Study of hydration and hardening processes of self-sensing cement-based materials with carbon black content


In this paper was assessed the influence of carbon black (CB) powder on the hydration and hardening processes of Portland cement (PC)-based materials; this type of materials is designed to be used as cement-based sensors in structural or pavement health monitoring. The rate of PC hydration and hardening processes is influenced by the amount of CB addition, with direct consequences on some properties of these materials, i.e., water for normal consistency and setting time. These results coupled with those obtained by thermal and X-ray diffraction analyses suggest a delaying effect of PC hydration and hardening processes in the presence of carbon black powder, most likely as a result of CB nanoparticles high adsorbent surface. Compressive strength values of PC mortars with CB content up to 1 mass%, assessed after 28 days of hardening, are comparable with those of PC mortars; thus, these materials can be successfully used in construction.


In the last years, there has been a growing concern regarding maintenance and monitoring of different infrastructures such as bridges, dams and buildings. In this context, a new concept has emerged named structural health monitoring (SHM) that sets to enable better, more effective means for evaluating stress and/or deformations that occur in various civil infrastructures. This evaluation can be done by adding different conductive fillers in order to decrease electrical resistivity of Portland cement (PC)-based materials (mortars or concrete) used in the construction of respective infrastructures [1,2,3].

One major reason for failure of conventional concrete-based structures is the lack of an effective mean for monitoring cumulative damage and its degradation through different periods of time [4, 5]. Within the scientific community, nowadays there are a number of studies that report the use of different fillers in order to increase electrical sensitivity of PC-based composites such as carbon fibers [4,5,6,7,8,9,10], carbon nanotubes [11,12,13,14,15], steel fibers [16, 17] and carbon black [4, 17,18,19].

Carbon black (CB) particles used in these types of composites present some advantages related to other additives, especially from an economical point of view, since this material is classified as a waste. The primary goal of using PC-based materials which possess electrical sensitivity in modern infrastructures is that one can gather different kinds of information such as service loads, cracks, corrosion and moisture via a computer software autonomously. As a result, since different parameters are monitored, an abrupt change can be easily detected and sent to an observer as an alarm [2, 17, 20, 21].

Hydration of Portland cement is a complex process that takes place in several steps and is related to a series of chemical reactions. The nature and amount of reaction products (hydrates) are responsible for the development of properties such as setting time or mechanical strength.

Although the number of scientific publications regarding the manufacture and characterization of self-sensing piezoresistive Portland cement composites with CB content increased in the past 3 years, to the best of our knowledge there is no information available regarding the influence of carbon black powder on the PC hydration and hardening processes.

Thermal analysis (TG–DTA) is a useful and reliable method used to assess the kinetic of PC hydration processes as well as the nature and amount of resulted hydrates [22,23,24,25,26,27,28,29].

In this context, the main objective of this research is to determine the influence of carbon black powder (CB) on the PC hydration and hardening processes, using different analytical techniques, i.e., thermal analysis (TG–DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Materials and methods

The materials used to prepare the cement-based self-sensing pastes are:

  • Portland cement (PC) CEM 42.5R with the following characteristics: CaO—63.78 mass%, SiO2—20.12 mass%, Al2O3—4.58 mass%, Fe2O3—3.99 mass%, MgO—1.20 mass%, SO3—2.61 mass%, L.O.I—3.72 mass%.

  • Carbon black nanopowder (CB) with a BET specific surface area of 72 m2 g−1 and an average size diameter of 53 nm.

Four types of binders were prepared by the partial replacement of Portland cement with various amounts of carbon black nanopowder, e.g., 0.5, 1, 3 and 5 mass%.

In order to assess the influence of carbon black addition on the Portland cement hydration processes, pastes with water-to-binder ratio of 0.5 were also prepared.

The pastes were hardened for different periods of time—3, 28, 60 and 90 days—and characterized by thermogravimetry and thermal analysis (TG–DTA) and X-ray diffraction (XRD). The XRD analysis was performed using a Shimadzu XRD 6000 diffractometer, with Ni-filtered CuKα radiation (α = 1.5406 Å), in 2θ = 5°–80° range.

The thermal analysis was performed using a DTG-TA-51H equipment, in air in the 20–1000 °C temperature range, with a heating rate of 10 °C min−1.

The microstructure of pastes without/with 5 mass% carbon black addition was assessed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray analysis (EDAX) using a Quanta Inspect F scanning electron microscope (1.2 nm resolution).

The water for normal consistency and setting time of cement pastes were determined in agreement with the requirements of European and corresponding Romanian norm SR EN 196-3 [30].

In order to assess the influence of carbon black nanopowder additions on the compressive strengths, mortar specimens with binder/sand = 1:3 and water-to-binder ratio of 0.5 were prepared. The aggregate was siliceous sand which fulfilled the requirements of European and corresponding Romanian norm SR EN 196-1 [31]. The fresh mortars were cast in rectangular molds (40 × 40 × 160 mm) and vibrated for 2 min; the specimens were cured in the molds the first 24 h, then demolded and cured up to 90 days in humid air (R.H. 90%). The compressive strengths were assessed using a Matest machine, and the values represent the average of least four strength values assessed on specimens cured in similar conditions.

Results and discussions

The water for normal (standard) consistency and setting time of cement pastes along with the mechanical strengths (assessed on mortars) are the main properties of Portland cement which are defined for its quality [32].

Therefore, in Fig. 1 is presented the influence of the partial replacement of Portland cement with various amounts of carbon black nanopowder on the amount of water for normal consistency (Wnc).

Fig. 1

Water for normal consistency (Wnc) and setting times versus amount of CB nanopowder (mass%)

As it can be seen, the amount of Wnc increases with the increase in CB dosage; this is the direct consequence of high water adsorption capacity of the CB nanopowder.

The initial setting time of cement pastes with various amounts of CB nanopowder is much shorter compared with reference (cement paste without CB)—Fig. 1. This variation can be explained by the initial adsorption of water on CB nanoparticles, which determines the increase in cement paste consistency (stiffening); in this case, one can speak of false set of cement. On the opposite side, the final setting time for the paste with CB additions is longer compared with reference. This delay is probably due to the releasing, after a while, of a significant amount of water in the system from carbon black nanoparticles; moreover, for higher CB dosages are also possible the agglomeration of CB nanoparticles at the surface of anhydrous cement particles and the formation of a film which prevents further hydration.

Figure 2 shows the evolution of compressive strengths versus curing time. As expected, the values of compressive strength increase versus curing time, mainly due to the development of Portland cement hydration and hardening processes.

Fig. 2

Compressive strength versus the amount of CB nanopowder (mass%)

The substitution of Portland cement with a lower amount of CB (0.5 and 1 mass%) has a limited influence of the compressive strength values; the increase in carbon black amount in mortar formulations (3 and 5 mass%) determines a decrease in compressive strength, as a consequence of a larger mass of cement substituted, as well as due to the increase in capillary porosity (consequence of a higher amount of water not bound in hydrates).

In order to better understand the influence of CB addition on the hydration processes and resulted hydrates, XRD analyses were performed on cement pastes, with/without CB addition, hardened for different periods of time—Fig. 3. XRD pattern presented in Fig. 3a shows the presence of the main mineralogical compounds specific for Portland cement, i.e., alite Ca3SiO5, JCPDS 42-0551, belite Ca2SiO4, JCPDS 33-0303 and calcium aluminate Ca3Al2O6, JCPDS 33-0251. The intensity of peaks specific for these anhydrous compounds assessed also on the XRD patterns of hydrated pastes (Fig. 3b, c) decreases versus time as a consequence of their consumption in the hydration process, and new peaks specific for the crystalline hydrates Ca(OH)2 (JCPDS 04-0733) and ettringite Ca6Al2(SO4)3(OH)2 (JCPDS 41-1451) are present.

Fig. 3

XRD patterns for anhydrous Portland cement (a) and hardened cements—0 mass% CB (b) and 5 mass% CB (c)

One can also notice the presence of CaCO3 (JCPDS 05-0586), especially after longer curing times. This compound results by the reaction of Ca(OH)2 with atmospheric CO2 [33].

For the pastes with various amounts of CB, hardened for 28 days (Fig. 4), the intensity of peaks specific for calcium hydroxide and ettringite (the crystalline hydrates resulted by cement hydration) seems to decrease with the increase in CB contents, suggesting a delay of this process.

Fig. 4

XRD patterns for pastes with various amounts of CB, hardened for 28 days

Thermal analysis (TG–DTA) can provide supplementary quantitative information regarding the rate of Portland cement hydration process and reaction products—both crystalline and gel-like [22,23,24,25,26,27,28,29, 33].

This analysis was performed on pastes without/with various amounts of CB (0.5, 1, 3 and 5 mass%) after 3, 28, 60, 90 days of hydration. Figure 5 presents, by way of example, the TG and DTA curves assessed on pastes hydrated for 28 days.

Fig. 5

Thermal analysis (DTA-TG) for pastes with/without CB, hardened for 28 days

For all studied specimens, one can assess on the DTA curves the following effects (Fig. 5b), attributed to the following processed [33]:

  • The large endothermic effect with maximum at approx. 120 °C and a shoulder at approx. 160 °C is attributed to the superposition of several processes, i.e., loss of moisture, loss of water bound in calcium silicate hydrates (C–S–H) with a low crystallinity and loss of water bound in ettringite.

  • The endo-effect with maxim between 452 and 468 °C is attributed to the calcium hydroxide dehydration.

  • The endothermic effect with maximum between 700 and 740 °C is due to the water loss from C–S–H with a higher crystallinity and decomposition of CaCO3, which resulted by the partial carbonation of Ca(OH)2.

On the corresponding TG curves (Fig. 5a), mass losses are recorded for these endothermic effects. The mass changes recorded for different temperature ranges are presented in Table 1.

Table 1 Mass changes recorded on TG curves for various temperature ranges

For the pastes with a higher amount of CB (3 and 5 mass%), one can notice on the DTA curves the presence of an exothermic effect at approx. 480 °C. This effect is due, in our opinion, to the oxidation of CB nanoparticles, with CO2 generation; the small mass increase recorded on the corresponding TG curves is due to the CO2 reaction with Ca(OH)2 and CaCO3 formation. The further increase in the temperature determines CaCO3 decomposition, illustrated by a higher mass loss (on TG curves—Fig. 5a and Table 1) and the shifting of endo-effect corresponding to this process to higher temperatures (Fig. 5b).

The mass loss corresponding to the temperature range 625–885 °C, which is due to the superposition of two processes, i.e., the water loss from C–S–H with a higher crystallinity degree and calcium carbonate decomposition [33], could provide a quantitative assessment of these processes.

In Fig. 6 is presented the difference between the mass losses (ΔML) recorded in this temperature range for the cement pastes with CB content and the corresponding mass loss assessed on the plain cement paste. Positive values of this difference imply the increase in this mass loss for the specimens with CB content compared with plain cement paste.

Fig. 6

ΔML for pastes with various amounts of CB hydrated for 3, 28, 60 and 90 days

The important increase in ΔML values can be noticed for the pastes with a higher amount of CB (3 and 5 mass%) which is in good agreement with the explanation mentioned above, i.e., the Ca(OH)2 carbonation.

The mass loss recorded on TG curves, in the temperature range 450–500 °C (corresponding to Ca(OH)2 decomposition), was used to calculate the amount of Ca(OH)2 present in the hydrated pastes—CHe; these values were compared with the theoretical amount of Ca(OH)2–CHt, i.e., the amount which should result by the hydration of the cement fraction present in the pastes with various amounts of CB. (CB partially replaces Portland cement in binder formulations.) Based on this assumption for Portland cement (without CB), CHe is equal to CHt.

As it can be seen from the data presented in Fig. 7, both CHt and CHe values decrease with the increase in CB amount and increase with the increase in curing time (due to increase in Portland cement hydration degree).

Fig. 7

Variation in Ca(OH)2 amount versus CB content in pastes hydrated for various times: a CHt, b CHe

It is important to note the lower CHe values assessed for the pastes with CB content as compared with the corresponding CHe. This difference increases with the increase in CB amount, thus demonstrating (i) the delaying effect on the cement hydration exerted by this nanopowder and (ii) the consumption of CH in the reaction with CO2 generated during the burning of CB—especially for the pastes with higher amount of CB (3 mass% and 5 mass%); the delay of cement hydration in the presence of CB is confirmed also by the evolution of final setting time versus CB amount, previously presented (Fig. 2).

The microstructure of mortar specimens without/with 5 mass% CB, hardened for 28 and 90 days, assessed by SEM, is presented in Figs. 8 and 9.

Fig. 8

SEM images of Portland cement paste hydrated for 28 days (a, b) and 90 days (c, d)

Fig. 9

SEM images of cement paste with 5 mass% CB hydrated for 28 days (a, b) and 90 days (c, d) and EDX analysis of zone 1 and zone 2 (e) from c

Irrespective of curing time, one can assess on SEM images of both mortars the presence of hydrates resulted by cement hydration, with specific morphology [34]:

  • calcium hydroxide—CH—hexagonal plates;

  • ettringite (AFt)—well-defined needle-shaped formations;

  • calcium silicate hydrates (C–S–H)—fine needles and films;

  • plate-like crystals characteristic for calcium carbonate (CC).

The assignation of hydrates based on their specific morphology was confirmed by EDAX analysis; for the purpose of illustration, the EDAX analysis presented in Fig. 9e confirms the presence of calcium carbonate, calcium silicate hydrates (C–S–H) and calcium sulfate aluminate hydrate (AFt).

As expected, the increase in curing time from 28 to 90 days determines an increase in sizes of hydrates and calcium carbonate crystals.


The experimental results presented in this article demonstrate the influence exerted by the carbon black (CB) powder on the main properties of PC-based materials; the increase in CB content (up to 5 mass%) increases the amount of water for normal consistency (Wnc) and delays the final setting time (with reference to plain Portland cement).

These results coupled with those obtained by DTA–TG and XRD analyses suggest a delaying effect of PC hydration and hardening processes in the presence of carbon black powder, most likely as a result of CB nanoparticles high adsorbent surface.

The compressive strength values of PC mortars with lower amount of CB (0.5–1 mass%) are comparable to the ones of reference (plain PC mortar); thus, these materials can be successfully used in construction.


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Correspondence to Georgeta Voicu.

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Pârvan, MG., Voicu, G. & Bădănoiu, AI. Study of hydration and hardening processes of self-sensing cement-based materials with carbon black content. J Therm Anal Calorim 139, 807–815 (2020).

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  • Cement
  • Carbon black
  • Hydration processes
  • Thermal analysis
  • Properties