Contribution of CNTs/CNFs morphology to reduction of autogenous shrinkage of Portland cement paste

Abstract

In this experimental study, carbon nanotubes (CNTs) and carbon nanofibers (CNFs) were dispersed by intensive sonication in water in the presence of superplasticizer and subsequently mixed with Portland cement with water/cement ratios varying between 0.3 and 0.4. The autogenous shrinkage in the fresh stage was investigated. The CNTs and CNFs were characterized by high resolution scanning electron microscopy (SEM) and the hydrated pastes were studied by X-ray diffraction and SEM. The results showed a reduction of the autogenous shrinkage by 50% for pastes containing small amounts (0.01 wt%) of nanomaterials. Higher additions appeared to be less effective. The highest reduction of shrinkage was observed for carbon nanofibers which were long, rather straight and had diameters of around 200 nm. The result showed that the addition of nanomaterials accelerated the hydration processes especially in the early stages of hydration. The effect was the most pronounced in the case of functionalized nanotubes. The proposed mechanism resulting in the reduction of the autogenous shrinkage was a combination of nano-reinforcing effects, alterations of hydration and microstructure of the hydrated matrix.

Keywords

carbon nanotubes / shrinkage / cement paste

Introduction

Autogenous shrinkage can be defined as an external volume change of cement or concrete without moisture transfer to and from the surrounding. Autogenous shrinkage should be limited as it might lead to cracking [ 1]. The volume change is related to the chemical shrinkage developing during hydration of Portland cement [ 2, 3]. Chemical shrinkage starts to develop just after mixing Portland cement with water and its amount can be calculated using the molecular weight and densities of the compounds forming during hydration. Autogenous shrinkage can develop already before the solid skeletal is formed, [ 4]. This phenomenon was recognized a long time ago but its importance in concrete technology is continuously increasing with the wider usage of very low water to cement ratio pastes and additions of ultrafine secondary binders such as, e.g., silica fume [ 5]. Although the physical bases of the mechanism behind the autogenous shrinkage is not yet fully understood several models has been proposed [ 1]. In general it is agreed that autogenous shrinkage can be related to changes of the relative humidity (RH) in pores of the hardening cement paste. The high strength concretes containing silica fume showed also increasing autogenous shrinkage and self-desiccation with lowering of the RH [ 4]. Other factors taken into consideration were changes in the surface tension of the solid gel particles, disjoining pressure as well as tension in capillary water. The autogenous shrinkage itself could be divided into a chemical shrinkage prior to setting, a chemical shrinkage after the final set and the self-desiccation [ 6]. Already in 1948 it had been calculated that the volume decrease due to hydration of Portland cement accounted for 6‒7 mL for each 100 g of hydrated cement [ 6, 7], found that the formation of ettringite contributed to the chemical shrinkage as well. Later it was calculated by Jensen and Hansen [ 4] that the reaction of silica fume with calcium hydroxide produced an estimated 20 mL of volume change for each 100 g of hydrated Portland cement. Autogenous shrinkage can be reduced by proper curing procedure and mix design (higher volume of coarse aggregates), low binder content and usage of expansive additives or shrinkage reducing admixtures [ 8, 9]. A new method for reduction of the autogenous shrinkage by addition of carbon nanotubes was described by Cwirzen [ 10]. The initial test results showed a potentially significant reduction of the early autogenous shrinkage. The application of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) in concrete technology, despite their extraordinary physical and mechanical properties [ 11], is still at a rather early stage. Some researchers studied the effects of CNT/CNFs on the mechanical properties of pastes and concretes [ 12‒ 14], frost durability of pastes and concretes [ 15] and sensing properties of concretes [ 16]. The hydration processes of cement appeared to be slightly accelerated by single walled carbon nanotubes (SWCNT), which could also potentially increase the autogenous shrinkage [ 17]. The effects of MWCNTs on autogenous shrinkage were investigated by e.g., Konsta-Gdotous et al. [ 18]. Measurements which started 6 h after casting showed lowering autogenous shrinkage values with increase content of MWCNTs in comparison with plain cement paste. Authors attributed this decrease to decrease of fine pores which lead to reduction of capillary stresses and thus lower autogenous strains.

The main objective of this paper was to study the potential effects of the CNTs and CNFs morphology on the autogenous shrinkage of low water to cement ratio Portland cement pastes.

Experimental setup

Sulfate resistant (SR) Portland cement (PC), from Finnsementti (CEM I 42, 5N-SR3) was used as binder. This SR cement was chosen due to its known weak interaction with polycarboxylate types of superplasticizers. All tests were performed using cement pastes and their compositions are shown in Table 1. 17 different mixtures having CNT or CNF concentrations between 0.005 and 0.1 wt% of the cement were produced, because earlier studies showed that CNTs improve the mechanical properties when added at low concentrations [ 13, 18‒ 20]. The water to cement ratio was 0.3 for most of the mixes and 0.4 for three additional mixes. The CNTs and CNFs were incorporated into the cement paste as dispersed in water in the presence of a polycarboxylate based superplasticizer (SP) Kolloment type produced by Grace Chemicals. The amount of SP varied between 0 and 0.2 wt% of cement.

Four different types of nanomaterials were used: non-purified carbon nanofibers synthesized at Aalto university (CNF1) and Pyrograf III carbon nanofibers from Pyrograf Products, Inc (CNF2), functionalized multiwalled carbon nanotubes (fCNT) and carbon nanotubes from Nanocyl type 7000 (nCNT). The functionalization of the CNTs (fCNT) was done by refluxing the nCNTs in HNO₃ (con. 63%) at about 110 °C for 4 h. The obtained solution was centrifuged and washed with deionized water to remove HNO₃ until neutral pH was reached. The produced functionalized fCNTs were dried at room temperature for at least 24 h.

The specimen preparation included dispersion of the CNTs/CNFs in water by intensive sonication in the presence of SP. A sonicating finger type Omni Sonic Ruptor 400 included in the Laser Diffraction Particle Size Analyzer (Beckman Coulter LS 13 320) was used. The sonicating power of the sonicator was 400 Watts and the frequency 20 kHz. The sonication time was 20 min in the case of all mixes. Additionally, one control mix where sonication time was 40 min was done. The agglomeration of CNTs/CNFs was estimated visually and by weighting filtered agglomerates remaining in the dispersion solution after sonication process.

Initial study revealed that application of 20 min of intensive sonication using sonication finger provided the best de-agglomeration and dispersion stability of all fours types of CNTs/CNFs. The autogenous shrinkage was measured using a modified method described by Jensen and Hansen [ 4]. For this purpose, the cement paste was cast into a transparent corrugated flexible tube having diameter of 2.1‒2.6 mm and enclosed at both ends with PVC elements. The strain was measured for at least 45 h starting 32 min after casting. The used setup is shown in Figure 1. The effect of CNTs and CNFs on the workability was measured by a non-standardized mini flow test method. Two perpendicular measurements were done for each sample: just after mixing and 20 min later. The same setup was used earlier by Cwirzen et al. [ 19]. The cone was 10 mm high and had upper and lower diameters of respectively 13 mm and 20 mm.

The flexural strength was determined using paste beams with dimensions of 10 mm ´ 10 mm ´ 60 mm. Teflon moulds were used to avoid contamination by the release oil. A low volume vacuum mixer was used to produce pastes. No vibration was applied to any of the specimens. Test was done by 3-point bending using Roell+Korthaus instrument, with a load rate of 0.35 mm/min.

The morphology of the used CNTs and CNFs as well as the microstructure of the fractured paste specimens was determined using a Field Emission Scanning Electron Microscope (FE-SEM) type Quant FEG 450 produced by FEI. The operating parameters were: accelerating voltage between 5–10 keV, chamber pressure of 10^–3 Pa and a working distance of 15 mm. No additional conductive coating was applied to the specimens’ surfaces. The secondary electron mode detector was used to obtain all shown images.

The chemical composition of the test pastes was determined by XRD analysis using a Philips PW1830 diffractrometer using powdered 28-day old paste samples. The accelerating voltage was 40 keV.

Results

Morphology and agglomeration of CNTs and CNFs

The FE-SEM investigation showed a significant difference in the morphology of the used CNTs/CNFs. FE-SEM images taken at three different magnifications enabled to obtain information about agglomeration degree, shape, length as well as the average diameter, Figs. 2–4. The observations are summarized in Table 2. The results showed that carbon nanotubes, both types fCNT and nCNT had diameters between 20–30 nm and lengths above 5 and 2 µm respectively. Both CNTs were slightly curved.

The functionalized CNTs marked as fCNT showed the highest degree of agglomeration and formed particle-like dense agglomerates having diameter between 10 and 200 µm, Fig. 2. Formation of densely packed agglomerates during functionalization processes was reported previously [ 19, 21]. The non-functionalized nCNTs appeared to form loosely packed ropes of nanotubes. Each rope had a diameter of approximately 2 µm and length of several tens of micrometers. The CNTs within these ropes appeared to be not well aligned but rather bundled in all directions. The formation of ropes was not observed in other studied nanofibers or nanotubes. The carbon nanofibers CNF1 showed to be highly agglomerated with agglomerate particle size of around 10–50 µm. The agglomerates were also densely packed similarly as observed in the case of the functionalized CNTs (fCNT). The single CNF1 nanofibers appeared to be the curliest of all studied materials, Fig. 3. Furthermore, the high magnification FE-SEM image shown in Fig. 4 indicated a high number of surface defects.

All used nanofibers and nanotubes fibers were dispersed in water solution by intensive sonication in the presence of a SP. The used procedure followed earlier studies [ 19]. The FE-SEM analysis showed that despite elongated sonication times of 20 or 40 min some agglomerates were still present in the solution. In order to estimate the amount of agglomerates all sonicated solutions were filtered through a laboratory quality filter having an average opening size between 20 and 40 µm. The obtained results are summarized in Table 3. Since the amount of SP was in all cases constant, a smaller added amount of CNT/CNF resulted in more SP polymers present in the dispersion. The results showed that at the lowest amount of CNT/CNF (0.005 wt%) no agglomerates larger than 20 µm were detected in any of the dispersions. Higher additions of fibers, 0.05 and 0.1 wt%, increased the amount of agglomerates remaining after sonication. The highest amounts of agglomerates were measured in the case of the carbon nanofibers CNF1 and CNF2. It accounted for 44 and 36 wt% at 0.1 wt% addition. The used sonication energy and sonication duration and the amount of SP were the same in all studied in this part cases. Only the additional sample containing CNF2 was also sonicated for 40 min. Consequently, the dispersion degree and thus the amount of present agglomerates can be directly related to the morphology of the nanofibers and nanotubes. The FE-SEM studies showed that the most significant difference between the CNFs and CNTs was their diameter which varied between 100 nm and 20 nm respectively. The used sonicating power of 400 Watts and frequency of 20 kHz was more suitable for smaller diameter CNTs as less agglomerates was found. This correlation between the CNTs diameter and the required sonication energy was also observed by others [ 22, 23], indicating a significantly higher energy requirement to disperse larger CNTs. The optimal required energy can be directly attributed to the van der Waals forces between the CNTs which are increased with the diameter and length, see the equation below [ 24].

(1) \mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{AL{d}^{\mathrm{1}/\mathrm{2}}{H}^{-\mathrm{3}/\mathrm{2}}}{\mathrm{24}}}

where A is Hamaker constant ~2 ´ 10^‒19 J, L is the length, d is the diameter, and H is the separation distance at the point of closest approach. This relationship could also explain why elongation of the sonication time from 20 to 40 min did not decrease the amount of weighted agglomerates of the CNF2. In this case the energy required to enhance the dispersion would have to be also increased. Higher amount of agglomerates measured in the case of CNF1, Table 3, confirm this conclusion as their diameter is also close to 90 nm and length exceeds 100 nm as in the case of CNF2.

The FE-SEM analysis was done on samples taken from the dispersion of CNTs/CNFs in water after the sonication to define the effects of the sonication on the morphology. A sampled dispersion droplet was deposited on a carbon tape and the water was removed by evaporation in an oven at 50 °C. Examples of the obtained high resolution FE-SEM images of carbon nanofibers CNF2 after sonication for 20 and 40 min are shown in Figure 5. SEM observations revealed that the sonication process not only exfoliated the CNTs/CNFs but also fractured and shortened the dispersed nanotubes and nanofibers. The initial length of a singular fiber decreased from an original 10‒20 μm to less than 5 μm. Sonication is known to result in a significant alteration of CNT/CNF morphology including shortening, as well as peeling of the layers in MWCNTs [ 25‒ 28]. The damage of CNTs and CNF is the result of the sonication’s dispersion mechanism. The sonication process generates alternating low- and high-pressure waves at frequencies corresponding to sound waves [ 24]. These lead to formation of vacuum bubbles and their subsequent violent collapse. These collapsing bubbles generate high forces, called cavitation, which can reach tens of GPa, which is in the same range as the tensile strength of CNTs and higher than the CNFs [ 29]. In the present study the elongation of the sonication time from 20 to 40 min did neither cause further shortening of the nanofibers nor further exfoliation of sonication agglomerates. Thus it can be concluded that for a specific combination of CNF/CNT and sonication power a limit in sonication time could exist. In that case elongation of the sonication time without changing other parameters would not improve the dispersion degree and would not cause more damage to the CNT/CNF. Similar conclusions were formulated by others [ 30‒ 32]. During sonication process the CNT/CNF length tends to reach a specific limit values and further changes in length do not occur. In the present study the main parameter during sonication process was the time required for the SP to diffuse to the surface of CNTs or CNFs. Unfortunately, the required 20 min to obtain a stabile dispersion caused shortening of their length.

Autogenous shrinkage

The measured values of the autogenous shrinkage are shown in Figs. 6–9. In general, addition of CNTs/CNFs decreased the measured shrinkage. The shrinkage value of the reference pure Portland cement paste, which was used as reference, was 13000 µm/m. An addition of 0.01 wt% of CNF1 lead to a shrinkage value of less than 6000 µm/m. Smaller and larger amounts of CNFs produced a smaller reduction of the measured shrinkage which ranged from 10000 to 7000 µm/m. Addition of CNF3 (Pyrograf carbon nano fibers) showed the highest reduction to 5500 µm/m for an addition of 0.01 wt%. Interestingly, addition of 0.1 wt% of CNF3 increased the recorded shrinkage. Additions of carbon nanotubes, having significantly smaller diameters in comparssion with CNFs, showed a smaller reduction of the autogenous shrinkage. In the case of nCNT changes in the amount of nanotubes added did not produce significant changes of the recorded values. In this case all recorded values oscillated around 8000 µm/m. Additions of 0.1 and 0.05 wt% resulted in a maximum shrinkage of around 8500 µm/m while for 0.005 and 0.01 wt% a value of 7500 µm/m was obtained. The test results showed that addition of functionalized carbon nanotubes fCNT reduced the shrinkage to a minimum value of around 6500 µm/m at an addition level of 0.01 wt%. Higher and smaller amounts added still reduced the shrinkage but to a lesser extent.

XRD test results

The XRD analysis was done only for pastes containing fCNT and CNF1. The obtained results were compared with the reference cement paste, Figs. 10 and 11. All pastes containing either CNTs or CNFs revealed higher peaks corresponding to Portlandite (P) and lower peaks corresponding to anhydrous cement and CSH (C₂S, C₂C₃CSH). The effect was the most pronounced in the case of the functionalized fCNT.

Analysis and discussion

The addition of CNFs and CNTs decreased the autogenous shrinkage of the studied cement pastes in most cases. The correlation between the recorded autogenous shrinkage values and the CNT/CNF type and their concentration is shown in Fig. 12. The uncorrected for agglomerates concentration of CNT/CNF is shown in Fig. 12a, while in Fig. 12b the weights of the agglomerates are subtracted. The comparison of these two figures revealed that the efficiency to reduce the autogenous shrinkage was in the case of nCNTs nearly constant for all concentrations. The amount of the agglomerated nCNTs remaining on filters appeared to be very low. In the case of dispersions containing 0.1wt% nanomaterials, the agglomerates accounted for only 1 wt% of the total added amount in the case of nCNTs versus 36 and 44 wt% in the case of CNF2 and CNF1. Thus, the carbon nanofibers were the least efficiently dispersed of all materials used. Despite that fact they provided the highest reduction of the autogenous shrinkage values. As shown in Fig. 12b the reduction of the autogenous shrinkage at higher additions, 0.05 wt% and 0.1 wt%, appeared not to be directly related to the amount of agglomerates present in the dispersion after sonication. Instead it was more related to their type. At the dosage of 0.01 wt% nearly all agglomerates were dispersed during sonication process in all studied types of CNTs/CNFs, which enabled to analyze the effect of the fiber type on the autogenous shrinkage. The results showed that the highest reduction occurred in the case of the carbon nanofibers, CNF1 and CNF2 followed by the functionalized fCNTs and the non-functionalized nCNTs. Additionally, larger CNF2 showed a slightly higher reduction.

The analysis of the autogenous shrinkage rate development was done based on the angle of tangent of the initial strain vs. time curve (nearly linear part of the curve). The relation between the CNT/CNF amount and the type vs. tangent is shown in Fig. 13. A smaller angle corresponded to a lower development rate. The effect on the CNT/CNF was the most pronounced at concentration of 0.005 wt%. The corresponding ultimate autogenous shrinkage values were also the lowest at this concentration. Furthermore, the development rate decreased depending on the type of the CNT/CNF in the following order from the lowest to the highest: CNF2, CNF1, fCNT and nCNT. Exactly the same order was observed by the recorded ultimate autogenous shrinkage values; CNF2 was the lowest and nCNT was the highest. In comparison with the reference paste addition of 0.05 and 0.01 wt% of any type of the CNT/CNF decreased the ultimate shrinkage and lowered the shrinkage development rate. The influence of the CNF/CNT type on the autogenous shrinkage could be to some extent related with the hydration of Portland cement [ 17, 33]. In these earlier studies the incorporation of SWCNT (single walled carbon nanotubes) produced higher initial mechanical properties which leveled back to the reference paste values after 15 days of hydration. The hydration heat development curves showed slightly higher temperatures and earlier maximum temperature peak in mixes with SWCNTs indicating more intensive hydration processes. The results of the present research comply with these observations. For example, the XRD spectra clearly indicated that the amount of the anhydrous cement (observed as increased amount of C3S) was lower in the case of pastes incorporating CNF1 and fCNT in comparison with the reference paste. The XRD peaks corresponding to Portlandite (P) were also higher indicating more extensive hydration. The acceleration of the hydration processes was the most pronounced in the case of the functionalized (carboxylated) nanotubes fCNTs, also observed by, e.g., Petrunin [ 14]. In that case the acceleration was more visible at early stages resulting in slightly higher compressive strength values at 7 days and slightly lower at 28 days. The acceleration mechanism could be attributed to the nucleation processes which are proven to control the hydration rate of Portland cement, [34]. The FE-SEM investigation of the fractured samples also confirmed that CNT/CNF could act as nucleation sites for formation of C-S-H. Fig. 14 shows functionalized CNTs (fCNT) covered with hydration products, predominantly C-S-H. Similar images were obtained by Makar [ 17] where SWCNTs were found to be encapsulated in a thick C-S-H layer. The authors stated that the usage of SWCNT, which are significantly smaller in comparison to MWCNT and CNF, also enabled their incorporation between the C-S-H layers. The present results did not show whether this is also possible with larger nanotubes or even nanofibers. Another, factor affecting the hydration of Portland cement could be a possible effect of CNT/CNF on the dissolution and hydration of various cement phases and thus on mechanical properties and shrinkage. Consequently, the ultimate autogenous shrinkage as well as the shrinkage development rate could depend on the cement composition. For instance, Justens et al. [ 3] reported that an increased content of C3S and C3A contributed to a higher early age chemical shrinkage. The dissolution of some cement phases could also result in a chemical shrinkage, [ 35]. Unfortunately, the present test results cannot address the combined effects of the cement composition and of the CNT/CNF type, as only one type of cement was used. However, the obtained results could indicate a potentially significant influence of the type and amount of the CNT/CNF on the dissolution and hydration, as for example revealed by the XRD studies. The XRD results indicated that the presence of smaller diameter CNTs resulted in a higher hydration rate of Portland cement at early age in comparison with CNFs. However, the effect of CNTs on the ultimate shrinkage and shrinkage development rate was significantly lower in comparison with larger CNFs. It can be concluded that larger CNFs accelerated the hydration processes to a lesser extent and thus did not increase the autogenous shrinkage to the same degree as smaller CNTs. A higher hydration rate appeared to increase the autogenous shrinkage in, e.g., silica fume concretes [ 36]. As shown in Fig. 15 addition of CNTs/CNF has also affected the flexural strength of the produced pastes. The shown values were measured after 15 days of wet curing. The highest flexural strength values were recorded for pastes incorporating 0.01 wt% of non-functionalized CNTs (nCNT) and large diameter CNFs (CNF2). In both cases SEM investigation showed less agglomerates and less extensive bundling, Figure 3 and Table 2 in comparison with functionalized CNTs (fCNT) and small diameter CNFs (CNF1). This could result in their better dispersion in the hydrated binder matrix and thus a better transfer of tensile stresses. The lowest flexural strength values were observed for mixes containing functionalized CNTs (fCNT) which can be directly related to their worst dispersion within the hydrated matrix and presence of larger amount of agglomerates and bundles, Figure 3. Pastes with higher additions of CNTs and CNFs showed in general lower flexural strength values thus further indicating that agglomeration and worse dispersion were affecting the reinforcing effectiveness.

The open question remains regarding the failure mechanism of pastes incorporating CNT/CNF. The FE-SEM analysis indicates clearly a pull out of the CNT/CNF from the hardened binder matrix, Fig. 16. There were no indications suggesting breakage of the fibers due to tensile stresses, which complies with earlier conclusions of, [ 17]. The load transfer occurs most probably through a classical reinforcing mechanism due to interfacial physical bonding related to, e.g., the shape of the fibers. Earlier research by Konsta-Gdotous, [ 18, 20], also indicated that presence of CNTs increased the stiffness of the hydrated binder matrix when measured with nanointendation.

Conclusions

The results obtained in this study showed that the addition of CNTs or CNFs resulted in most cases in a significant reduction of the autogenous shrinkage. The morphology, including diameter, length and shape of the used CNTs and CNFs as well as the amount affected the observed effect. The highest reduction of the autogenous shrinkage was observed for carbon nanofibers (CNF2) which were long, rather straight and had diameters of around 200 nm. The results showed that the addition of both CNTs and CNFs accelerated hydration processes especially in the early stages of hydration. Two main mechanisms leading to the observed effects were proposed. The first mechanisms assumed that a well dispersed network of CNT/CNF could physically interconnect the hydration products forming at the early stage. Densification of the interstitial pore solution and phases forming in-between the CNT/CNF and their bundles could reinforce and stiffen the forming binder matrix, which in turn could limit the autogenous shrinkage.

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