat ratio, which is the ratio of heat release of the SCM in the cementitious pastes (at 7 days) to the heat release from the pozzolanic test (at 10 days). The second measure of the degree of reaction is the calcium hydroxide ratio, which is the ratio of calcium hydroxide consumption in the cementitious pastes to the calcium hydroxide consumption in the pozzolanic test. For cements with SCMs, a filler effect of 5% was assumed while calculating the calcium hydroxide and heat release produced by cement hydration [42,44,47]. The heat release and the calcium hydroxide consumption for the SCMs in the cementitious pastes were calculated using Eqs. (2) and (3), respectively.

Q SCM ¼

Q cementitious  Q control  f  ð1  RÞ R

CHSCM ¼

ð2Þ

w CHcontrol  f  ð1  RÞ  CHcementitious  ð1 þ cm Þ R

ð3Þ

phous portion of the SCM (g/100 g SCM), and MSiO2 is the molar mass of SiO2 (60.08 g/mol). For the calculations performed, the bulk Ca/Si ratio of the SCM is obtained from the XRF data and the SiO2 in the SCM is assumed to be amorphous.

3. Results and discussion 3.1. Pozzolanic test Fig. 1 shows the heat release and calcium hydroxide consumption from the pozzolanic test plotted against each other. The SCMs ordered in descending order of heat release are SF > MK > SL > FA. The SCMs ordered in descending order of calcium hydroxide consumption are SF > MK > FA > SL. These results are in general agreement with literature [19,21,23,35]. An assumption is made here that the SCM reaction is complete at the end of the testing duration due to the excessive amount of calcium hydroxide, the temperature of testing (50 °C), the high pH (13.5), and the absence of cement. Therefore, the values obtained from heat release and calcium hydroxide consumption are assumed to be values that would be obtained from complete reaction of SCM. The fitted line shown here is not the best fit line for the data points plotted here. Rather, the best fit line is from a previous study [23] for a larger data set and does not include slag data. 3.2. Heat flow of cementitious pastes Fig. 2a and b show the heat flow of the cementitious pastes (w/cm 0.35) at 23 °C and 50 °C, respectively. The cementitious pastes ordered in descending order of peak time (the time at which the heat flow is the highest) are MK > SF > Control > SL > FA at both temperatures. This peak time ranges from 8.5 h for MK to 11.25 h for FA at 23 °C. At 50 °C, the time ranges from 1.75 h for MK to 4.25 h for FA. Cementitious pastes with MK and SF have earlier

where Q SCM is the heat released by the SCM (J/g SCM), Q cementitious is the heat release in the cementitious paste (J/g cementitious), Q control is the heat release in the control cement paste (J/g cement), CHSCM is the calcium hydroxide consumption by the SCM in the cementitious paste (g/100 g SCM), CHcontrol is the calcium hydroxide content in the control cement paste (g/100 g cement paste), CHcementitious is the calcium hydroxide content in the cementitious paste (g/100 g cementitious paste), f is the factor considered for the filler effect (here, f = 1.05), R is the SCM mass replacement w level (here, 0.20), and cm is the water-to-cementitious material ratio (here, 0.35 or 0.50). All values obtained are at 7 days. The degree of reaction values were compared with those obtained from another method by adapting the procedure stated in Durdzinksi et al. using Eq. (4) [26]:

a¼

DmCH MCH

þf



mCH;SCM MCH

f

w

þ

mlime;SCM MCaO

SiO2 ;SCM

M SiO

2




þ

mC

Ca Si CSH

2  Ca Si

 Ca

2 S;SCM

MC

2S







CSH

 100%

ð4Þ

Si SCM

where a is the degree of reaction of the SCM (%), DmCH is the difference in the amount of calcium hydroxide in the cementitious paste (with SCM) and cement paste (no SCM) (g/100 g anhydrous binder) and the vales are obtained from the TGA (for cement paste, a 5% filler effect is assumed), MCH is the molar mass of calcium hydroxide (74.09 g/mol), mlime,SCM is the free lime content in the SCM (g/100 g unreacted SCM), MCaO is the molar mass of CaO (56.08 g/mol), mC 2 S;SCM is the mass of C2S in the SCM (g/100 g unreacted SCM), MC2S is the molar mass of C2S (172.24 g/mol), (Ca/Si)C-S-H is the calcium to silica ratio of the C-S-H (here, assumed as 1.82 in pastes without SCMs), wSiO2 ;SCM is the mass of silica in the amor-

Fig. 1. Plot of heat release against calcium hydroxide consumption.

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Fig. 2. Heat flow of cementitious pastes (w/cm = 0.35) at (a) 23 °C and (b) 50 °C.

peak times as compared to the other pastes. This is likely due to acceleration of cement hydration by SF and MK due to the filler effect (although this depends on SCM fineness and replacement level [44]). FA shows a delay in peak time, compared to Control, consistent with literature [45,47,48]. Multiple peaks are observed in SL pastes. The descending order of cementitious pastes based on peak height (the maximum magnitude of the heat flow) is SF > MK > Control > SL > FA at 23 °C and SF > Control > MK > SL > FA at 50 °C. SF shows a greater peak height compared to Control, consistent with results described elsewhere [45]. Even though the peak times of Control and SF are similar, the peak height is approximately 15% greater for SF. Other SCMs show a similar or lower peak height, possibly due to the dilution of the cement. FA shows substantially lower peak height, especially at 50 °C. Fig. 3a and b show the heat flow of the cementitious pastes (w/cm 0.50) at 23 °C and 50 °C, respectively. The cementitious pastes ordered in descending order of peak time are: MK > SF > Control > SL > FA. The peak time varies from 9.75 h for MK to 12.5 h for FA at 23 °C, and varies from 2.25 h for MK to 4.75 h for FA at 50 °C. The descending order of cementitious pastes based on peak height is SF > Control > MK > SL > FA. The results are in general similar to those at w/cm 0.35. 3.3. Effect of temperature and w/cm on the heat flow All pastes show an obvious acceleration of the hydration as temperature is increased. Peak times at 50 °C are 5–8 h earlier than

Fig. 3. Heat flow curves for cementitious pastes (w/cm = 0.50) at (a) 23 °C and (b) 50 °C.

those at 23 °C. In the pastes at 50 °C, an obvious separation between silicate and aluminate peaks is not seen, consistent with literature [48–51]. The peak heights at 50 °C are 3–7 times the values at 23 °C. The peak width at 50 °C is much narrower compared to that of 23 °C, as a significant portion of the heat release occurs within the first ten hours compared to that of 23 °C. The trends observed between the various pastes seem to be independent of w/cm. The peak height for SF is 28% greater at w/cm 0.50 compared to w/cm 0.35 at 23 °C, likely due to better dispersion of the SF. For the other pastes, the difference is not as large. In general, for the two w/cm values, there are no major differences in the heat flow characteristics at a given temperature. Similar conclusions have been drawn for different w/cm values in literature [49,52]. 3.4. Heat release of cementitious pastes The (cumulative) heat release of the cementitious pastes (w/cm 0.35) at 23 °C and 50 °C are shown in Fig. 4a and b, respectively. The cementitious pastes arranged from the highest to the lowest heat release are SL > Control > MK > SF > FA at 23 °C, and SL > Control > SF > FA > MK at 50 °C. The values range from 238 to 281 J/g cementitious material at 23 °C, and 289 to 320 J/g cementitious material at 50 °C. In general, SL has the highest heat release at the end of 7 days, although MK and SF have a higher heat release in the first 24 h. The heat release values for SL are similar to those

S. Ramanathan et al. / Construction and Building Materials 204 (2019) 621–630

625

Fig. 4. Heat release for cementitious pastes (w/cm = 0.35) at (a) 23 °C and (b) 50 °C.

Fig. 5. Heat release for cementitious pastes (w/cm = 0.50) at (a) 23 °C and (b) 50 °C.

reported elsewhere [27,45,50]. The heat release of Control is lower than SL. SF shows a lower heat release, presumably due to the low w/cm and the high replacement level, consequently resulting in high water demand and reduction in cement degree of hydration. FA has relatively lower heat release, as expected [45,47]. As the temperature is increased, the heat release is increased, similar to the results from others [43,51]. The increase in heat release at 50 °C, when compared to 23 °C for the various pastes are MK (9%), SL (16%), Control (19%), FA (23%), and SF (23%), respectively. The reason for the increase in the heat can be attributed to the increase in the rates of the hydration and pozzolanic reactions at increased temperatures [45,52]. Fig. 5a and b show the heat release of the cementitious pastes (w/cm 0.50) at 23 °C and 50 °C, respectively. The heat release of cementitious pastes from the highest to the lowest is Control > SL > SF > MK > FA at 23 °C, and SL > MK > Control > SF > FA at 50 °C. The heat release ranges from 276 J/g cementitious material (FA) to 320 J/g of cementitious material (Control) at 23 °C, and from 327 J/g cementitious material (FA) to 375 J/g of cementitious material (SL) at 50 °C. SF has a higher heat release in the first 24 h. At the end of 7 days, all pastes except FA show heat release in a relatively narrow range of 306–320 J/g cementitious material at 23 °C. At 50 °C, Control, MK and SL have a higher heat release compared to SF and FA, ranging between 364 and 375 J/g cementitious material.

3.5. Effect of temperature and w/cm on heat release There is an increase in the heat release with an increase in temperature. The increase in heat release at 50 °C, when compared to 23 °C for the various pastes at w/cm = 0.35 is 14%, 18%, 8%, 16%, and 19% for SL, FA, MK, Control, and SF respectively and at w/cm = 0.50 is 17%, 16%, 16%, 12%, and 8% at SL, FA, MK, Control, and SF respectively. This increase in the heat release can be attributed to acceleration of hydration and pozzolanic reactions as stated in the previous sections. Control, FA, and SL show similar increases as the temperature increases for both w/cm values. SF shows a greater increase at lower w/cm and MK shows a greater increase at higher w/cm. As the w/cm increases, the heat release generally increases. At 23 °C, the increases in heat release when comparing w/cm 0.35 and w/cm 0.50 are 11%, 15%, 15%, 19%, and 30% for SL, FA, MK, Control, and SF, respectively. At 50 °C, the increases in heat release are 11%, 14%, 15%, 15%, and 26% for Control, SL, FA, MK, and SF, respectively. At higher w/cm, there is increased availability of water and additional space for the growth of hydrates. This is supported by conclusions from literature [53,55]. The specific trends seen for the various SCMs may be because of differences in water demand and activation energies of the SCMs. The greatest effect of w/cm is seen for SF, likely because of greater dispersion at the higher w/cm.

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3.6. Calcium hydroxide contents The calcium hydroxide contents at w/cm 0.35 and w/cm 0.50 are shown in Fig. 6a and b, respectively. At 23 °C, the descending order of calcium hydroxide content is Control > SL > FA > SF > MK (w/cm 0.35), and Control > FA > SL > SF > MK (w/cm 0.50). At 50 °C, the order is Control > FA > SL > MK > SF, at both w/cm. The values at 23 °C range from 6.54 g/100 g paste (SF at w/cm 0.35) to 11.94 g/100 g paste (Control at w/cm 0.35). At 50 °C, the values range from 0.82 g/100 g (SF at w/cm 0.35) to 14.51 g/100 g paste (Control at w/cm 0.50). In general, the highest value is seen with Control and the lowest values for SF and MK, due to a combined effect of the pozzolanic reaction and dilution. Similar conclusions are stated in literature [54].

The w/cm does not seem to have a large impact on calcium hydroxide contents at either temperature. In general, calcium hydroxide contents change by less than 1 g/100 g paste as the w/cm increases.

3.8. Degree of reaction of SCMs

As temperature increases, the calcium hydroxide content increases by 2 g/100 g paste for the Control paste. However, with SCMs, there is a decrease in the calcium hydroxide content with an increase in temperature as the pozzolanic reaction is accelerated. At 23 °C, the calcium hydroxide contents of pastes with SCMs lie in a relatively narrow range of 6.41 g/100 g paste (MK, w/cm 0.35) to 10.07 g/100 g paste (FA, w/cm 0.50). However, this range becomes broader at 50 °C, with the values being 0.82/100 g paste (SF, w/cm 0.35) to 8.63 g/100 g paste (FA, w/cm 0.35). These values show no obvious correlation with the heat release. SF and MK show the greatest impact of temperature on calcium hydroxide contents.

The degree of reaction values of the SCMs at 23 °C (w/cm = 0.35) using the two methods are shown in Fig. 7a. The SCMs ordered from the highest to the lowest degree of reaction using the heat ratio method are SL > MK > FA > SF, and the values range from 12 to 54%. The SCMs ordered from the highest to the lowest degree of reaction using the calcium hydroxide ratio method are MK > SL > SF > FA, and the values range from 11 to 24%. At the replacement level tested (20%), for w/cm 0.35, MK and SL have the highest degree of reaction values; these values are comparable and somewhat higher than values for FA and SF. While typically SF and MK are considered to be more reactive, reactivity also depends on replacement levels, and at the same replacement level, for example, a slag could have greater degree of reaction than a silica fume. For practical applications, metakaolin and silica fume are used at lower replacement levels than fly ash and slag. The low degree of reaction of SF can be attributed to the low w/cm and the high water demand of the silica fume [40,53]. In general, the values obtained using the heat ratio are greater than those obtained using the calcium hydroxide ratio, perhaps because a uniform filler effect is assumed for both these ratios. It may be that the

Fig. 6. Calcium hydroxide content for cementitious pastes at (a) w/cm = 0.35 and (b) w/cm = 0.50.

Fig. 7. Degree of reaction of SCMs for w/cm = 0.35 at (a) 23 °C and (b) 50 °C.

3.7. Effect of temperature and w/cm on calcium hydroxide contents

S. Ramanathan et al. / Construction and Building Materials 204 (2019) 621–630

filler effect factor may be different for heat release and for calcium hydroxide consumption. The degree of reaction of SCMs at 50 °C (w/cm = 0.35) using the two methods is shown in Fig. 7b. The SCMs ordered from the highest to the lowest degree of reaction using the heat ratio method are SL > FA > SF > MK, and the values range from 19 to 55%. The SCMs ordered from the highest to the lowest degree of reaction using the calcium hydroxide ratio method are SL > SF > MK > FA and the values range from 35 to 79%. As the temperature increases, the pozzolanic reaction occurs at a faster rate and hence, the degree of reaction values are higher. For the SCMs, the heat ratio increases on average 2% as the temperature increases, however, the calcium hydroxide ratio increases on average 34%. This can also be inferred from Fig. 7, as at 23 °C, the heat ratio is greater than the calcium hydroxide ratio; however, the reverse is true at 50 °C. The degree of reaction of SCMs at 23 °C (w/cm = 0.50) using the two methods is shown in Fig. 8a. SCMs ordered from the highest to the lowest degree of reaction using the heat ratio method are SL > SF > MK > FA, and the values range from 12 to 42%. The SCMs ordered from the highest to the lowest degree of reaction using the calcium hydroxide ratio method are MK > SF > SL > FA and the values range from 5 to 20%. The corresponding degree of reactions at 50 °C are shown in Fig. 8b. The SCMs ordered from the highest to the lowest degree of reaction using the heat ratio method are SL > MK > FA > SF, and the values range from 28 to 67%. Using the calcium hydroxide ratio method, the corresponding order is SL > MK > SF > FA, and the values range from 53 to 109%. Negative values and values greater than 100 are obtained for the

Fig. 8. Degree of reaction of SCMs for w/cm = 0.50 at (a) 23 °C and (b) 50 °C.

627

degree of reaction in some cases, which could be because of assumptions regarding the filler effect. This could also be because the reaction is incomplete in the pozzolanic test. As the temperature increases, the heat ratio increases on average 15%, however, the calcium hydroxide ratio increases on average 65%. As temperature increases, the heat ratio does not increase significantly. This is possibly because of acceleration in the aluminate reaction in pastes with SCMs as the temperature increases [43,57,58], which may not be fully captured by using the same filler effect value. Pane and Hansen also stated that degree of reaction values obtained from isothermal calorimetry are greater than those from thermogravimetry at 23 °C [43]. Similar observations at 50 °C are not available in literature. Comparing at the same temperature for the two w/cm values, there is no obvious correlation in the degree of reaction of SCMs. For SL at 23 °C, the degree of reaction does not increase with increase in w/cm, contrary to literature [37]. 3.9. Comparison of values with other methods and literature The degree of reaction values obtained by heat release and calcium hydroxide ratios are plotted against the ‘‘portlandite consumption” method computed using mass balance from Durdzinski et al. [26] in Fig. 9. Best fit lines to the data are also shown; these lines are fitted to pass through the origin and exclude slag data. There appears to be a strong correlation between the calcium hydroxide ratio and the ‘‘portlandite consumption” method for pozzolanic materials like FA, SF, and MK, but not for hydraulic materials (SL). In addition, the best fit line shows that the correlation is almost 1:1. The heat ratio does not show a correlation with the ‘‘portlandite consumption” method. The R2 values is 0.87 for the calcium hydroxide ratio; no linear correlation exists for the heat ratio. A strong correlation for the calcium hydroxide ratio is observed presumably because the calculations stated in Ref. [26] also rely on calcium hydroxide consumption. The absolute average difference obtained by subtracting the values obtained from the ratios and the degree of reaction from Ref. [26] is 7% for the calcium hydroxide ratio and 13% for the heat ratio. For SL, the average difference is 94%, possibly because the assumptions lead to larger errors for hydraulic materials. A direct comparison of the DoR values with literature is not trivial as these values depend on several factors such as the chemical composition of the SCMs, fineness, the replacement levels, test method

Fig. 9. Degree of reaction compared with degree of reaction calculated from literature [26].

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Table 2 Comparison of obtained values of degree of reaction of SCMs at 7 days and 23 °C with literature (r refers to SCM mass replacement level and DoR to degree of reaction). Method

Selective dissolution, Portlandite consumption, SEM, XRD [26] Selective dissolution, SEM [28] SEM [37] Isothermal calorimetry, TGA [43] TGA [58] Selective dissolution, Portlandite consumption, SEM, XRD [26] Selective dissolution, dissolution in diluted alkaline solutions, SEM [29] Isothermal calorimetry, TGA [43] Isothermal calorimetry, TGA [43] Mass balance method, Thermodynamic modeling, XRD [60] * à d

SCM, r

DOR (%)

SL, 40% SL, 40% SL, 20%–60% SL, 25% SL, 20% FA, 30% FA, 35% FA, 25% SF, 10% MK, 30%

11–45% 40–60% 10–20%* 40–70%à 42% 0–17% 0–12% 40–65%à 60–75%à 25–90%d

DOR (w/cm 0.35)

DOR (w/cm 0.50)

Heat ratio

Calcium hydroxide ratio

Heat ratio

Calcium hydroxide ratio

54%

22%

42%

4%

23%

11%

12%

5%

12% 40%

16% 24%

37% 36%

11% 20%

Values interpolated from 3 and 28-day data; the results for w/cm = 0.35 and 0.50 are presented here for comparison. The upper bound values are results from isothermal calorimetry and the lower bound values from TGA. Ternary blend containing 15% limestone and 30% calcined clay.

etc. However, a general comparison of degree of reaction values with those for these SCMs from literature is shown in Table 2. The values in literature vary widely, ranging from 11 to 70% and depend strongly on the replacement level and the method used [26]. While there is scatter, the values obtained here are in general agreement with the range of values from literature. Table 2 and Fig. 9 suggest that the methods developed here show promise, and that a pozzolanic test can potentially be used to determine the degree of reaction of supplementary cementitious materials in cement pastes. Additional testing on a wide variety of SCMs is being carried out in order to better validate the developed methods.

To address these shortcomings, further studies are being conducted to assess the corrections that need to be made for the pozzolanic test and to better quantify the filler effect. In addition, it is pointed out that this study is somewhat preliminary and is only a part of studies being carried out to compare pozzolanic testing with early and later age properties of cementitious materials. Testing is currently being carried out to extend pozzolanic results to more SCMs [59], to compare pozzolanic test results with compressive strength results, and to better explain the roles of SCM fineness and replacement level on SCM reactivity.

4. Conclusions 3.10. Limitations of the current study and further work Three major limitations of the methods proposed here should be noted: 1. The degree of reaction values obtained depend on the filler effect, which may in part explain negative values and values over 100. The filler effect needs to be quantified/calibrated for various SCM types, fineness, replacements, w/cm values, and temperatures. In addition, separate filler effects for calcium hydroxide consumption and heat release, may be required. While these approaches will increase the accuracy of the methods proposed here, they also make the method tedious. The method proposed cannot be used to study the degree of reaction of inert fillers (even though filler materials can be classified as ‘‘inert” based on the pozzolanic test). 2. It is not clear if the reaction in the pozzolanic test proceeds to completion, and this may also explain why values greater than 100 for the degree of reaction are obtained. Preliminary results suggest that this may not always be the case [59]. Quantitative x-ray diffraction and electron microscopy may be used to assess whether reaction proceeds to completion in the pozzolanic test. If the reaction in the pozzolanic test does not proceed to completion, then those effects need to be corrected for in the degree of reaction calculations. Long term pozzolanic tests are being carried out in our laboratory currently to better under the reaction progress in the pozzolanic tests. 3. Both methods result in very different values of SCM degree of reaction and it is not obvious if one is better than the other. This may be a general shortcoming of tests determining SCM degree of reaction – the value determined may depend very strongly on the test method used (which certainly seems to be the case from Table 2).

In this work, the degree of reaction of different SCMs is studied using two methods – heat ratio and calcium hydroxide ratio at two different w/cm and two different temperatures using the pozzolanic test result as a reference system where full reaction is assumed. The significant conclusions from this work are as follows:
As the temperature increases, the heat release increases due to an acceleration in the rate of reaction and the calcium hydroxide content in pastes with SCMs decreases due to an acceleration in the pozzolanic reaction.
As w/cm increases, the heat release increases, and there is a delay in the peak time in the heat flow curves. There is no obvious increase in the calcium hydroxide contents as w/cm increases.
The degree of reaction values increase as the temperature increases but do not strongly depend on w/cm. The degree of reaction values obtained by the heat ratio and calcium hydroxide ratio show correlation with values from literature. However, the values obtained from this work and from literature show significant scatter and the values appear to depend strongly on replacement level and method used to determine the degree of reaction.
While the use of the pozzolanic test appears to be promising, some limitations exist. In order to improve the results from the methods suggested, the filler effect of the SCMs should be better quantified. In addition, corrections should be made if the reaction in the pozzolanic test does not proceed to completion. Conflict of interest The authors have no conflicts of interest to declare.

S. Ramanathan et al. / Construction and Building Materials 204 (2019) 621–630

Acknowledgements The authors acknowledge funding from a grant (18TBIPC111160-03) from Technology Business Innovation Program (TBIP) funded by Ministry of Land, Infrastructure and Transport of Korean government.

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