The influence of graphene admixture on strength properties of cement-based composites

Cement-based composites pose a long tradition in the construction industry and are used in various forms such as structural concretes, masonry and plaster mortars, injection and special mixtures for three-dimensional concrete printing. Fine granular composites intended for injection and printing activities may have increased tend to shrinkage and cracking. The application of nanomaterials may bring a partial solution to toughening of cement-based composites. For this reason, the effect of the addition of various amounts of graphene admixture up to 0.03 wt.% on the strength properties of fine granular cement-based composites was researched. All prepared fresh mixes had the constant w/c ratio, and the behavior of modified composites was compared with plain reference material. On hardened test specimens, compressive strengths obtained after 1, 7 and 28 days of curing time were evaluated. The experimental investigation revealed, on the one hand, the partial worsening of workability of fresh mixes being compensated with an increasing amount of a superplasticizer and, on the other hand, the increment of compressive strength up to 12.5 % even for specimens with the lowest addition of graphene.


Introduction
Composite systems on a cement basis have been very valued materials in the construction industry due to their versatile utilization from bearing structures up to mortars or plaster mixes of a surface finish of buildings [1].Nowadays, three-dimensional concrete printing has become very attractive due to its effective layered extrusion manufacturing via a robotic arm [2].Nevertheless, composites used for this specific application differ in their composition from traditionally used mixes.To ensure a high rate of extrudability for thin printing of architectural elements, such materials contain fine-granular particles [3] that may cause increased shrinkage and cracking.Reinforcing techniques using nanomaterials can provide a way to modify the structure of cement-based fine granular composites and thus mitigate their adverse behaviour [4].Graphene is one of the possible materials to be applied [5].Generally, it constitutes a type of carbon-based two-dimensional material with one-layer thickness.However, considering its price and a practical applicability, multi-layer graphene nanoplatelets including a few up to 100 graphene sheets are produced.Usually, such advanced materials have the thickness around of several micrometers [6].Their mechanism of action was described as the control of polyhedral crystal growth during Portland cement hydration, and thus graphene contributes to the enhanced toughness of cement composites [7].
In this paper the influence of different amounts of graphene admixture on strength properties of fundamental formulations of fine-grained concrete mixture intended for three-dimensional printing via layered extrusion technique was researched.Performed tests revealed that even very low concentrations of graphene particles may considerably increase the compressive strength of hardened composites after the initial hydration period.

Experimental part
As the fundamental component for formulation of cement-based composites, Portland cement of the grade 42.5 R (CEM I) was used.The cement, provided by Českomoravský cement, Ltd., Czech Republic, contains in minimum 95% of Portland clinker and thus initiates considerable hydration heat of 332 J•g -1 in the first 7 days.Used cement was supplemented by fine silica powder (SF) produced by Av Eko-color, Ltd., Czech Republic, which contains up to 98 wt.% of pure silica dioxide in its amorphous form.Due to its active character, SF is recommended by its producer to modify concretes formulation.The chemical composition of CEM I provided by its producer and selected physical properties of both aforementioned powders are outlined in tables 1 and 2. Another fine component, GUP Admixture, provided by GrapheneUP SE, Ltd., Czech Republic, was applied to verify its direct influence on the strength properties of designed composites after different curing times.GUP Admixture constitutes the water suspension in which graphene particles in the concentration of 1.2 wt.% are dispersed in demi-water.The final product is represented by black viscous fluid (viscosity <120 mPa•s at 25 °C) having a density of 1.25 g•cm -3 .Distributed particles are composed of a few layers of graphene (2-5 layers) with low oxygen content, as stated by the producer.The powder density of input materials was measured according to the standard EN 1097-3 [8].Apparent density values were determined by the pycnometric measurement technique described in the EN 1097-7 [9].To obtain the specific surface area, the Blaine apparatus and corresponding procedure ) were applied.Particle size distribution curves were obtained via the laser diffraction technique with the device Cilas LD 1090 (Cilasariane Group, France), disposing the measuring range of 0.4 -500 μm.The composition of fine-grained cement-based composites used throughout experimental investigation is given in table 3.In total, four mixes were prepared, whereas three different graphene powder concentrations, namely 0.01; 0.02 and 0.03 wt.% of binder content (CEM I and SF), were considered.The amount of GUP Admixture was added depending on the desired graphene concentration in the particular mix, and the rest of demi-water to be part of suspension was deducted from the dosage of batch water.For all prepared mixes the same amount of batch water was kept, however, for maintaining constant consistency of 140 × 140 ± 10 mm (EN 1015-3 [11]) the super-plasticizer (SP) MasterGlenuim ACE 430 (Master Builders Solutions CZ, Ltd., Czech Republic) was incorporated.Silica sand in the fraction of 0.0/0.5 mm, delivered by Filtrační písky Ltd., Czech Republic, as filler material was used.In the laboratory mixer E095 (Matest S.p.A., Italy), fresh mixes were prepared according to the procedure specified in the EN 196-1 [12].The fresh mix was placed into iron molds of prismatic shape with dimensions (40 × 40 × 160) mm 3 in two independent layers, whereas each layer was compacted on vibrating table Matest C278-1 (Matest, S.p.A., Italy) for 30 sec.Prepared specimens were subsequently covered by plastic foil and left in laboratory conditions at 20 ± 1 °C and 45 ± 5 % of relative humidity for 24 hours.Then, partially hardened prisms were unmolded and stored in water for other 27 days in the laboratory environment.
The influence of different graphene additions into produced composites via their compressive strengths was evaluated.In this context, the hydraulic press Servo Plus Evolution (Matest, S.p.A., Italy) was used and in minimum six independent measurements on prismatic samples according to the EN 1015-11 [13] were done.

Results and discussion
Particle size distribution of both binding components, as well as graphene suspension, are given in table 4.
From the presented data is evident the increase in particle size of applied silica fume powder (SF) and graphene in the comparison with Portland cement.Although SF showed the important specific surface area of 5 210 m 2 •kg -1 (ultrafine particles are assumed) and, furthermore, the lateral size of graphene particles of around 20-50 μm (product list from the producer), it has to be taken into consideration the flocculation effect of ultrafine particles induced by their static surface charge or van der Waals forces [14].In this context, Lin and Du pointed out the importance of mechanical mixing with homogenizers and ultrasonic dispersion technique for better graphene redistribution in water [15].On the other hand, the increasing size of particles of used components may bring a synergic filler effect contributing to the smooth grain curve and promoting the strength properties of produced composites.The development of compressive strengths of graphene adjusted test specimens together with REF after 1 day of the hydration process is shown in figure 1. Obtained data clearly indicate the improvement of compressive strength of 4.8 % related to plain hardened specimens were recorded only in the case of specimens containing graphene only in 0.01 wt.%.Comparable values with REF were obtained for specimen G 0.02 (0.02% of graphene).On the contrary, the highest graphene dosage (0.03 wt.%) induced the decrease of average compressive strength of hardened sampled by 3.7 % versus REF.This negative phenomenon may be caused by insufficient dispersion of graphene particles in a mass of cement-based composite, and thus weak zones in hardened skeleton appear [16] or due to the significant deceleration of the hydration process caused by the higher presence of graphene particles.The change in compressive strengths of prepared specimens after 7 days of hardening is shown in figure 2.
As is clearly evident, prolonged water curing for 6 days helped to effectively promote mechanical resistance of all graphene enriched test specimens in contrast with above discussed values obtained for 24 hours of cured samples.Accordingly, hardened composites containing 0.01 wt.% and 0.02 wt.% of carbon-based admixture showed about 9.2 % and even 10.0 % improved compressive strength with respect to REF.It is worth noting that the highest dosage of graphene (0.03 wt.%) with no positive effect after 24 hours curing caused that prolonged hydration helped to increase the average compressive strength of specimens close to 55.0 MPa.In other words, a strength improvement of 6.5 % versus reference state was recorded.The graphical representation of compressive strengths of graphene admixture enriched samples and reference material at a period of 28 days is outlined in figure 3. Plotted strength values clearly confirmed the positive effect of graphene particles on the hydration processes of Portland cement-based systems, particularly taking place in the water environment for a more extended curing period of several weeks.Looking closely, there is an evident considerable increase in average compressive strength value of about 12.5 % when the graphene incorporation in hardened samples corresponded to 0.01 wt.% of the binder amount.With increasing concentrations of graphene particles in composites formulations, compressive strength contributions were detected to be slightly lower.Nevertheless, compared with REF, at 0.02 wt.% and 0.03 wt.% of graphene additions, compressive strength gains still reached 11.0 % and 9.9 %, respectively.As has been indicated with the help of experimentally obtained data, graphene-based admixtures represent specific materials which, on the one hand, require an advanced approach in view of their application into cement-based composites, however, on the other hand, even in very low dosages may importantly promote their mechanical resistance.

Conclusion
The effect of various amounts of graphene-based admixture on the strength properties development at different curing periods of fine-grained cement-based composites was researched in this paper.Particle size distribution of fine powdered input materials revealed possible negative flocculation effect of the fine silica fume and graphene particles caused by their surface static charge and/or van der Waals forces.Flocculated fine particles, however, to some extent, may contribute to beneficial filler effect and suitably supplement and create a smooth particle size distribution curve.In view of strength properties, the improvement of compressive strength of 4.8 % after 24 hours of cured specimens was recorded only in the case of the specimens containing the lowest dosage of graphene (0.01 %).After prolonged curing periods (7 and 28 days), the positive influence of the water environment helped to promote the mutual action of cement and graphene particles and the positive effect was detected in all graphene-dosed specimens.In summary, graphene-based admixture may be used as an effective tool to increase the mechanical resistance of fine granular cement-based composites even at very low additions of 0.01 wt.%, however, the admixture efficiency strongly depends on its mechanical processing and curing related environment of test specimens.

Figure 1 .
Figure 1.Compressive strength development of graphene dosed samples and reference material after 1day of the hydration process.

Figure 2 .
Figure 2. Compressive strength development of graphene dosed samples and reference material after 7 days of the hydration process.

Figure 3 .
Figure 3. Compressive strength development of graphene dosed samples and reference material after 28 days of the hydration process.

Table 1 .
The chemical composition of used Portland cement.

Table 2 .
Selected physical parameters of CEM I and silica powder (SF).

Table 3 .
The composition of prepared mixes with different amounts of graphene admixture.

Table 4 .
Particle size distribution of powders used for composites specification.