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Autors: V.A. Ushkov1, O.L. Figovsky2, G.V. Nalbandyan1, S.V. Samchenko1, E.V. Korolev3
Evaluation of the effectiveness of various ways to improve the performance characteristics of repair compositions
1. National Research Moscow State University of Civil Engineering (NRU MGSU)
2. Israeli Association of Inventors
3. Saint Petersburg State University of Architecture and Civil Engineering (SPbGASU)
Annotation. The article considers the influence of the processing conditions of raw materials components (cement, quartz sand and sealing water) with lo w-temperature nonequilibrium plasma (NTNP), the use of dispersed reinforcement (glass and basalt fibers, polypropylene fibers, steel fiber and fiber from structured ferromagnetic micropipe) and aqueous suspension of nanocrystalline cellulose on the strength of repair compositions. It is shown that the combined use of the above methods of modification of raw materials components allows to increase the strength of repair cement compositions in 35-45%.
Keywords: sealing water, quartz sand, nanocrystalline cellulose, plasma treatment, strength, repair composition, fiber
One of the actively developing areas in building materials science is to improve the operational properties of fine-grained concrete and repair compounds based on Portland cement. An increase in the physical and mechanical properties of building composites is achieved most often by the activation of raw materials [1-9], dispersed reinforcement [10-15] or the use of various nanoscale additives [16-20]. For example, mechanochemical activation of cement increases its specific surface area and leads to amorphization of the surface layer of its particles. Mechanochemical activation of cement is carried out in vortex layer apparatuses or with the help of vibrating mills of various designs. Mechanochemical activation of quartz sand is carried out in centrifugal planetary mills, as well as in vortex layer apparatuses. The issues of activation of the sealing water are considered in detail in [3, 5]. Of particular interest is the plasma modification of raw materials (cement, quartz sand and mixing water) to improve the performance properties of cement materials, in particular, repair compounds [21-24].
As a fiber for the production of fine-grained concrete and repair compounds, cut fibers of various chemical nature with a diameter from 10-15 microns to 0.7-0.9 mm and a length of up to 50 mm are widely used. Most often, metal (steel), polymer (polypropylene, polyethylene terephthalate or glass composite), mineral (glass or basalt) and vegetable (cellulose) fiber are used for these purposes. Fiber gives cement composites increased resistance to shock loads and high tensile and bending strength, reduces shrinkage and cracking of fine-grained concrete and mortar during hardening. The effectiveness of the fiber in this case depends on the chemical nature of the fiber from which it is made, and, consequently, the adhesion of the fiber to the cement stone, the diameter and length of the fiber, its volume content and distribution in the mineral matrix [10-15].
To improve the physical and mechanical characteristics of cement-based building composites, ultra- and nano-additives of various chemical nature are widely used [16-20]. At the same time, the use of nanoadditives of fulleroid structure, the maximum size of which does not exceed several hundred nanometers, is of great interest. They are a special form of carbon. Among the nanocellulose additives, a special place is occupied by spherical or needle-shaped nanocellulose (NC), which is used in the form of an aqueous suspension, which allows preserving the characteristic features of nanocellulose. Nanocellulose is obtained by acid hydrolysis of microcrystalline cellulose (MCC) or cotton cellulose fibers in 61-65% H2SO4 solution at a temperature of 25-50 ° C, followed by high-frequency processing in an ultrasonic field (ultrasonic dispersion). The concentration of NC in an aqueous suspension reaches up to 5%. During acid hydrolysis, the fibers of MCC or cotton cellulose are destroyed into nanoscale particles. By selecting the appropriate hydrolysis conditions, NC is synthesized with various physico-chemical properties depending on the type of cellulose fibers and the place of rupture of the macro chain, which determines the reactivity of nanocellulose particles. The rheological properties of aqueous suspensions of spherical (amorphous structure) and needle-shaped (crystalline structure) NC particles at a temperature of 25-70 °C were studied in detail in [25], and the structure and morphology of nanocellulose used as a micro additive for building composites were studied in [26]. The influence of NC on the cement hydration process and the properties of cement composites are considered in [26-29].
It should be noted that in the publications reviewed, the authors focus on improving the physical and mechanical properties of fine-grained concrete. Therefore, it seemed interesting to compare the effectiveness of the methods discussed above to improve the technological properties and increase the operational characteristics of cement composites in relation to repair mortars used in the restoration and repair of communication collectors.
Materials and methods of research. Portland cement of the CEM I 42.5N brand (GOST 31108-2016), fractionated quartz sand of class I (GOST 8736-2014) consisting of fractions of 0.5 mm and 1.5 mm and sealing water (GOST 23732-2011) were used for the manufacture of the compositions. Fractionated quartz sand contributes to the formation of a denser packing of particles in the hardened construction composite, increases the water-holding capacity. Cement, quartz sand and mixing water were treated in a low-temperature nonequilibrium plasma (NTNP) in a flow mode on a laboratory installation according to the operating procedure [24]. Polypropylene, steel, glass, basalt fiber and fiber made of structured ferromagnetic microconducting (SFMP) were used as fibers, the physical and mechanical properties of which are shown in Table 1. Glass alkali-resistant fiber, manufactured by Owens Corning (Spain), basalt fiber, manufactured by Alliance-Construction Technologies LLC, polypropylene fiber, manufactured by Fiber Lux LLC were used for the manufacture of fiber.
Table 1 — Physical and mechanical properties of the fiber used in the work
Fiber name Fiber dimensions Density, kg/m3 Tensile strength, MPa Elongation at break, % Tensile modulus of elasticity, GPa
Length, mm Diameter, microns
Steel 15 300 7800 1870 3,6 200,6
Glass 6 14 2680 2500 2,5 72,4
Basalt 6 16 2670 2200 2,5 76,9
Polypropylene grade HSM 6(0,6) 6 20 910 240 212 3,8
Fiber from structured ferromagnetic microwire 10 15,2 7300 3500 3,2 154,7
Samples made from repair mortars hardened under normal heat and humidity conditions. The strength of the repair mortar samples during compression, bending and stretching was determined after 1; 3; 7; 14 and 28 days of their hardening using the Unstron 3382 hydraulic press and the WDW – 100E breaking machine according to GOST 5802-86, and the setting time according to GOST R 56587-2015.
The results of the study. Processing of NTNP Portland cement in the flow mode practically does not affect its mineralogical composition, slightly reduces (by 3.7%) the maximum size and increases (by 4.3%) the concentration of binder particles with a size of less than 2 microns, increases the specific surface area of Portland cement by 4%. This reduces the normal density of cement dough (from 0.3 to 0.26), increases (by 35.7%) the total heat release during hydration of Portland cement, reduces the start and end of setting (respectively from 60 and 180 minutes to 15 and 60 minutes). At the same time, the strength of cement stone increases (by 16.2 – 20.4%) and by 18.1 – 22.3% of cement-sand mortar based on it.
Figure 1 — Dependence of the strength of cement stone under compression (1,1′) and bending (2,2′) on the duration of hardening of Portland cement: 1, 2 — cement grade CEM I 42.5N; 1′, 2′ — cement grade CEM I 42.5N after treatment with NTNP
The treatment of quartz sand with NTNP reduces the specific surface area of its grains by 10.6-20.3% and the pore surface area by 8.4-14.1%. At the same time, a significant decrease in the pore surface area and specific surface area of quartz sand occurs with an increase in the size of its grains, which is due to the melting of their surface when interacting with the streamer. When processing quartz sand with NTNP, the surface of its particles is amorphized. The processing of quartz sand by NTNP also increases the physico-mechanical properties of cement-sand solutions. Thus, the compressive strength of the samples hardened in 28 days under normal conditions, higher strength of the control specimens 20.4% (from 23.5 to 28.3 MPa) using quartz sand, past a single plasma treatment, and at two — and three-time processing of quartz sand NTP strength of these compounds increases by 13.3% and 17%, that is to 33.1 38.2 MPa (Fig. 2).
Figure 2 – the Strength of the cement-sand mortar, depending on the ratio of plasma processing of quartz sand: 1 — triple treatment; 2 — double treatment; 3 — single treatment; 4 — control composition
A single treatment of the NTNP mixing water increases the compressive strength of cement-sand solutions by 13.5% (from 18.5 to 21.1 MPa) compared to the control composition. The sealing water, which has undergone two- and three-time treatment in the flow mode in the NTNP installation, increases the compressive strength of repair compounds by 4.5-6%. The combined use of untreated and plasma-modified mixing water in a ratio of 1:1 more significantly increases the strength of cement-sand mortars due to the formation of a fine-crystalline structure of solidified mortars.
The combined use of processed NTNP mixing water and plasmodified quartz sand leads to a synergistic effect to increase the strength of mortar (Fig. 3). Should be specified not only from a comparison of the strength of samples, and analysis of the empirical coefficients of the kinetic equation [30]:
where Rmax – maximum strength; α – coefficient characterizing the rate of curing; n is the structural index (the values of empirical coefficients are given in table 2).
Table 2 — Values of empirical coefficients of kinetic equation
n/a Composition Empirical coefficients
1 Solution based on quartz sand, which has undergone 2-fold treatment with NTNP and mixing water, consisting of a mixture of untreated and plasma-modified water with their ratio equal to 1:1 38,5 7,43 1,69
2 Solution based on mixing water consisting of a mixture of untreated and plasma-modified water with their ratio equal to 1:1 30,0 7,39 1,23
3 A solution based on quartz sand that has undergone 2-fold treatment with NTNP 30,4 16,18 1,32
4 Solution based on plasmodified Portland cement 24,6 12,35 1,20
5 Control composition 23,3 15,83 1,14
The data in Table 2 demonstrate:
1) Various mechanisms of influence on the structure formation of cement stone components treated in NTNP. This follows from the comparison of the values of the coefficients α for compositions with No. 2-5. So, for the control composition and compositions containing dispersed components treated with NTNP (Portland cement or quartz sand), the value of the coefficient α varies in the range of 12 …16. When mixing water is added, partially containing water treated in NTNP (composition No. 2), the value of α decreases to 7.5. Moreover, a similar value of α is characteristic of composition No. 1.
2) The values of the coefficients n for the studied compositions with the use of components processed in NTNP increases. However, a greater increment of the coefficient n is observed for formulations containing dispersed components treated with NTNP (formulations No. 3 and 4).
3) The comparison of the values of the coefficients n for compositions No. 2 and 3 with n for composition No. 1 indicates a mutual strengthening of the influence on the structure formation of cement stone of components treated in NTNP and in different aggregate states. Moreover, the value of α is dominated by the mixing water, partially containing water treated in NTNP, and the coefficient n is affected by the treated dispersed components (in particular, quartz sand).
Figure 3 — Dynamics of strength gain of cement-sand mortars: 1 — a solution based on quartz sand, which has undergone 2-fold treatment with NTNP and mixing water, consisting of a mixture of untreated and plasma-modified water with their ratio equal to 1:1; 2 — a solution based on mixing water consisting of a mixture of untreated and plasma-modified water with their ratio equal to 1:1; 3 — a solution based on quartz sand that has undergone 2-fold treatment with NTNP; 4 — a solution based on plasmodified Portland cement; 5 — control composition
As noted above, an effective method of increasing the strength of fine-grained concrete and repair compounds is their dispersed reinforcement. Figure 4 shows the dependence of the tensile strength of building mortars during bending on the type and volume content of metal, polymer and natural fiber. It follows from Fig. 4 that the use of steel fiber and SFMP fiber, basalt and glass fiber with their volume content of 0.4% increases the tensile strength when bending repair compounds, respectively, by 74.6; 63.5; 50.8 and 47.6%, (from 6.3 MPa to 11; 10.3; 9.5 and 9.3 MPa), and polypropylene fiber — by 34.9% (up to 8.5 MPa). The observed effect is due, in our opinion, to a higher modulus of elasticity when stretching a metal fiber compared to a natural or polymer fiber. The lower strength of repair compounds containing polypropylene fiber may also be due to its low adhesion to cement stone. At the same time, there is a linear dependence of the bending strength of repair compounds on the value of the Mupr fiber (Fig. 5).
Figure 4 — Dependence of the tensile strength during bending of the repair composition on the type and volume content of the fiber: 1 — steel fiber; 2 — SFMP fiber; 3 — basalt fiber; 4 — glass fiber; 5 — polypropylene fiber
Figure 5 — Dependence of the tensile strength during bending of the repair composition on the elastic modulus during stretching of the fiber and its volume content: 1 – 0,5 %; 2 – 0,4 %; 3 – 0,3 %; 4 – 0,2 %; 5 – 0,1 %
The effectiveness of volumetric dispersed reinforcement of repair compounds can be estimated by comparing the angle of inclination of the straight lines indicated in Fig. 5 to the abscissa axis — the ratio of the magnitude of the increase in bending strength of repair compounds to the difference in elastic modulus under tension of the studied types of fiber (tga). It follows from Figure 6 that the value of tga increases linearly with an increase in the degree of volumetric fiber content. This indicates the additive effect of fiber on the strength of the material and the absence (or insignificance in magnitude) of interaction in the «cement stone ‒ fiber» contact zone.
Figure 6 — Dependence of tgα on the volume content of fiber in the repair composition
Among the known nanoscale additives, the use of nanocrystalline cellulose (NCC) in the form of aqueous suspensions containing up to 4.8% NCC deserves special attention. It was found that with an increase in the concentration of NCC to 0.14%, the bending strength of cement stone and repair compound having a strength class of B25 (Figure 7) increases by 21.3 and 20.3% (respectively from 6.1 and 6.4 MPa to 7.4 and 7.7 MPa), and the compressive strength (Figure 8) increases by 29.4 and 28.8% (respectively from 51.1 and 32.3 MPa to 66.4 and 41.6 MPa).
Figure 7 — Dependence of the bending tensile strength of the repair compound (1) and cement stone (2) after 28 days of hardening under normal conditions on the concentration of NCC
Figure 8 — Dependence of compressive strength after 28 days of hardening under normal conditions of cement stone (1) and repair composition (2) on the concentration of NCC
The optimal concentration of NCC in cement composites is 0.14…0.16%. It should be noted that with an increase in the concentration of NCC, the setting time of cement stone and repair compounds increases. This shift in the setting time may be due to the high ability of nanocellulose to retain water in the mixture. This leads to an increase in the degree of hydration of Portland cement, a decrease in porosity and an increase in the uniformity of cement stone. The dynamics of strength gain of repair compounds modified with nanocrystalline and microcrystalline cellulose are shown in Figure 9.
Figure 9 — Dynamics of the tensile strength gain during bending of the repair compound modified with micro- and nanocrystalline cellulose: 1 — repair compound containing 0.14% nanocrystalline cellulose; 2 — repair compound containing 0.5% microcrystalline cellulose; 3 — control composition
The increase in the strength of cement stone and repair compounds modified with nanocrystalline cellulose, in our opinion, is due to an increase in the density of cement stone (a decrease in porosity) due to an increase in the degree of hydration of Portland cement. At the same time, the rigidity and fragility of building composite samples significantly increases, and bending deformations decrease linearly with an increase in the concentration of nanocrystalline cellulose.
Thus, it is established that the use of plasma modification (processing in a low-temperature nonequilibrium plasma) of the components of cement composites leads to an increase in their physical and mechanical properties. At the same time, the effect of introducing modified components in this way depends on their aggregate state. Treatment in a low-temperature nonequilibrium plasma naturally has different effects on dispersed components and water. For the first of these components of cement composites, there is a change in the parameters of the crystal structure (amorphization) of the surface layer and (or) a change in the relief of the surface of the particles (in particular, for quartz sand). Water treatment leads to a change in its hardness, the consequence of this is a slowdown in the processes of structure formation of cement stone (almost a twofold decrease in the value of the coefficient α). When the components processed in NTNP are used together, there is a mutual strengthening of their influence on the structure formation of cement stone.
Dispersed reinforcement of cement composites containing plasma-modified components has a natural effect on the mechanical properties of such materials, depending on the type of fiber. The use of nanoscale cellulose makes it possible to increase the density and uniformity of the cement stone structure, which, together with an increase in the degree of hydration of Portland cement, also provides an increase in the strength of the materials being developed.
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About the authors
Ushkov Valentin Anatolyevich — Doctor of Technical Sciences, Associate Professor, National Research Moscow State University of Civil Engineering
Figovsky Oleg Lvovich — Doctor of Technical Sciences, Academician of the Russian Academy of Architecture and Construction Sciences, President of Israeli Association of Inventors
Nalbandyan Grigor Vilenovich — Postgraduate Student, National Research Moscow State University of Civil Engineering
Samchenko Svetlana Vasilyevna — Doctor of Technical Sciences, Professor, National Research Moscow State University of Civil Engineering
Korolev Evgeny Valeryevich — Doctor of Technical Sciences, Professor, St. Petersburg State University of Architecture and Civil Engineering
Illustrations: B-Plaster B25
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