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Application of isothermal and isoperibolic calorimetry to assess the effect of zinc on cement hydration

  • Pavel Šiler
  • Iva Kolářová
  • Radoslav Novotný
  • Jiří Másilko
  • Jaromír Pořízka
  • Jan Bednárek
  • Jiří Švec
  • Tomáš Opravil
Article

Abstract

The amount of zinc in the clinker or in the secondary raw materials has been increasing in recent years. Zinc can get to Portland cement from solid waste or tires which are widely used as a fuel for burning in a rotary kiln. The aim of this work was to determine the effect of zinc on Portland cement hydration. This effect was studied by isothermal and isoperibolic calorimetry. Both calorimetry methods are suitable for measurements during the first days of hydration. Isoperibolic calorimetry monitors hydration process in real-life conditions, while isothermal calorimetry does it at a defined chosen temperature. Zinc was added to the cement in the form of two soluble salts of Zn(NO3)2, ZnCl2 and a poorly soluble compound ZnO. The concentration of zinc added was chosen as 0.05, 0.1, 0.5 and 1 mass%. The results show that increasing amounts of zinc ions in cement pastes lead to hydration retardation and reduce both the maximum temperature and the maximum heat flow due to the retarding effect of zinc. The newly formed compounds during hydration were identified by X-ray diffraction method.

Keywords

Portland cement Zinc Isothermal calorimetry Solution calorimetry Hydration 

Introduction

Portland cement is a hydraulic binder produced by grinding cement clinker and minor amounts of gypsum [1]. Clinker phases are tricalcium silicate (abbr. C3S–3CaO·SiO2 or Ca3SiO5, alite), dicalcium silicate (abbr. C2S–2CaO·SiO2 or Ca2SiO4, belite), tricalcium aluminate (abbr. C3A–3CaO·Al2O3 or Ca3Al2O6, celite) and calcium aluminoferrite (abbr. C4AF–4CaO·Al2O3·Fe2O3 or Ca4Al2Fe2O10, brownmillerite) [2, 3]. Sulfate ions from gypsum (CaSO4·2H2O) influence the reaction kinetics of C3A and Ca(OH)2 (CH, portlandite), to form ettringite (C6A \(\overline{\text{S}}\) 3H32), associated with the retarding effect. In the absence of CaSO4 or other regulators of setting, C3A responds to hydration very quickly to form stable C3AH6, which results in too rapid, so-called “quick” setting. In contrast, there is a “false” setting in the case of large content of sulfate ions compared to aluminate ions [4, 5].

The effect of zinc on the clinker properties was investigated in works [6, 7, 8]. The clinker was burned together with metal, or metal was also used as an admixture added to industrially produced cement. During mixing as an admixture, Zn2+ ions were immediately available for reaction with cement. When zinc was incorporated into the clinker phase, the release of Zn2+ ions depended on the dissolution rate of the zinc-rich phases. However, zinc is mostly bound to the interstitial phase of the clinker [9]. Some interstitial phases containing Ca6Zn3Al4O15 were still encapsulated in silicate grains, making it impossible to react.

Arliguie published the effect of zinc on the hydration of C3S and C3A. The hydration of C3A in the presence of zinc is affected by the concentration of sulfates in the cement. When the sulfate concentration is higher than 2.5%, the hydration of C3A is slowed down [10]. Zinc retards the initial hydration of C3S, and a Zn2(OH)6·2H2O can precipitate [11].

Many other studies have dealt with the action of zinc during hydration reactions where zinc in water at ambient temperature is rapidly hydrolyzed and absorbed on the surface of cement grains. The zinc hydroxyl anions Zn(OH) 3 a Zn(OH) 4 2− are found in the system between pH 12 and 13. Subsequently, they are transformed into insoluble CaZn2(OH)6·2H2O before portlandite. Thus, hydration can be inhibited until these reactions are complete, since both OH and Ca2+ are present in low concentrations. Then the formation of portlandite depends on Ca2+ and OH ions concentration in the surrounding solution. When these ions are used to form insoluble zinc compounds, they can cause saturation delay of the surrounding solution, thereby delaying the precipitation of portlandite and CSH gel, until all zinc is converted into an insoluble hydroxide compound. Ca(Zn(OH)3)2·2H2O creation may occur during the induction period, which leads to delaying the supersaturation of the pore solution and hence retarding the hydration reactions. The behavior of the double membrane formed with calcium and metallic hydroxide on the surface of C3S is not sufficiently explained yet [7, 11, 12, 13, 14, 15, 16]:
$${\text{Zn}}^{2 + } + 2{\text{OH}}^{ - } \to {\text{Zn}}\left( {\text{OH}} \right)_{2}$$
(1)
$${\text{Zn}}\left( {\text{OH}} \right)_{2} + 2{\text{OH}}^{ - } \to 2{\text{H}}_{2} {\text{O}} + {\text{ZnO}}_{2}^{2 - }$$
(2)
$$2{\text{ZnO}}_{2}^{2 - } + {\text{C}}_{3} {\text{S}}/{\text{O}} - {\text{Ca}}^{2 + } + 6{\text{H}}_{2} {\text{O}} \to {\text{C}}_{3} {\text{S}}/{\text{O}} - {\text{CaZn}}_{2} \left( {\text{OH}} \right)_{6} \cdot 2{\text{H}}_{2} {\text{O}} + 2{\text{OH}}^{ - }$$
(3)
Dashes in Eq. (3) indicate bonding of Ca through oxygen.

Lieber also found the existence of CaZn2(OH)6·2H2O but did not associate it with the depletion of Ca2+ and OH ions from the solution during its formation and with likely prevention of saturation of the solution by these ions. Taplin suggested that retardation may not happen due to the binding of compounds to cement grains, but their presence may cause the retardation effect. Lieber further published the difference between the retardation effects of zinc compounds. According to his explanation, the difference is due to the process of conversion of all forms of zinc to zincate, using available ions from the solution, which causes retardation rather than zinc alone [17]. The strength of cement pastes decreases with 5 mass% zinc, but with fly ash content already at 2 mass%. Zinc increases cement penetration, probably by supporting ettringite formation, but does not release itself from the matrix [18, 19, 20, 21].

Other scientists have published the most likely idea of zinc binding to CSH structures by incorporating of Zn2+ ions into the CSH interlayer or sorption on CSH internal crystalline surfaces. However, the zinc may be adsorbed into CSH interlayers as ZnO4 on the Si–O sites from SiO4 chains oriented to the interlayer. Furthermore, by XAFS measurements, there is the possibility of Si–O–Zn bonds [22, 23]. The third option is that zinc bonds to internal surfaces between ordered domains in the CSH gel. The structure of the CSH gel has more or less two-dimensional arrangements, so the existence of internal surfaces and micropores cannot be ruled out. Here, zinc can also be adsorbed onto Si–O sites. The fourth option is the precipitation of zinc-containing silicates [24, 25, 26, 27].

Gineys et al. [28] studied the effect of addition of Cu, Cd, Ni, Pb and Zn nitrate salts on PC properties. According to the results of the SEM and XRD analyses, copper and lead are preferentially absorbed into the CSH gel, while cadmium, nickel and zinc precipitate as hydroxides into the space between the crystals. Therefore, the elements that precipitate have only a slight effect on the final strength of the created material.

A group of scientists led by Nochaiy published a study on the impact of ZnO on the properties of Portland cement in 2015. They found that due to zinc retardation, mechanical strengths after 3 days were less than zinc-free cement samples. After 1 week, the strength was increasing and after 28 days it was higher than for the control sample. This effect was attributed to the fill action of ZnO nanoparticles. In addition, decreasing porosity with an increasing amount of ZnO was noted. FTIR analysis confirmed the presence of zinc hydroxide compounds and CaZn2(OH)6·2H2O as inhibitors of hydration reactions. As a result, they found that ZnO influences the hydration reaction predominantly in the early reactions [29].

This study was undertaken to illustrate the effect of zinc using ZnO, Zn(NO3)2 and ZnCl2 at different concentrations on the Portland cement hydration. Isothermal and isoperibolic calorimeters were used for monitoring of cement hydration. It is clear that not only the type of zinc compounds (due to their solubility) but also their concentration have significant effect on the hydration of Portland cement. The scientific background consists of determining the different products newly formed due to the presence of zinc. The duration of induction period (retardation effect) was expressed as a mathematical function. These results can serve to optimize the quantity of alternative fuels for the burning of clinker in a rotary kiln.

Experimental

Calorimetry

The calorimeters used in this kind of testing are constructed as adiabatic (preventing the heat exchange with the surroundings) or semiadiabatic, isoperibol or isothermal (at constant temperature of the environment) and conductive (measuring the rate of heat evolution from the temperature gradient between the sample and the metal envelope of constant temperature). Each of these methods has its advantages and supplies specific data describing individual perspectives on the rate of hydration under different ambient temperature and curing conditions [30].

The semiadiabatic method (EN 196-9) [31] is based on the determination of the evolved heat by measuring the temperature development of the freshly made sample to a given time point during setting and hardening. This direct method is more popular and more indicative because the curves show the course of reactions running immediately after mixing the components under given conditions [32, 33, 34].

For a determination of the effect of additive on the course of hydration, various methods based on calorimetry proved to be useful for their fastness, simplicity of analysis and high informative value [35, 36].

Isothermal calorimetry

Isothermal calorimetry has been used by several researchers to quantify the hydraulic activity of Portland cement, the pozzolanic activity of fly ash or the latent hydraulic properties of slag [37, 38]. Calorimetry provides continuous measurements and is therefore a convenient method to study the early stage of hydration where the heat rate is relatively high.

The measurement of hydration heat evolution was taken using isothermal calorimeter TAM Air (TA Instruments). The measurement was based on ASTM C 1679. Quartz sand was used as a reference. The measurement was taken at 25 °C. The introduction of the sample preparation is mentioned below.

Isoperibolic calorimetry

Very good representative values of the hydration reactions can be acquired using multicell isoperibolic calorimeter [30]. This simple method makes it possible to monitor the progress of hydration of several samples simultaneously and to estimate the beginning of setting or hardening. It is also possible to monitor the impact of the amount and type of admixtures and additives [39, 40].

The following data characterizing the influence of different additives on the course of hydration of the cement–water system can be obtained from calorimetric curves: induction period (or more precisely the setting start time), which can be estimated as the time from water addition to the beginning of the second peak on the calorimetric curve, the start of hardening which corresponds with the end of setting time and the maximum temperature reached. Values obtained by this way correspond to values obtained by Vicat needle method [41].

The main difference between the calorimeters used is their design. In the isoperibolic calorimeter, the sample temperature is measured at a constant temperature of the surroundings. For isothermal calorimetry, the heat flow needed to maintain the sample at the same temperature as the reference material is measured.

By choosing the calorimeter temperature, the isothermal calorimeter allows the choice to accelerate or slow down the reaction inside the calorimeter and also allows measurements to be taken for a longer period. Thanks to these possibilities, it is possible to monitor reactions that cannot be monitored by means of an isoperibolic calorimeter, such as the conversion of ettringite to monosulfate or the reactions in later stages of hydration.

Isoperibolic calorimeter is a simpler device which also means it is a lower-cost device. The main advantage is the measurement under real-life conditions, which cannot be achieved by isothermal calorimeter. Due to the warming of the sample during hydration, it is possible to set and harden samples which do not hydrate under the simulated conditions of the isothermal calorimeter.

For the study of hydration reactions, it is therefore advantageous to use a combination of both these calorimetric methods.

X-ray diffraction analysis (XRD)

It is a rapid analytical technique primarily used for phase identification of a crystalline material. The analyzed material is finely ground and homogenized, and average bulk composition is determined.

X-ray diffraction analysis was performed with X-ray diffractometer Empyrean (Panalytical) with Bragg–Brentano parafocusing geometry and using CuKa radiation. The measurements were taken within the range from 5° to 120° 2Θ with angular step 0.013° 2Θ and 25 s duration using automatic divergence slits to maintain the constant irradiation of the sample area. The measurements were repeated four times and then summed.

Inductively coupled plasma atomic emission spectroscopy (ICP-OES)

It is an analytical technique used for the detection of trace metals. The analyzed samples were in liquid form of aqueous extract. The measurement was taken on water extracts with Horiba Jobin Yvone Ultima 2 spectrometer.

Sample preparation

All mixtures was prepared from the cement CEM I 42,5 R Mokrá—Českomoravský cement, a.s., Heidelberg Cement Czech republic. The chemical and phase composition of the cement is shown in Tables 1 and 2.
Table 1

Chemical compositions of the components used

Component

%

CaO

65

SiO2

19

Al2O3

4

Fe2O3

3

MgO

1

SO3

3

S2−

0.04

Cl

0.051

K2O

0.75

Na2O

0.15

ZnO

0.00005

LOI

3.1

Table 2

Mineralogical compositions of the clinker

Component

%

C3S

67

C2S

11

C3A

7

C4A

11

Zinc was added in the form of soluble compounds Zn(NO3)2·6H2O, ZnCl2 and insoluble ZnO in the amount from 0.05 to 1 mass% of the cement substitution. (The percentages of the replacement were always calculated on pure zinc in the binder.) The pastes were mixed with distilled water on water-to-binder ratio equal to 0.4.

For measurements using an isoperibolic calorimeter, 300 g of the mixture was weighed into a polystyrene crucible provided with a thermo-insulating jacket and a thermocouple. Calorimetric measurements and calorimetric curves were obtained. For isothermal calorimetry, 7 g of the mixture was dosed to a glass ampoule, which was then inserted into the instrument.

The pastes for ICP-OES determination were embedded into 100 × 20 × 20 mm molds and stored in wet conditions for sample analysis. The prepared test pieces were leached according to CSN EN 12457. One test sample was placed in the sampler, and water was added to have a water-to-solid phase ratio of 10:1. The temperature of the water used for the preparation of the leachates was kept between 15 and 25° C. The sample boxes were placed in a shaker after filling with the sample and water. The leaching was performed by continuously rotating the sample and water at a rate of 5–10 revolutions per minute for 24 h ± 0.5 h. The sample was allowed to settle for 15 min (± 5 min) after the rotary movement of the sampler was stopped. All aqueous extracts of the samples were filtered through a 0.45-μm membrane filter by vacuum filtration.

Samples for phase determination were prepared with 5 mass% zinc replacement. Addition of up to 5 mass% of zinc was used because of the resolution of the X-ray diffraction apparatus itself. After mixing, the samples were placed in polystyrene containers and stored in a humid environment until the time of measurement. Thereafter, they were milled in a vibratory mill and the hydration was stopped by washing in acetone. Residual acetone was removed by evaporating at 50 °C.

The same samples were used also for TGA/DTA analysis (Q600; TA Instruments).

Results and discussion

Due to the preparation of samples outside the calorimeters, the first peak was not completely recorded. This peak is caused mainly by the wetting and dissolving of the cement phases, especially the beginning of the C3A hydration. The effect of this peak is very small, and it contributes only slightly to the total amount of developed heat.

Another peak which divides the main hydration peak into sulfate depletion peak and real main hydration peak was discovered in the spectrum during measurement by an isothermal calorimeter. Its emergence is illustrated by several hypotheses. The first one deals with the hydration of C3A in cement. It can be caused by higher C3A content (≥ 12%) [42]. In earlier studies, this peak was considered to be the hydration peak of ettringite transformation to monosulfate [43, 44]. This peak was detected in Portland cement containing fewer amounts C3A than 7.5% [45].

At low concentrations of zinc, the achieved temperature and the developed heat were increasing with increasing addition. From a certain value (0.1 mass% for isoperibolic curves and 0.5 mass% for isothermal curves except for samples with ZnO), the measured values decrease. Decreasing heat flow can be caused by the incorporation of zinc ions into the membrane covering cement grains [11]. With an increasing amount of zinc, there may be a loss of sites capable of binding this element, thereby reducing heat generation. Another factor is the faster response associated with the peak narrowing after the induction period. There is no significant reduction in the heat flow between the zinc mixtures in amounts of 0.5 and 1 mass% in case of ZnO. This may be related to the gradual reaction of ZnO itself due to lower solubility. Total heat released during isothermal measurements was not affected by zinc addition. Differences in isoperibolic calorimetry were mainly due to the lower sensitivity of the device over longer measurement times.

CEM I 42,5 R (Ref I)

For all prepared samples, the same following dependencies were observed. An increasing zinc concentration results in the first peak increasing in the isoperibolic measurements. This phenomenon is due to the elevated temperature of the solution used due to the exothermic reactions of the compounds used.

The reduction in total developed heat during hydration reactions with an increasing amount of zinc was mainly due to its retardation effect since all values measured by isothermal calorimetry were similar. Differences in isoperibolic calorimetry were mainly due to the lower sensitivity of the device over longer measurement times.

During hydration, zinc is formed into leachable compounds. Just after 24 h, a small amount of zinc ions were found by the ICP-OES method in the aqueous extract of samples. At first, the formation of amorphous structures occurs. These structures are little organized and can be transforming over time into more energy-efficient compounds (trying to achieve the lowest entropy) from which zinc cannot be so easily released. This fact was confirmed by the gradual decrease in the amount of zinc in aqueous extracts by ICP-OES and increasing amount of crystalline compounds by XRD. Thus, on the surface of the allite grain, zinc compounds will be formed [11, 17] and subsequently precipitated into crystalline compounds. However, only a small amount of these compounds is produced and therefore their effect on the total released heat is negligible.

CEM I + Zn(NO3)2·6H2O

Isoperibolic calorimetry

A low exothermic peak was found in this mixture between 20th and 30th hours for the addition of 1 mass% Zn. This could be related to the dissolution of Zn(NO3)2 or the formation of compounds on the surface of the hydrating grains [11, 17]. Another possible explanation could be the rapid aluminate desiccation [46]. Due to the high inhibitory effect in the 1 mass% zinc sample, lower maximum temperature development and therefore slower hydration reactions occur (lower and wider peak) (Fig. 1).
Fig. 1

Isoperibolic calorimetry curves for samples with Zn(NO3)2

After 7 days of hydration, a crystalline compound 3CaO·Al2O3·0.83Ca(NO3)2·0.17Ca(OH)2·9.5H2O was found in the XRD sample.

Isothermal calorimetry

The main hydration peak and sulfate depletion peak [46] can be seen on the figures from isothermal calorimetry at low zinc concentration. For the samples with the lowest concentrations of zinc (0.05 and 0.1 mass%), the main hydration peak was recorded together with a small sulfate depletion peak as well as for reference sample. With an increasing amount of zinc, the sulfate depletion peak is increasing and the main hydration peak decreasing to a concentration up to 0.5 mass% where only a single peak is visible. The overlapping of both peaks can be attributed to the inhibitory effect of zinc itself, which subsequently results in faster hydration reactions (narrower peaks). Another reason can be the depletion of sulfate ions from the solution during prolonged hydration, which may lead to simultaneous reactions of silicates and conversion of ettringite to monosulfate [43, 44]. In case of 1 mass% of zinc, another peak around 22nd hour was recorded as well as for isoperibolic calorimetry. This could be related to the dissolution of Zn(NO3)2 or the formation of compounds on the surface of hydrating grains [11, 17]. In view of the results obtained by X-ray diffraction, where the formation of zinc compounds was not confirmed after 24 h, zinc binding can be assumed to amorphous compounds formed on the surface of the hydrated grains. It could be also caused by rapid alumina hydration. By zinc addition, portlandite precipitation is delayed by the uptake of released Ca2+ ions [6, 7, 11, 18]. Portlandite can be forming only in very small quantities below the detection limit of the X-ray diffraction method. Furthermore, there is other peak for the specimen with a low zinc concentration between 30th and 40th hours. This peak is probably related to the conversion of ettringite to monosulfate [43, 44] (Fig. 2).
Fig. 2

Isothermal calorimetry curves for samples with Zn(NO3)2

Mixture with 1 mass% of zinc showed a different behavior again due to the high zinc inhibiting action. This mixture has the most significant influence on the environment of the hydration reactions by changing the pH value, lower heat, and the influence of the Zn2+ and NO3 ions themselves. There may be a slower precipitation of zinc compounds and therefore slower hydration [11]. Subsequently, monosulfate is formed and nitrate ions will be incorporated.

In comparison with isoperibolic calorimetry, retardation is prolonged with higher zinc additions (from 0.5 mass%). This change may be due to the temperature in calorimeter chamber which was set at 25 °C, while the isoperibolic measurement was significantly warmer. According to these findings, the hydration reaction at higher zinc concentrations is slower at lower temperatures. Ambient temperature can influence the inhibitory effect itself, the dissolution of Zn(NO3)2, the kinetics of incorporation of zinc into amorphous structures and consequently its partial precipitation into crystalline structures.

An increasing inhibitory effect of zinc is due to its probable presence in the amorphous phase because no new crystalline compound has been detected after 24 h. After 7 days a monosulfate analog, 3CaO·Al2O3·0.83Ca(NO3)2·0.17Ca(OH)2·9.5H2O, in a 5% addition was found. In the first stages of hydration, Zn(NO3)2 was dissolving. Subsequently, it reacted wherever it could bind to the surface of the hydrating grains and form an impermeable layer where Ca[Zn2(OH)6](H2O)2 [11] was precipitating later. Because no portlandite was detected, the pH of the hydration environment is also affected because amorphous zinc compounds can consume Ca2+ ions [7, 11, 12, 13, 14, 15, 16].

CEM I + ZnCl2

Isoperibolic calorimetry

A small peak between 20th and 30th hours was also found in this mixture as well as in the sample with Zn(NO3)2. The measured curves were very similar to the curves for Zn(NO3)2 at low Zn concentrations (up to 0.1 mass%). 1 mass% of zinc in the form of ZnCl2 did not have a significant retardation effect like the 0.5 mass% did. This effect is due to the function of chloride ions as a hydration accelerator, which has been shown at a higher dose (Fig. 3).
Fig. 3

Isoperibolic calorimetry curves for samples with ZnCl2

The crystalline zinc compounds—Zn5(OH)8Cl2H2O and Ca2Al(OH)6Cl(H2O)2—were detected after 24 h. But the amount of zinc in the crystalline compound does not account with the doped amount, so part of the zinc remains in the amorphous phase. This part of zinc is probably bound to the grain coating and prevents the hydration reaction. Assuming the possibility of producing only a certain amount of Zn5(OH)8Cl2H2O, the amount of which does not change over time, it can be said that the inhibition is dependent on the amount of zinc in the amorphous phase. After increasing of the concentration above 0.5 mass% of zinc, no further prolongation of hydration was observed. There were two distinct peaks in the graph which indicate further reactions, suggesting a greater complexity of hydration. There was a significant increase in released heat after 80 h.

Two identical trends were found for soluble compounds Zn(NO3)2 and ZnCl2. The first is the prolongation of the induction period with the increasing amount of zinc in the system due to the higher amount of zinc in the amorphous phase. Furthermore, the increasing concentration of zinc (from 0.5 mass%) decreases the maximum temperature in the sample. Slightly different salt effects can also be caused by the presence of anions from test compounds of both nitrates and chlorides that can alter the environment of hydration reactions.

Isothermal calorimetry

Due to doping of zinc to the samples, the main hydration peak was gradually decreasing and then merges as well as samples with Zn(NO3)2. The course of hydration was similar for both soluble compounds (Fig. 4).
Fig. 4

Isothermal calorimetry curves for samples with ZnCl2

No peak was found on the calorimetry curve for 1 mass% zinc in the mixture, probably due to a low temperature in a calorimeter (25 °C). In the isoperibolic calorimeter, significant warming occurred in the first stages of hydration, allowing subsequent solidification and hardening of the mixture.

CEM I + ZnO

Isoperibolic calorimetry

Maximum sample temperature decreasing below the reference sample value was observed at all zinc concentrations. This would correspond to a slower ZnO reaction rate due to its low solubility. ZnO was also detected on XRD after 90 days. Ca[Zn2(OH)6](H2O)2 was measured after 24 h in an amount which does not change. Further dissolution of ZnO leads to the incorporation of Zn2+ ions into the amorphous phase. As the amount of zinc increases, there is no significant change in the highest achieved temperature because the differences are small. The decrease was mainly due to the highest rate of retardation associated with the gradual dissolution of ZnO, which takes longer than the measurement time period. A slight decrease can also be attributed to the formation of a crystalline compound of Ca[Zn2(OH)6](H2O)2 or to the binding of zinc into amorphous compounds [7, 11, 12, 13, 14, 15, 16] (Fig. 5).
Fig. 5

Isoperibolic calorimetry curves for samples with ZnO

ZnO caused the highest recorded retardation (approximately twice as long as the soluble salts due to its very low solubility).

The peak between 20th and 30th hours was not found due to the slow dissolution of ZnO as opposed to soluble compounds.

Isothermal calorimetry

Despite the significant differences measured by isoperibolic calorimetry, the curves measured by isothermal calorimetry for zinc in the form of ZnO at zinc concentration of up to 0.1 mass% were very similar to the previous samples (Fig. 6).
Fig. 6

Isothermal calorimetry curves for samples with ZnO

The sulfate depletion peak was found only with the lowest amount of zinc (0.05 mass%). At higher concentrations, the main hydration and sulfate depletion peaks were merging. As in the previous cases, this phenomenon can be explained by inhibition of hydration reactions or by simultaneous reactions, which would correspond to a narrower peak. Due to the late formation of ettringite under other conditions, a rapid recrystallization of the monosulfate can occur and thus the simultaneous course of these reactions.

Increasing zinc concentration increases the heat flow in the samples as well as in the case of soluble compounds, but high heat flow was also measured for the sample with 1 mass% of zinc thanks to its pronounced retardation effect and the possibility of reacting of larger amounts of the binder even after a longer hydration time. No peak was also found between 20th and 30th hours of hydration.

Compared to the results of previous samples, almost doubled prolongation of hydration was observed.

Despite the different conditions in both calorimetric measurements, the data measured at the same temperature of 25 °C have some consistent trends. The times when maximum temperatures were achieved, or heat flows in the first 48 h of hydration, were almost identical for all samples. The developed heat differs primarily due to higher temperatures of samples in isoperibolic calorimeter which causes a faster increase in hydrating heat and steeper curves.

A partial conversion during hydration (the difference between sulfate depletion peak and main hydration peak or conversion of ettringite to monosulfate [46]) and a repeatable measurement after a longer hydration time (within a few weeks) can be observed by using isothermal calorimetry.

Due to the slow gradual dissolution of ZnO, a decrease in the maximum temperature below the value of pure cement in isoperibolic calorimetry was observed while there was no such decrease for isothermal calorimetry curves. In the isothermal calorimeter conditions, it was not possible to achieve solidification and hardening in this sample, which shows the importance of comparison of measurements in real and simulated conditions.

The comparison of induction period length

To illustrate the effect of zinc, the lengths of the induction periods were determined and plotted in relation to the amount of zinc in the samples in Fig. 7. The events taking place in space most often exhibit exponential dependence. For this reason, exponential dependence was chosen for interleaving of the measured values. Equation (4) was chosen from possible equations in origin software because it showed the smallest errors. For this reason, it was chosen to interleave the acquired values.
$$y = A_{1} \cdot \exp \left( {\frac{ - x}{{t_{1} }}} \right) + y_{0}$$
(4)
Since the t 1 value was negative for all samples, the equation is given as Eq. (5).
$$y = A_{1} \cdot \exp \left( {\frac{x}{{t_{1} }}} \right) + y_{0}$$
(5)
By matching the obtained values with this exponential function, a very good match of the measured points with the regression curve was obtained, confirming the exponential dependence. Graphs obtained by isoperibolic and isothermal calorimetry are similar. It can be seen from the graphs that the hydration was most prolonged by the very low soluble ZnO and at least by Zn(NO3)2. The accelerating effect of chloride ions showed only in isoperibolic calorimetry with 1 mass% ZnCl2 addition, while no curves on isothermal calorimeter were measured.
Fig. 7

Graphical comparison of the ends of the induction periods for isoperibolic (left) and isothermal (right) calorimetry

XRD and ICP-OES results

The XRD and ICP-OES methods were used to better understand the mechanism of zinc action in the cement matrix. The values from XRD analysis were evaluated by using Rietveld Analysis of XRD Patterns. The results are shown in Tables 36. The results from ICP-OES are shown in Tables 710.
Table 3

Phase composition of pure cement samples

Hydration/days

Ca3SiO5

Hatrurite (alite)

Ca2(SiO4)

Larnite (belite)

Ca2(Fe2O5)

Brownmillerite

C6A\(\overline{\text{S}}\) 3H32

Ettringite

Ca(OH)2

Portlandite

Ca(CO3)

Calcite

1

13.1

40.1

7.3

4.2

28.5

8.0

7

13.9

36.3

7.9

4.9

31.3

10.2

28

13.2

31.7

6.4

4.3

34.0

13.2

90

10.8

32.4

6.2

5.8

37.8

13.3

Table 4

Phase composition of sample with Zn(NO3)2·6 H2O

Hydration/days

Ca3SiO5

Hatrurite (alite)

Ca2(SiO4)

Larnite (belite)

FeAlO3(CaO)2

Brownmillerite

Ca(CO3)

Calcite

C6A\(\overline{\text{S}}\) 3H32

Ettringite

3CaO·Al2O3·0.83Ca(NO3)2·0.17Ca(OH)2·9,5H2O

Ca[Zn2(OH)6](H2O)2

Ca8Al2Fe2O12CO3(OH)2·22 H2O

1

14.9

67.7

3.2

6.9

7.3

7

14.9

56.2

3.1

11.4

10.2

5.3

28

16.3

56.1

2.0

11.3

10.8

6.4

2.0

90

12.1

56.0

2.1

10.5

10.2

5.4

2.1

2.2

Table 5

Phase composition of sample with ZnCl2

Hydration/days

Ca3SiO5

Hatrurite (alite)

Ca2(SiO4)

Larnite (belite)

FeAlO3(CaO)2

Brownmillerite

Ca(CO3)

Calcite

C6A\(\overline{\text{S}}\) 3H32

Ettringite

Ca2Al(OH)6Cl(H2O)2

Hydrocalumite

Zn5(OH)8Cl2H2O

Simonkolleite

1

14.3

69.0

3.2

5.0

6.4

2.4

1.0

7

16.4

60.4

3.0

6.2

7.2

5.5

3.2

28

15.6

62.5

2.4

7.3

7.3

5.6

3.4

90

13.2

61.7

2.3

8.6

6.4

6.3

2.0

Table 6

Phase composition of sample with ZnO

Hydration/days

Ca3SiO5

Hatrurite (alite)

Ca2(SiO4)

Larnite (belite)

FeAlO3(CaO)2

Brownmillerite

C6A\(\overline{\text{S}}\) 3H32

Ettringite

Ca(CO3)

Calcite

Ca(CO3)·H2O

Monohydrocalcite

CaSO4·2 H2O

Gypsum

ZnO

Ca[Zn2(OH)6](H2O)2

Ca(OH)2

Portlandite

3CaO·Al2O3·0.17CaSO4·

0.17Ca(OH)2·0.66CaCO3·xH2O

Ca8Al2Fe2O12CO3(OH)2·22H2O

1

16.3

54.2

6.6

6.1

8.2

2.0

5.2

3.4

7

16.2

57.0

5.4

7.0

8.2

1.2

5.4

3.1

28

10.0

51.9

5.3

8.9

7.6

9.0

0.5

2.4

3.2

8.0

0.5

90

6.3

37.4

5.3

8.1

9.8

1.1

1.3

2.1

18.6

9.2

Table 7

ICP-OES results of pure cement samples

Hydration time/days

pH

Ca/mg dm−3

RSD/%

Si/mg dm−3

RSD/%

1

11.9

100.1

4.6

1.7

2.9

7

11.8

45.1

6.8

10.1

0.8

28

11.6

24.2

9.3

24.5

2.6

90

11.4

19.9

2.3

29.6

2.2

Table 8

ICP-OES results of samples with Zn(NO3)2

Zn/hm.%

Hydration time/days

pH

Ca/mg dm−3

RSD/%

Si/mg dm−3

RSD/%

Zn/mg dm−3

RSD/%

0.05

1

11.9

119.7

8.8

3.8

1.6

0.4

7

11.8

86.0

1.7

7.8

0.8

0.5

28

11.7

64.7

2.1

17.1

3.3

0.3

90

11.5

19.4

4.5

29.6

2.0

0.3

0.1

1

12.0

576.5

1.9

1.9

2.5

0.6

7

11.8

157.0

9.6

5.6

1.0

0.4

28

11.6

147.7

5.2

17.1

2.8

0.1

90

11.4

14.7

9.0

22.0

1.6

0.1

0.5

1

12.4

808.0

3.9

1.6

0.7

3.9

7

11.7

192.3

0.8

11.8

1.4

1.7

28

11.5

92.1

0.4

12.7

2.4

1.7

90

11.3

18.0

6.1

30.4

2.0

1.2

1

2

12.3

461.5

2.3

2.1

2.2

0.01

0.2

7

11.8

148.2

6.0

7.5

5.2

0.5

28

11.5

40.1

2.6

21.3

0.2

0.7

90

11.3

13.8

4.7

22.9

0.3

0.5

Table 9

ICP-OES results of samples with ZnCl2

Zn/hm.%

Hydration time/days

pH

Ca/mg dm−3

RSD/%

Si/mg dm−3

RSD/%

Zn/mg dm−3

RSD/%

0.05

1

11.8

120.3

1.8

4.0

2.3

0.2

7

11.7

90.5

3.7

10.9

2.8

0.3

28

11.6

41.7

0.4

21.8

1.1

0.6

90

11.4

24.2

7.0

26.5

3.2

0.1

0.1

1

12.4

192.3

4.9

1.0

0.1

0.5

7

11.7

84.0

0.8

18.9

0.6

1.2

28

11.5

38.3

0.9

23.9

0.8

0.8

90

11.3

28.3

0.5

24.2

0.1

1.5

0.5

2

12.2

590.3

2.5

1.8

0.1

2.1

7

11.8

99.7

6.5

11.3

0.4

2.5

28

11.5

62.0

4.8

21.9

0.5

1.4

90

11.3

21.0

4.8

28.0

2.9

1.8

1

2

12.3

364.9

5.1

5.3

4.0

0.01

0.3

7

11.9

72.9

6.2

7.1

4.9

2.8

28

11.7

22.4

5.0

22.3

3.9

2.3

90

11.5

22.9

2.8

28.4

3.1

2.4

Table 10

2 ICP-OES results of samples with ZnO

Zn/hm.%

Hydration time/days

pH

Ca/mg dm−3

RSD/%

Si/mg dm−3

RSD/%

Zn/mg dm−3

RSD/%

0.05

1

12.4

526.0

5.9

1.2

2.6

0.2

7

11.9

148.7

6.1

5.6

0.3

0.1

28

11.9

46.7

4.7

17.3

2.4

0.2

90

11.5

16.0

2.5

31.0

1.6

2.3

0.1

1

12.6

135.3

3.7

0.3

4.7

0.4

7

11.8

131.7

1.7

16.6

3.6

0.1

28

11.5

34.2

7.3

23.8

3.4

0.3

90

11.3

30.9

2.9

26.0

1.1

2.8

0.5

2

12.4

999.5

7.0

0.3

1.1

0.13

0.5

7

12.0

243.7

2.1

18.7

0.4

1.4

28

11.7

73.0

2.2

24.

0.2

0

2.1

90

11.3

25.3

2.2

27.8

0.8

2.1

1

3

12.6

913.1

4.1

0.8

3.7

0.14

2.1

7

12.4

372.0

2.2

10.9

2.9

2.3

28

11.9

113.0

6.5

23.4

4.1

1.3

90

11.3

23.1

3.4

29.6

0.9

1.5

DTA/TGA results

In the picture above, it is possible to see the gradual samples mass decreasing during heating. The sharp mass drop around 100 °C is caused by the release of free water from the samples. The water in samples may remain as capillary water, or as part of CSH gel, ettringite or hydrates despite the crushing of the sample and washing with acetone. In the figure we can observe the endothermic reactions that take place in the systems. The small endothermic peak around 370 °C could be due to complete loss of free water as no water can be present after exceeding the temperature of 374 °C. Another visible mass loss was observed above 400 °C, where the thermal decomposition of portlandite happens. The last endothermic peak is visible around 700 °C due to the decomposition of carbonates. While to the amount of water and carbonates in the sample primarily influencing its storage, the amount of portlandite can be affected by the presence of zinc (Figs. 9 and 10).
Fig. 8

DTA and DTA curves of sample with 1% of ZnO after 28 days of hydration

The differences between pure cement and cement with 1% ZnO are shown on TGA curves in Figs. 9 and 10.

Amount of portlandite in the samples was calculated according to mass loss around 400 °C. The results are shown in Table 11. The samples with ZnO cannot be measured after 1 day because of hydration delaying (Figs. 9, 10).
Table 11

Amount of portlandite in the pure cement sample and sample with 1% ZnO

Sample

Days of hydration

Portlandite amount

Cement

1

3.096

7

3.340

28

3.418

90

2.950

Cement + ZnO

4

1.759

7

2.350

28

1.833

90

1.752

Fig. 9

TGA curves of pure cement

Fig. 10

TGA curves of samples with 1% of ZnO

According to TGA analysis, the amount of portlandite found in the sample with ZnO is lower than that of the pure cement sample. This is probably due to the preferential precipitation of zinc hydroxyl anions in the presence of zinc [11]. No significant decrease indicating a measurable amount of portlandite was found in the sample with ZnCl2 and Zn(NO3)2, which was also confirmed by XRD analysis. The results measured by XRD and TGA analysis are different because the XRD analysis shows only the content of crystalline components, but the TGA analysis shows the total amount of portlandite.

Mutual comparison of these samples is not possible due to a significant retardation of hydration due to the zinc retardation effect.

Conclusions

The aim of this work was to monitor zinc effect on Portland cement hydration by using isothermal and isoperibolic calorimetry. Zinc was doped in the form of two soluble salts Zn(NO3)2·6H2O, ZnCl2 and ZnO which is very poorly soluble.

An inhibitory effect of zinc has been shown using both types of calorimetry. The retardation effect of zinc with increasing concentration was not observed in isoperibolic measurements of the sample with 1 mass% Zn in the form of ZnCl2. Here, the effect of chloride ions was shown as hydration accelerators, but this action was only at this dose and with this type of calorimetry.

The induction period lengths were plotted as a function of zinc content and intertwined with exponential dependence. For all tested additives and for all the investigated compounds, both the calorimetric methods found very good agreement between the experimental data and the interleaved exponential function.

Very similar retardation effect was measured for both soluble salts. The largest extension of induction periods was demonstrated in ZnO specimens, where longer retardation effect was observed due to its very slow dissolution.

The composition of samples after hydration was checked by XRD, ICP-OES and TGA/DTA method.

Despite the different conditions in both calorimetric measurements, the data measured at the same temperature of 25 °C have some consistent trends. The times when maximum temperatures were achieved, or heat flows in the first 48 h of hydration were almost identical for all samples. The developed heats differ primarily due to higher temperatures of samples in isoperibolic calorimeter which causes a faster increase in hydrating heat and steeper curves.

Due to better temperature control, isothermal calorimetry allows monitoring of hydration for a longer time and monitoring of partial reactions not visible by isoperibolic calorimetry. On the other hand, isoperibolic calorimetry allows the observation of the reactions in real conditions and thus the observation of the reactions which cannot occur in simulated conditions.

Notes

Acknowledgements

This work was financially supported by the project Materials Research Centre at FCH BUT—Sustainability and Development. REG LO1211 with financial support from National Program for Sustainability I (Ministry of Education Youth and Sports).

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2017

Authors and Affiliations

  • Pavel Šiler
    • 1
  • Iva Kolářová
    • 1
  • Radoslav Novotný
    • 1
  • Jiří Másilko
    • 1
  • Jaromír Pořízka
    • 1
  • Jan Bednárek
    • 1
  • Jiří Švec
    • 1
  • Tomáš Opravil
    • 1
  1. 1.Faculty of Chemistry, Materials Research CentreBrno University of TechnologyBrnoCzech Republic

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