Effect of structurally different aluminosilicates on early-age hydration of calcium aluminate cement depending on temperature

Construction and Building Materials 235 (2020) 117404

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Effect of structurally different aluminosilicates on early-age hydration of calcium aluminate cement depending on temperature Mariola Nowacka ⇑, Barbara Pacewska Warsaw University of Technology, Faculty of Civil Engineering, Mechanics and Petrochemistry, Institute of Chemistry, 17 Łukasiewicza St., 09-400 Płock, Poland

h i g h l i g h t s  The enhanced conductometric method was used to track early hydration of CAC mixes.  The non-zeolitic addition accelerates setting and hardening of CAC.  The used zeolite retards or accelerates CAC hydration depending on temperature.  The zeolite can change the ‘‘abnormal” setting of CAC depending on temperature.  The studied aluminosilicates can prevent the initial formation of hydrogarnet.

a r t i c l e

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Article history: Received 10 April 2019 Received in revised form 5 October 2019 Accepted 26 October 2019

Keywords: Calcium aluminate cement Aluminosilicate addition Curing temperature Setting and hardening Hydration products

a b s t r a c t The work is concerned with the effect of two kinds of aluminosilicate addition with zeolitic and nonzeolitic structure on early-age hydration of calcium aluminate cement at different curing temperatures. The hydration kinetics was examined by conductometry and the formed hydrates were recognized by thermal analysis, X-ray diffraction and FTIR spectroscopy. In the conductometric investigations a specific cell was designed to examine the setting and hardening of cement paste by simultaneous monitoring electrical conductivity and internal temperature versus time. It was found that the studied aluminosilicates have a different interaction mechanism in the induction stage and their influence on the kinetics of the hydration process varies with temperature. The nonzeolitic addition, regardless of the curing temperature, acts as an accelerator but the zeolitic, depending on the temperature, acts as a retarder or an accelerator of setting and hardening of calcium aluminate cement. The effect was mainly attributed to the different structure of the aluminosilicates. It was observed that both mineral additions reduce the self-heating and thus can prevent the hydrogarnet formation already during early hydration process. However, the initially formed hydration products depend principally on the temperature than on the used amount of the aluminosilicates. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction It is well known that in case of every process its conditions are determining factors in the course of reactions and thus the process results. The same goes for cement hydration process. A type of cement, quantity and quality of water, presence of mineral additions or/and chemical admixtures as well as environmental circumstances are all important for hydration and consequently properties of the hardened material. Among different cements a calcium aluminate cements (CAC) hydration is exceptionally sensitive to the process conditions, especially to curing temperature. The curing temperature decides primarily about the formed CAC ⇑ Corresponding author. E-mail address: [email protected] (M. Nowacka). https://doi.org/10.1016/j.conbuildmat.2019.117404 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

hydration phases [1–9]. According to [3]: below ca. 15 °C metastable CAH10 is produced, at about ca. 15–25 °C mixtures of metastable CAH10 and C2AH8 are formed with an increase in the quantity of C2AH8 when the temperature rises (according to abbreviations used in cement chemistry: C-CaO, A-Al2O3, H-H2O, S-SiO2, F-Fe2O3). Above ca. 25 °C to at least 40 °C, C2AH8 is the main hydration product formed together with hydrous alumina, firstly as alumina gel, which crystallizes with time to gibbsite AH3. Above 40 °C and particularly 60 °C, the hydrogarnet (C3AH6) and AH3 are formed. The above temperature ranges are rather approximate and in literature different information could be found, e.g. [8]. Although C2AH8 dehydrates to C2AH7.5 within a few hours [6], it has been mostly described as C2AH8. The CAH10 and C2AH8 are thermodynamically metastable products and they inevitably convert to the stable C3AH6 and AH3 phases with the rate depending

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essentially on temperature and also time, humidity or pH [1–9]. Another metastable C4AHx (x = 13 or 19 depending on the relative humidity) can also initially form, mainly in small amounts and its formation is relatively unimportant in relation to other calcium aluminate hydrates with regard to setting and early strength development [6]. The curing temperature is also important for CAC hydration kinetics and the ‘‘anomalous” setting behavior is observed in CAC, when the increase of the curing temperature within certain range (about 25–30 °C) results in an elongation of the setting time rather than the expected decrease [3–5]. On the other hand because of exothermic character of hydration process the internal temperature of material should be also included. It is especially significant for CAC, because the calcium aluminate cement is a fast-hardening hydraulic binder with a quick and much heat release rate in the first 24 h of hydration, which can generate considerable so-called a self-heating effect [1,5,10]. This rapidity of the process producing a characteristic rapid strength development of material leads to a great importance of the studies of early-age CAC hydration. Such studies are interesting not only due to refractory applications of CAC but also in context of its growing usage in numerous building chemistry products of often high specialization, like mixtures for fast repair, enabling the use of CAC for repair work of highways, airport’s runways, internal and external building surfaces [1,3,13,23]. Among many various research techniques employed to investigate initial hydration, a calorimetry is the most popular method allowing to monitor the heat change during the process [7,10– 12,25]. Considering an ion-nature of the hydration, the electrical conductivity measurement is valuable way to track the reactions, too [13–20,25]. The conductometry is successfully used to testing cement suspensions [13], pastes [5,14–19,25] or concretes [20], made of not only calcium aluminate cement [5,13–15,25] but also other kinds of cements [16–20]. In our previous studies [5,21] as well as in this work we take advantage of enhanced conductometric method thanks to specific designed cell enabling to monitor the electrical conductivity simultaneously with internal temperature of cement paste within time of setting and hardening. Many different mineral additions have been proposed to be used with calcium aluminate cement in literature [1,7,11,12,14,21–27]. Such action is fully consistent with the principles of sustainable development and the usage of waste gives even greater profit economically and ecologically. It is important to these minerals to be not only a filler which sealing cement matrix, but their active participation in the reactions providing efficient modification and improved properties of the binder is recommended. Most often a silica-containing minerals, e.g. silica fume, fly ash or blast furnace slag have been described as reactive in CAC hydration and thus they ensure better features of the composite [1,14,22–26]. However, to our knowledge, there are no data concerning the effect of minerals depending on their structure on setting and hardening of calcium aluminate cement at different temperatures. The aim of this work was to study the influence of two structurally different aluminosilicates (zeolite and non-zeolite) on early-age CAC hydration depending on curing temperature in the range from 6 to 60 °C with particular emphasis the setting and hardening monitored by the mentioned conductometry. Moreover, the formed hydrates were examined by thermal analysis (TG/DTG), X-ray diffraction (XRD) and infrared spectroscopy (FTIR). Taking into account a high, comparable and especially early activity of the chosen minerals proven in mixtures with calcium hydroxide and Portland cements, their reactivity at initial hydration of CAC could also be supposed and it is worth examining.

2. Experimental 2.1. Materials The studies were carried out with calcium aluminate cement Górkal 40 (called further as CAC40) produced by Górka Cement Sp. z o.o., Poland, and two kinds of aluminosilicates as an addition: waste from the petrochemical industry - spent catalyst from the fluid catalytic cracking installation (FBCC) and commercial mineral addition for cements – metakaolin (MK). For CAC40 the content of components recalculated into oxides, according to producer data [28], is: Al2O3 min. 40%, CaO min. 36%, SiO2 2–4%, Fe2O3 max. 15 mass%, and mineralogical composition is: CA as a primary phase and C4AF, C12A7 and C2AS as concurrent phases; its Blain fineness 3100–3800 cm2/g, grain size in the range 0–63 mm min. 80%, bulk density 1.1 g/cm3 and specific gravity 3.0 g/cm3. The mineralogical composition of the CAC40 determined by XRD was shown in our previous works [9,12]. Cement pastes were made with water/binder ratio = 0.5 (binder = cement + addition). The addition (FBCC or MK) was introduced as replacement part of 10, 15 or 25% by mass of cement. Their studies were carried out in comparison of reference sample with 0% of the addition (100% CAC40). The curing temperature was from the range of 6 to 60 °C. In case of hydration products studies one of the series of cement pastes stored at temperature of 25 °C was about three times larger than those in other samples enabling to get the self heating of paste similarly as in conductometry results (maximum internal temperature of 100% CAC40 paste was about 55 °C (Fig. 4b)) and this condition was marked as temperature of 25 °C++. 2.2. Methods The setting and hardening of cement pastes was monitored by conductometric method including simultaneous measurement of electrical conductivity and internal temperature of sample versus time. The experimental equipment for the conductometric measurement was described in detail in our previous work [5,21]. A weighed amount of binder was vigorously stirred with water for 1 min and then the paste was put in a measuring cell maintaining a constant volume of the sample (about 55 cm3). The measurement data of electrical conductivity and temperature of the paste were continuously recorded by the conductometer from 2 min to 1440 min after mixing the binder with water. Repeatability of the data was checked by doubling the experiments. Usage the same cell in the presented coupled measurement allowed to compare the results. The hydrates, which already formed during the first day of hydration, were recognized by X-ray diffraction method (XRD), thermal analysis (TG/DTG) and Fourier-transform infrared spectroscopy (FTIR). The cement pastes were closed in polyethylene bags and stored at definite curing temperature. Before the analysis, hydration process of the pastes was inhibited with acetone. The XRD analysis was performed by HZG–4 diffractometer (anticathode cooper CuKa, anode voltage 30 kV, anode current 25 mA) from a 2h value of 5 to 50° with a step of 0.04° and time step of 4 s. For thermal analysis TA Instruments SDT 2960 thermoanalyser (heating rate 10 °C/min in nitrogen atmosphere, mass of sample: 9–13 mg) was used. The FTIR spectra were taken with FTIR spectrometer Genesis II, produced by Mattson (4000–400 cm 1, sample preparation–pelletizing with KBr). For particle size analysis of the additions laser size analyser Cilas 1090 LD was used. The chemical composition of the aluminosilicates was marked by X-ray fluorescence (XRF) method according to standard ISO 29851-2:2010.

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3. Results 3.1. Characteristic of the studied additions The spent catalyst is a waste material (EWC code 16 08 04), while the metakaolin is a commercial product belonged to calcined natural pozzolans [29,30]. Chemical composition and some properties of FBCC and MK are presented in Table 1. The XRD and FTIR analysis of the additions are shown in Figs. 1 and 2, respectively, and their grain size distribution is presented in Fig. 3. Both the studied additions are non-hazardous, creamy-white aluminosilicates of similar chemical composition (Table 1) but differ strongly in mineralogical structure (Figs. 1 and 2) and grain size distribution (Fig. 3). FBCC grains are spherical and reach an average size of about 43 lm, while MK grains are finer and lamellar with an average diameter of about 19 lm (Table 1). MK is visibly more fine-grained with a significant advantage of the fraction: 0.04 lm 20 lm compared to FBCC, in which the fraction of 20 lm 50 lm predominates (Fig. 3). The waste catalyst retains a zeolitic structure similar to the zeolite Y of faujasite type. The metkaolin is not a zeolitic material but it has a structure of muscovite belonging to the mica group (Fig. 1). These differences are clearly confirmed by the FTIR analysis (Fig. 2). Bands with maxima at wave numbers 1205 cm 1, 835 cm 1, 611 cm 1 and 530 cm 1 corresponding to stretching and bending vibrations of the aluminosilicate framework in Y-zeolite are observed only in the FBCC spectrum [29]. On the other hand the band at 802 cm 1 characteristic of Al-O stretching vibrations in AlO4 of mica and metakaolinite, the band at 473 cm 1 corresponding to Si-O vibrations of mica and the band at 554 cm 1 corresponding to Si-O-Al vibrations of kaolinite are observed only in the MK spectrum. The band at about 1080 cm 1 corresponding to the vibration of Si-O-Si (Al) bonds is clearly visible in the spectra of both the aluminosilicates (Fig. 2). The zeolitic structure justifies tenfold higher value of the BET specific surface area of FBCC in comparison with MK (Table 1).

Fig. 1. XRD analysis of FBCC and MK.

Fig. 2. FTIR spectra of FBCC and MK.

3.2. Characteristic of the setting and hardening behavior The changes of electrical conductivity and internal temperature of cement pastes containing different amount of FBCC or MK dur-

Table 1 Chemical composition and some properties of FBCC and MK. Oxides (mass%)

FBCC

MK

SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O P2O5 TiO2 ZrO2 BaO NiO, ZnO ZnO, Mn2O3, SrO, HfO2 La-compounds

52.86 39.32 0.81 0.08 0.05 0.02 0.01 0.13 0.11 1.34 0.01 0.01 <0.01 – 0.65 (estimated) 0.01 (traces)

53.19 42.07 1.08 0.06 0.27 – 1.10 – 0.10 0.50 0.02 0.05 – <0.01 –

2.35 170 42.76 3.4 4.96

2.54 17 19.42 1.3 5.69

Ga-compounds Properties Density [g/cm3] Specific surface area (BET) [m2/g] Average particle size [mm] Calcination loss at 1000 °C [%] pH of suspension with the addition:water ratio = 1:2

–

Fig. 3. The grain size distribution for FBCC and MK.

ing the setting and hardening at curing temperature of 6 °C, 25 °C and 40 °C are presented in Fig. 4. Additionally, in Fig. 4 five stages of the early hydration of reference sample (100% CAC40) at each curing temperature are marked. These stages were distinguished

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Fig. 4. Electrical conductivity and internal temperature within time of setting and hardening of cement pastes containing 10%, 15% or 25% of FBCC or MK as CAC40 substitution in comparison with reference sample (100%CAC40) (w/b = 0.5) at the curing temperature of: a) 6 °C, b) 25 °C, c) 40 °C.

based on the temperature-time curves by analogous with calorimetry. This way of the analysis was discussed in detail in

our previous work [5]. A characteristic of the maximum internal temperature (Tmax) of the studied cement pastes is shown in Fig. 5.

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Fig. 5. Characteristic of the maximum internal temperature (Tmax) of the tested cement pastes depending on the curing temperature.

Considering the analogous samples, the increase in the curing temperature causes an increase in electrical conductivity, especially in the first and second stages, and the rise of internal temperature in the third stage. The higher the value of electrical conductivity is reached in the stage 2, the bigger its drop within the stage 3. The bigger the fall in conductivity within the stage 3, the higher the Tmax of paste is obtained (Fig. 4). Regardless of the curing temperature, the studied aluminosilicates lead to decrease of maximal values of electrical conductivity and internal temperature of cement pastes compared to reference. The larger amount of the additions, the more intensive these effects are. Moreover, these effects are greater in case of samples with FBCC than MK (Figs. 4 and 5). For example, 15% of FBCC causes a reduction in maximal electrical conductivity, compared to reference sample, more of about 27% at 6 °C and 10% at 25 °C and 40 °C than 15% of MK. At the same time the Tmax reduction, in comparison with reference, is bigger for 15% FBCC than for 15% MK of about 9%, 2% and 1% at 6 °C, 25 °C and 40 °C respectively. Since the first value of electrical conductivity was obtained 2 min after mixing cement with water so there are missed some initial dissolution effects and this value is high. Although the presence of FBCC reduces electrical conductivity significantly, this aluminosilicate results in distinct initial gain in electrical conductivity and internal temperature of pastes. Therefore the stage 1 is clearly prolonged for the samples with FBCC. In addition, because of obtaining a small maximum of the internal temperature in this stage, there is difficult to distinguish the next stage for the FBCCcontaining samples, especially at higher temperature (Fig. 4). Stage 2, which is characterized by high and almost constant value of electrical conductivity and none thermal effects in reference paste, maintains this specific curves shape only in case of samples with MK. Metakaolin reduces electrical conductivity in the stage 2 and shortens this stage compared to reference paste at all of the studied curing temperatures. The higher the MK content is, the bigger these effects are. The waste catalyst also reduces electrical conductivity in the stage 2, but the electrical

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conductivity-time dependence for samples with FBCC is considerably different in comparison to reference paste and depends strongly on curing temperature. At 6 °C in the second stage the electrical conductivity is clearly stabilized and duration of this period is considerably lengthened by the presence of FBCC. On the contrary, at curing temperature of 25 °C and 40 °C the electrical conductivity-time curves attain maximum here and duration of the stage 2 is clearly shortened by FBCC compared to reference. Irrespective of curing temperature, the larger FBCC content is, the bigger the discussed effects are. Moreover, the more FBCC is, the higher internal temperature during the period 2 keeps up (Fig. 4). In the third stage a reduction of the drop in electrical conductivity and decrease of the Tmax in comparison with reference paste due to both the aluminosilicates are observed regardless of the curing temperature. The greater content of the minerals is, the more intensive the above effects are. However, FBCC causes clearly smaller the drop in electrical conductivity and significantly lower the Tmax than the same amount of MK (Fig. 4). For example, the reduction of Tmax compared to reference sample in case of 15% of the addition equals 30% and 21% at 6 °C, 6% and 4% at 25 °C and 19% and 18% at 40 °C for FBCC and MK, respectively (Figs. 4 and 5). The effect of the aluminosilicates on the time of Tmax occurrence is even more interesting. While MK accelerates the Tmax in comparison to reference paste at each of curing temperature, FBCC accelerates it considerably at ambient and higher temperatures (25 °C and 40 °C) but retards it significantly at temperature of 6 °C (Figs. 4 and 5). The higher content of the addition is, the more intensive these changes are. A big difference of the influence on the time of Tmax versus the curing temperature between FBCC and MK is found (Fig. 5). The effect of MK on the time of Tmax is similar to the calculated from reference results (grey line in Fig. 5), while the effect of FBCC is quite different. For example: for pastes containing 15% of the aluminosilicate at 6 °C Tmax occurs about 120% later in case of FBCC than MK but at curing temperature of 25 °C and 40 °C about 25% and 59% earlier, respectively (Figs. 4 and 5).

3.3. Characteristic of the formed hydrates As the larger amount of the aluminosilicates the more their effect is, the pastes with 25% of the additions was chosen for the investigation of their impact on the hydrates forming during the first 24 h of hydration at different curing temperatures (w/ b = 0.5). A difference of mass loss at definite temperature ranges according to TG results is shown in Fig. 6, while Fig. 7 presents the corresponding DTG curves for these pastes with identification of the formed hydrates. The same way of recognition of DTG peaks was applied as in our previous studies [7,9,11]. The X-ray patterns and FTIR findings of the studied samples are demonstrated in Figs. 8 and 9, respectively. The XRD results clearly show a presence of unreacted cement phases: ferrite (C4AF) and gehlenite (C2AS) in all of the tested pastes (Fig. 8). Neither the different curing temperatures nor the aluminosilicates changes intensity of their patterns. On the contrary, a reflections of CA are clearly reduced with rise of the curing temperature and do not occur in case of the pastes hydrated at temperature of 40 °C and 60 °C. The aluminosilicates also lead to decrease of CA patterns intensity compared do reference sample. The reduction is rather slight at temperature of 6 °C and almost total at temperature of 30 °C for both the minerals, while at temperature of 25 °C and 25 °C++ it depends on kind of the addition and it is significantly larger in case of FBCC than MK. None of the characteristic X-ray patterns or FTIR spectra of the aluminosilicates (Figs. 1 and 2) are visible in the cement pastes results (Figs. 8, 9). Irrespective of the studied additions a decrease of mass loss in the range of 20–180 °C simultaneously with an increase of mass

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Fig. 6. The TG mass losses of the pastes containing 25% of FBCC or MK as replacement part of cement compared to reference sample (100% CAC40) (w/b = 0.5) hydrated 24 h at different temperatures.

Fig. 7. DTG analysis of the pastes containing 25% of FBCC or MK as replacement part of cement compared to reference sample (100% CAC40) (w/b = 0.5) after 24 h of hydration at different temperatures.

loss in the range of 220–370 °C with rise of the curing temperature are observed (Fig. 6). At curing temperature of 6 °C a formation of crystalline CAH10 is clearly confirmed by its high X-ray diffraction patterns (Fig. 8a), clear and wide DTG peak at temperature of about 100 °C (Fig. 7) and bands with maxima at wave numbers 3500 cm 1, 1016 cm 1, 975 cm 1 and 575 cm 1 (Fig. 9a) [31]. Similar observations concern the paste with 25% of MK. In case of sample containing 25% of FBCC additionally C2AH8 reflection of very low intensity (Fig. 8a) and a very little inflexion at about 1080 cm 1 (Fig. 9a) are observed. Moreover, mass loss in the range of 20–180 °C is bigger for pastes with FBCC than MK (Fig. 6). At curing temperature of 25 °C, beside CAH10, C2AH8 arises in all of studied samples (Figs. 7, 8b, 9b). The amount of C2AH8 increases at the cost of lowering CAH10 content with rise in temperature both inside (selfheating 25 °C++) and outside (30 °C), so at curing temperature of 30 °C the C2AH8 is the main formed hydrate (Figs. 7, 8c, d, 9b, c). The DTG peak of maximum at about 235 °C (Fig. 7) and a small bands with maxima at wave numbers of 3624 cm 1, 3524 cm 1, 3477 cm 1, 1020 cm 1 and 969 cm 1 (Fig. 9b, c) indicate a formation of AH3. As the temperature rising the more intensive these effects are. A lack of AH3 reflections on XRD results (Fig. 8b, c, d) is a prove of gel or semi-crystalline form of the gibbsite created at the discussed conditions. In addition, in case of only the reference paste hydrated at 25 °C++ formation of little amount of C3AH6 is confirmed by DTG and XRD data. Both the DTG peak of hydrogarnet at about 268 °C and its X-ray patterns are of very low intensity here (Figs. 7, 8c). Both the studied minerals cause visible increasing of mass loss in the range of 20–180 °C compared to reference sample, especially at curing temperature of 25 °C++ (Fig. 6). However, only in case of pastes with MK hydrated both at temperature of 25 °C and 25 °C++ a band at about 1090 cm 1 is observed (Fig. 9b, c). This band corresponds probably to Si-O vibrations in CS-H [32]. At curing temperature of 40 °C a crystalline AH3 and C3AH6 dominate in the reference and MK-containing pastes, but in case of sample with FBCC the C2AH8 is still the main crystalline hydrate (Fig. 8e). Thus, a mass loss in the range of 20–180 °C is more significant for the paste containing FBCC than for the other studied pastes (Fig. 6). The DTG peaks at about 257 °C and 282 °C indicating AH3 and C3AH6 respectively, as well as bands at 3622 cm 1, 3529 cm 1, 3469 cm 1, 1022 cm 1, 969 cm 1 and 533 cm 1 referring to AH3 are all of lower intensity in case of FBCC-containing paste than in the other pastes (Figs. 7, 9d). The band characteristic to C3AH6 with maximum at 3661 cm 1 is of high intensity in

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Fig. 8. XRD analysis of the pastes containing 25% of FBCC or MK as replacement part of cement compared to reference sample (100% CAC40) (w/b = 0.5) after 24 h of hydration at temperature of: a) 6 °C, b) 25 °C, c) 25 °C++, d) 30 °C, e) 40 °C, f) 60 °C.

reference and CAC 40-MK samples, but it is not observed in the paste with FBCC (Fig. 9d). At curing temperature of 60 °C there is no difference in the kind of the formed hydrates between the studied pastes (Figs. 7, 8f, 9e). The gibbsite and hydrogarnet are the main products of hydration and neither CAH10 nor C2AH8 are identified regardless of the presence of the additions in the pastes. There is only a small increase in

mass loss in the range of 20–180 °C in case of FBCC-containing paste compared to reference and sample with MK (Fig. 6). 4. Discussion Regardless of the curing temperature, just after addition of water to calcium aluminate cement a hydroxylation of cement

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Fig. 9. FTIR spectra of the pastes containing 25% of FBCC or MK as replacement part of cement compared to reference sample (100% CAC40) (w/b = 0.5) after 24 h of hydration at temperature of: a) 6 °C, b) 25 °C, c) 25 °C++, d) 40 °C, e) 60 °C.

grains, surface dissolution and hydrolysis of clinker phases lead to release of Ca2+ and Al(OH)4 and some Al3+ ions into solution with simultaneous increasing of pH and also some heat generation [2,8,13,33]. These initial effects have been described in literature as a wetting (pre-induction) period (stage I in Fig. 4). In this stage a significant and fast rise of electrical conductivity is characteristic due to the intensive ions releasing. At lower temperature lower solubility of cement delivers less ions and less mobility of ions and water molecules occurs than at higher temperature, so a reduction of electrical conductivity with decrease of the curing temperature is fully understandable and refers to different types of cement [14,17,18]. The rise of temperature favors cement dissolution and a degree of cement hydration increases. It is well known and referred to different cements [44]. Therefore, as temperature rising the higher electrical conductivity is obtained simultaneously with the larger consumption of CA from the studied pastes (Figs. 4, 8). The ferrite and gehlenite clinker phases are inactive at this early age of hydration, so their X-ray patterns did not change (Fig. 8) [1,3,35]. In the next stage (stage II in Fig. 4) high and almost

constant electrical conductivity maintains for a while without any thermal effects at each of the curing temperature. This stage is called an induction (or dormant) period. At this stage the following dissolution of anhydrous calcium aluminates as well as nucleation of hydrates progress. The concentrations of ions stay on near their maximum values and the rates of dissolution and precipitation thus being approximately equal [2,5,8,13,34]. When the nuclei growth to their critical size and saturation point is reached, the massive precipitation of hydrates takes place [2,5,33,34]. A sudden drop of the electrical conductivity simultaneous with increase of internal temperature to Tmax is observed then (stage III in Fig. 4). These intensive changes results from a development of a rigid crystalline network and closure of the connectivity of open pores to interconnected capillary pore system [19,20]. In the following stages (IV and V in Fig. 4) further transformations of a product growing and structure thickening occur [2,5,8,33,34]. Because of Tmax corresponds with the end of hydrates precipitation, the time of Tmax occurrence can be related to the setting time [33]. The time of Tmax directly depends on a duration of induction stage (Fig. 4), so

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this period is the most important for kinetics of cement hydration. Many different factors have influence to this stage. A characteristic of solids (size of particles, the grade of surface: defects, dislocations) and availability of space to the reactions influence the dissolution and modify the nucleation rate. These decide of the local equilibrium state with the aqueous phase and of the growth of the nuclei and the hydrates. Moreover, when considering that a layer of hydrated products is formed around cement particles the diffusion of ions also plays a role [2,10,13,34]. Considering the above, when CAC particles are replaced by the studied finegrained additions their physical effects could be more decisive in initial hydration than their chemical interaction. Based on data presented in Figs. 4 and 5 it could be noticed that the studied aluminosilicates influence the setting and hardening of CAC but in the way depending on their kind, content and the curing temperature. A higher content of both FBCC and MK intensifies the effects regardless of temperature. Already at the beginning, both the minerals provide additional heat generation compared to reference sample but considerably greater for FBCC than MK at each of the curing temperatures (Fig. 4). This thermal effect results primarily from a developed wetting of the fine-grained additions. Although a particles of MK are finer than FBCC (Fig. 3), the more important here is the tenfold higher specific surface area of FBCC than MK (Table 1) enabling very intensive interaction of FBCC with water. The produced heat, clearly evident in the tested internal temperature (Fig. 4), favors additional dissolution of CA and explains its bigger reduction in the pastes with FBCC than MK (Fig. 8). However, the electrical conductivity at stage I and II is reduced in the pastes containing the aluminosilicates in comparison with reference sample regardless of the temperature (Fig. 4). Such impact is mainly because a part of CAC is substituted with the additions so less cement grains participate in reactions thus less ions are released into solution. This phenomena described as ‘‘dilution of cement” is known in literature and related to different mineral additions mixed with various cements [13–20]. Considering an acidic pH of suspensions of the additions (Table 1) some ions might be released from the studied aluminosilicates too. In an alkaline environment of the hydrating cement an aluminosilicate structure could be destroyed and its depolymerization and dissolution provide the [SiO(OH)3] and [Al(OH)4] ions into solution [36]. However the solubility of the minerals is not as high as clinker phases so they cannot fully compensate loss of cement and thus the reduction of conductivity is observed in the pastes with FBCC and MK. In addition, decrease of effective w/b ratio due to consumption of water in wetting of the fine-grained aluminosilicates leading to limit of ions and its mobility can also contribute to the electrical conductivity reduction. Despite of the same content of the minerals in the cement pastes a large difference in the action of FBCC and MK at the stage I and II is observed (Fig. 4). Although the electrical conductivity clearly grows in time only in the pastes with FBCC, its maximal value is much more lower in samples with FBCC than MK. While the changes during setting and hardening of MK-containing pastes are similar to reference at each of curing temperature, a new course of electrical conductivity-time curves in case of FBCCcontaining samples is obtained (Fig. 4). These behaviour can be mainly attributed to a structure difference of the aluminosilicates because MK is non-zeolite, while FBCC is a zeolite. As a zeolite with ions exchanging skills FBCC participates actively in the ionic reactions during initial CAC hydration, thus quite different the dissolution-precipitation equilibrium state at stage II is observed in the pastes with FBCC compared to reference and MKcontaining samples (Fig. 4). A capacity for absorbing water is also higher for FBCC than MK particles so the effective w/b ratio is lower in FBCC than MK-containing pastes. The lower pH value of suspension for FBCC than MK indicates higher activity in ionic

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solution of FBCC than MK, too. It is known that on the zeolite surface a dissociation of Si-OH and Al-OH groups to Si-O and Al-O provides an acidic centers capable to pozzolanic reaction [37,38]. That centers actively adsorb Ca2+ ions from solution what clarifies the decrease of electrical conductivity and additional consumption of CA in the pastes with FBCC. According to [39] MK can also perform the pozzolanic reaction by adsorption of calcium ions. However, because of the curves of electrical conductivity-time of pastes with MK remain as characteristic as in reference the change of ions concentrations in solution is not as effective as in presence of the zeolite. Consequently, the discussed differences of the studied additions result in different their impact on a kinetics of hydration. Metakaolin acts as an accelerant irrespective of the curing temperature, while the waste catalyst at temperature of 6 °C acts as a retarder but at ambient and higher temperatures (25 °C, 40 °C) it acts as an accelerant the setting and hardening of CAC (Figs. 4, 5). Similar FBCC action on hydration kinetics was also observed in our previous studies [12] and in case of white calcium aluminate cement (CAC 70) [11,21]. The driving force of the massive precipitation in stage III is lower solubility of the hydrates in relation to the solubility of the anhydrous phases. The most important four hydrates of CAC present the following order of solubility: C3AH6 < C2AH8 < CAH10 < AH3. The formation of less soluble hydrates will mean an increase in the precipitation. In contrast, the most soluble hydrate (AH3) requires a longer induction period [33]. However, different CAC hydrates depending on temperature can form, what has been already described in the introduction. Thus the ‘‘abnormal (anomalous)” setting of calcium aluminate cement at temperature of about 25–30 °C has some interpretative difficulties itself [3–5,34]. The results presented in this work confirm both the ‘‘abnormal” setting of CAC (Figs. 4, 5) and the intensive changes of the formed hydrates with rising of the temperature (Figs. 6–9). Analysis of the time of Tmax (Fig. 5), which expresses the setting time of the pastes, allows to notice that the effect of the aluminosilicates on the ‘‘anomalous” setting of CAC depends on kind of the addition and its content in the pastes. The presence of MK has no effect on the ‘‘abnormal” setting. But the ‘‘anomalous” setting behaviour is clearly confined by 15% of FBCC and it fades away in case of 25% of FBCC. Thus for the paste containing 25% of FBCC as replacement part of CAC 40 a typical shortening of the setting time with increase of the curing temperature is observed. According to [4] the anomalous setting behaviour of CAC is because none of the hydrated calcium aluminates can form readily in the range 25–30 °C. The nucleation of CAH10 is no longer favored on thermodynamic grounds, whilst the rate of formation of C2AH8 is very sluggish. However, in [6] there are pointed that the nucleation rate of C2AH8 does not decide about the massive precipitation of hydrates and the nucleation rate of AH3 is more important for the setting than the nucleation of C2AH8 and as important as nucleation of CAH10. Moreover, the crystallinity and solubility of AH3 influence strongly the stability of CAH10 relative to C3AH6 and AH3. The crystallinity of AH3 also changes with increase of temperature and time i.e. the AH3 transform to gibbsite from firstly amorphous or microcrystalline form [1–3,6]. Based on our previous studies [5] the importance of a self heating should be also considered. The rise of internal temperature of sample can decide about formation of hydrogarnet even at ambient temperature, when Tmax obtained is high enough to cause the conversion of metastable hydrates firstly precipitated to the thermodynamically stable C3AH6 within the first hours of the hydration. The results of phase composition the studied pastes (Figs. 6–9) prove that the formed products during the first 24 h of hydration depend principally on the temperature and the effect of 25% of the aluminosilicates is rather negligible. At each of the curing

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temperature the hydrates are similar to those in reference, especially in samples with MK. None of the crystalline silicacontaining hydrate like C2ASH8 has not been identified yet, so precipitation of typical CAC hydrates probably is deciding factor about the setting time of the pastes with the additions too. The replacement part of CAC by the aluminosilicates has three main consequences for the induction stage: reduction of ions from clinker phases, limitation of availability of water and place to reactions due to their fine particles and ability of providing an additional nucleation centers. Therefore a saturation point providing precipitation of hydrates should be reached easier and faster in pastes with the additions. However, it is found that only MK accelerates the setting of CAC regardless of temperature. The very finegrained particles of MK become an additional and effective nucleation centers of the hydrates so their massive precipitation follows faster. Such additional nucleation centers for the formation of hydration products leading also to an increase in their amount [45]. The reason of the different behaviour of FBCC could be attributed to its zeolitic structure thus the ions exchange capacity of FBCC predominates. The FBCC particles can also play a nuclei role, however the consumption of Ca2+ ions by the zeolite causing to change of Ca/Al ratio seems to be more important. At temperature of 6 °C, when nucleation of CAH10 determines the setting, a reduction of Ca2+ concentration in solution makes impossible to reach saturation point of the CAH10 and hence the induction stage lasts clearly longer in the pastes with FBCC (Fig. 4a). On the other hand adsorption of calcium ions on FBCC surface provides its as high local concentration as enough to precipitate of C2AH8 and little X-ray pattern of C2AH8 is detected in the FBCC-containing paste (Fig. 8a). Moreover, elongation of the induction period duration favors the reactions of FBCC beside cement reactions thus formation of very small amount of C-S-H is found in paste with this addition (Fig. 9a). Concerning the hydration products MK is not as effective as FBCC at this curing temperature. A lower reactivity of MK than FBCC at low temperature was found at early age hydration of Portland cement too [40]. On the contrary at ambient curing temperatures higher activity of MK than FBCC is proved by recognition of C-S-H phase in the samples with MK (Fig. 9b, c). It has been also confirmed by other studies that the rising of temperature enhances pozzolanic activity of metakaolin [40,43], while for FBCC the increase of temperature is not as important and to get better reactivity of the waste catalyst its grinding is recommended [38]. Certainly, clearly finer particles in case of MK than FBCC (Fig. 3) leads to better availability of MK than FBCC to reactions. The rate of the pozzolanic reaction increases with increasing the surface area available for the reaction [44,45]. On the other hand taking into account that at ambient temperatures FBCC accelerates precipitation of hydrates, even more intensive than MK (Fig. 4b), the reactions of FBCC cannot develop as much as in case of hydration at 6 °C. However, because of comparable the TG mass loss in the ranges from 20 to 220 °C for pastes with the additions (Fig. 6) the activity of FBCC at temperature of 25 °C and 25 °C++ producing some amorphous phases could be noticed. As the formation of calcium aluminate hydrates strongly depends on temperature the effect of the aluminosilicates on the internal temperature cannot be neglected. Both the FBCC and MK reduce Tmax of sample at each of the studied conditions (Fig. 5) therefore at temperature of 25 °C+ + they prevent formation of C3AH6. The self heating of the pastes with the additions becomes not enough to get conversion of firstly precipitated thermodynamically unstable products to hydrogarnet within the first hours of hydration. Thermodynamics calculations published in [42] provide that at 25 °C in the CaO-Al2O3-SiO2-H2O system many different products can create including C-S-H, C2ASH8, C3AH6, C3ASxH6-2x (0.2 < x < 1) or some zeolite-type compounds but the authors

concluded that the addition of SiO2 to the CaO-Al2O3-H2O system indicates that C2ASH8 and C3ASxH6-2x may form instead of C3AH6. As a product of the hydration of aluminate cement with silicacontaining addition the most often the strätlingite has been given out [1,14,22–26,41]. The members of the solid solution C3AS3-xH2x (0 < x < 3) have been also identified in systems with silica fume and fly ash [22]. However, it should be noticed that formation of such products, which depends on kind of mineral, its quantity and its ability to silica release, has been found especially at later ages of hydration [1,14,22–26,41]. Therefore the studied time or/and the used amount of the aluminosilicates are probably insufficient to create these phases including crystalline C2ASH8 at first 24 h of hydration at ambient temperature in the pastes with the additions. Even at temperature of 40 °C, when the waste catalyst limits the hydrogarnet formation very effective, although the stable C3AH6 and AH3 are produced in the MK-containing and reference pastes, the crystalline strätlingite is not identified in the sample with FBCC but C2AH8 (Fig. 8e). The formation of C2AH8 still in temperature of 40 °C confirms the previously discussed mechanism of FBCC interaction in CAC pastes. Certainly, the adsorption of calcium ions on the zeolite particles lowering the Ca2+ concentration is mainly responsible for precipitation of C2AH8 instead of C3AH6 in the pastes with FBCC. Intensification of cement reactions with increase of curing temperature over the reactions of the aluminosilicates provides to production of crystalline gibbsite and hydrogarnet in all of studied pastes hydrated at 60 °C what brings these additions to primarily filler role then. Although the authors of [43] indicate the hydrogarnet (C3ASH6) formation at this temperature after 21 h as a result of the pozzolanic reaction between MK and lime there is none changes in our XRD or FTIR results of MK-containing pastes compared to reference sample proving the Si-hydrogarnet production in these samples. It could be summarized that the reactivity of the aluminosilicates in early-age CAC hydration at the studied conditions depends on the temperature as well as on the kind of the addition. These are deciding about an adequate synchronization of cement reactions rate with the mineral reactions rate. For the cement reactions the temperature is the most important while in case of the addition its availability and ability to reactions at the definite temperature seem to be the most essential. 5. Conclusions Both the aluminosilicates influence the early-age hydration of calcium aluminate cement but their effect is greater on the process kinetics than on the initially formed hydrates. Irrespective of the kind of the studied additions a higher their content leads to their stronger impact on setting and hardening of CAC but the effect is different between FBCC and MK and depends strongly on the curing temperature. MK acts only as an accelerator, while FBCC acts as a retarder at low temperature (6 °C) and as an accelerator at ambient and higher temperature (25 °C, 40 °C). The effect is mainly attributed to the different structure of the aluminosilicates. MK is a non-zeolitic while FBCC is a zeolitic material. Regardless of the temperature and the additions, the hydration kinetics changes are realized by duration of the induction stage. The aluminosilicates show different interaction mechanism at this stage. As FBCC is a zeolite its impact on dissolution-precipitation equilibrium state in the induction stage is intensive and significant. The enhanced conductometric method consisting in the simultaneous measurement of electrical conductivity and internal temperature of cement paste within time is an usable way to monitor setting and hardening of CAC mixtures enabling study not only the

M. Nowacka, B. Pacewska / Construction and Building Materials 235 (2020) 117404

kinetics changes and thermal effects but also the course of the ionic reactions. FBCC mixed with CAC can overcome the ‘‘abnormal‘‘ setting of CAC, so typical decrease of setting time with rising of temperature can be obtained in the CAC-FBCC mixtures. Both the studied additions reduce the self-heating of cement paste and thus can prevent the hydrogarnet formation already during initial hydration. However the initially formed hydration products depend principally on the temperature than on the used amount of the aluminosilicates. Only at curing temperature of 40 °C an exceptionally significant effect of FBCC on the formed hydrates prevailing over the effect of the temperature was obtained. In the pastes with the studied content of the aluminosilicates (25%) none of crystalline C-A-S-H type hydrate was identified after 24 h of hydration, regardless of the curing temperature. However, the formation of very little amount of amorphous C-S-H type phases in these pastes depending on the curing temperature was noticed. This reactivity of the aluminosilicates including bigger effect of FBCC than MK at low temperature (6 °C) and MK than FBCC at ambient temperature (25 °C, 25 °C++) was connected with the size of their particles and their influence on setting time. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The studies presented in this work has been partly co-financed from the European Union funds granted by the European Social Fund. Agreement no. 235/ES/ZS-III/W-POKL/14 as part of the system project of the Mazovian Voivodeship Local Government (Poland). References [1] K.L. Scrivener, A. Capmas Calcium aluminate cements, Chapter 13, in: P.C. Hewlett (Ed.), Lea’s Chemistry of Cement and Concrete, John Wiley & Sons, New York, 1998. [2] H.F.W. Taylor, Cement chemistry, 2nd ed., Thomas Telford, London, 1998. [3] J. Bensted, Scientific aspects of high alumina cement, Cem. Lime Concr. 3 (2004) 109–133. [4] S.M. Bushnell-Watson, J.H. Sharp, On the cause of the anomalous setting behaviour with respect to temperature of calcium aluminate cements, Cem. Concr. Res. 20 (1990) 677–686. [5] M. Nowacka, B. Pacewska, Enhanced conductometric method in the studies of calcium aluminate cement hydration process, Cem. Lime Concr. 4 (2015) 225– 234. [6] B. Lothenbach, L. Pelletier-Chaignat, F. Winnefeld, Stability in the system CaO– Al2O3–H2O, Cem. Concr. Res. 42 (2012) 1621–1634, https://doi.org/10.1016/j. cemconres.2012.09.002. [7] M. Nowacka, Studies of the effect of aluminosilicate addition on calcium aluminate cement hydration process, PhD thesis, 2015, Warsaw University of Technology, Faculty of Chemistry, Warsaw (in Polish). [8] C. Parr, Calcium aluminate cement - what happens when things go wrong. Presented at IRE annual conference, Rotherham UK, 2008. [9] B. Pacewska, M. Nowacka, Studies of conversion progress of calcium aluminate cement hydrates by thermal analysis method, J. Therm. Anal. Calorim. 117 (2014) 653–660, https://doi.org/10.1007/s10973-014-3804-5. [10] Ch. Gosselin, E. Gallucci, K. Scrivener, Influence of self heating and Li2SO4 addition on the microstructural development of calcium aluminate cement, Cem. Concr. Res. 40 (2010) 1555–1570, https://doi.org/10.1016/j. cemconres.2010.06.012. [11] B. Pacewska, M. Nowacka, V. Antonovicˇ, M. Aleknevicˇius, Investigation of early hydration of high aluminate cement-based binder at different ambient temperatures, J. Therm. Anal. Calorim. 109 (2012) 717–726, https://doi.org/ 10.1007/s10973-012-2233-6. [12] B. Pacewska, M. Nowacka, M. Aleknevicˇius, V. Antonovicˇ, Early Hydration of Calcium Aluminate Cement Blended with Spent FCC Catalyst at Two Temperatures, Proced. Eng. 57 (2013) 844–850, https://doi.org/10.1016/j. proeng.2013.04.107.

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