Development of a concrete mixture composition utilizing a nanostructuring additive for 3D printing of small architectural forms

Original article

In recent years, additive technologies have become an important tool in construction, including the creation of small architectural forms, opening up new horizons for design and implementation [1–9]. To create small architectural forms, building mixes are used on various types of binder, but products made with cement binder are considered the most durable and durable. Construction 3D printing using concrete mixes allows you to create complex and unique objects, not limited to traditional methods. This approach not only reduces construction time, but also significantly reduces material costs.

The development of compositions for concrete mixtures used in construction 3D printing is an exciting and promising area of modern construction technologies. Depending on the requirements of the project, the concrete composition must provide the necessary mechanical properties, solidification rate and resistance to external influences.

The key role is played by the choice of binders, additives and fillers, which affect the fluidity of the mixture and its adhesion to the 3D printer. An important aspect is the optimization of the formulation to ensure the uniformity and stability of the material flow, which is critical for the output layers of the structure.

Currently, many studies are focused on the creation of modifying additives that improve the characteristics of building mixes, as well as on the development of optimal formulations for 3D printing used in construction. In this regard, the process of selecting components for a building mix designed for 3D printing of small architectural objects is relevant [10–22].

METHODS AND MATERIALS

The following materials were used in the research:

– nanostructuring additive;

– additive-free Portland cement 52.5N;

– complex concrete additive Sika Antifreeze N9;

– the sand of the Ukhta deposit;

– sand of the Chapaevsky deposit;

– Axton superplasticizer;

– superplasticizer C-3.

The nanostructuring additive was obtained from an aluminosilicate precipitate, which is dried and ground to a powder with the required specific surface area [23].

The specific surface area of bulk materials was measured using the PSX-9 device [24].

To determine the water retention capacity, an installation consisting of a glass plate with a thickness of 5mm and a size of (150×150) mm; blotting paper with a size of (150×150) mm; a metal ring with a wall thickness of 5 mm, a height of 12 mm and a diameter of 100 mm and a gauze cloth gasket (250×350) mm was

used [25]. The calculation was carried out according to the formula:

， -(1 。。 тп^—тп^ 1 。。) ,                (1)

where m ₁ – the weight of the blotting paper size (150×150) mm before testing, g;

m ₂ – the weight of the blotting paper after the test, g;

m ₃ – installation weight without mortar mixture, g;

m ₄ – the mass of the installation with a mortar mixture, g.

The kinetics of water absorption of the composition was determined by the method specified in GOST 5802 [25]. To do this, the samples were pre-dried to a constant mass and placed in a vessel filled with water t = (20 ± 2) °C. The mass of the samples was measured on a scale every hour, the results were recorded with an accuracy of ±0.1%. The tests continued until the difference between two consecutive weighings did not exceed 0.1%.

Water absorption by weight (Wm) as a percentage was determined using the formula:

И4n =

叫一 2 I00 ,

人

where m_d – mass of the dried sample, g;

m_w – mass of the water–saturated sample, g.

The compressive strength of the samples (MPa) was determined in accordance with GOST 5802 [25]. To determine the strength, samples with a size of 100×100×100 mm and an IR 5057-50 testing machine were used. The load rate on the test sample was set to 10 mm/min. Calculations were carried out according to the formula:

R_с = P / F ,

where P – destructive load, N;

F – sample surface area, m².

Capillary ( P_cap ), contractional ( Pcon ) and gel ( P_gel ) porosities of the compositions were calculated as a percentage in accordance with the methodology [26] and formulas:

%=絵鳶.1°°,(4)

Р"詳訖・1。。,(5)

p2=面鼻.1°°,(6)

where W/C – water-cement ratio;

The hydration of cement (α) was calculated from the value of chemically bound water ( Wn ) in the studied samples [26]:

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α = 4· W_n .                                         (7)

The open porosity ( P_o ), taking into account the natural volume of the material ( V_n, m³ ), was calculated using the formula [26]:

2 - [^ ' 1000 * ]0 。% ,                (8)

where m_s and m_d is the mass of the material in a saturated state and mass of the material in a dry state, kg.

The total porosity ( P_g ) as a percentage was calculated using the formula [26]:

P = //C-°'216. 100,                   (9)

g W//C+0,32

The change in the normal density and setting time of the cement dough was determined using the Vika device [27]. The test solution was placed in the bowl, leveled, then the pestle of the Vika device was brought into contact with the solution, the initial value on the scale of the device was fixed and the locking mechanism was removed. After 5 minutes, the final value was measured on the scale of the device. The difference between the initial and final values, equal to (5–7) mm, was considered the normal density of the solution.

The change in the setting time of the composition was measured by the Vika device using a needle. The solution was placed in the bowl of the device and evenly distributed. The needle of the device was brought to the surface of the solution and the locking mechanism was released. The beginning of the setting process was considered to be the moment when the needle did not reach the bottom of the bowl by 1–2 mm, and the end was the moment when the needle did not go deeper into the solution by more than 1–2 mm.

The intergranular voidness of a fine filler in an uncompacted state ( V_i.v. ) was calculated [28]:

%.=аү ) .іоо%,            (10)

where ρ _b and ρ _t – bulk density and true density of the material, g/cm³.

The true density ( с_t ) was determined using the Le Chatelier device [29] and the formula:

с_t = ( m ₁– m ₂)/ V ,                                 (11)

where V – volume of the poured powder, cm³;

m ₁ is the initial mass of the powder with a cup, g;

m ₂ – mass of the remaining powder with a cup, g;

The bulk density ( с_b ) was determined according to GOST 9758 [30] using the formula:

с_b = m / V ,                                        (12)

where m is the mass of the material, g;

V –volume of the vessel, cm³.

RESULTS AND DISCUSSIONS

To optimize the composition of the concrete mix for a 3D printer, it is necessary to investigate the effect of each component on the system and determine its optimal content in the formulation.

In order to identify the optimal size of the specific surface of the additive for its use in the composition of the concrete mixture, an assessment of the hardening change was carried out (Fig. 1) and water absorption (Fig. 2) of a cement composite using a synthesized additive of different dispersion in the formulation.

The analysis of Fig. 1 showed that the value of the specific surface of the nanostructuring additive has an effect on the structure formation of cement compositions. It was revealed that a nanostructuring additive with a surface of Sud = 1.03 m2/g and Sud = 0.69 m2/g in the studied compositions favorably affects an increase in compressive strength by 7% and 17.8%, but an increase in the specific surface of the additive leads to a decrease in strength by 35% and 31% compared with the additive-free composition. Thus, a nanomodifying additive with a high dispersion value contributes to the creation of favorable conditions for the strength gain of the composition using cement.

Analysis of the data presented in Fig. 2 showed that samples with an additive with a specific surface area of Sud = 0.69 m2/g, compared with samples with Sud = 0.1 m2/g, have 9.6% lower water absorption. That is, it is possible to identify a pattern that the higher the dispersion value of the additive, the lower the water absorption of the studied compositions.

Thus, for subsequent tests, the optimal surface size of the nanostructuring additive is Sud = 1.03 m2/g.

The results of the effect of the amount of nanostructuring additive, depending on the mass of cement in the cement system, on the setting time and the normal density of the cement dough are shown in Table 1.

It was found that the compositions in the presence of a nanostructuring additive have a higher value of the normal density of the cement paste and a shorter setting time. At the same time, an increase in the percentage of the nanomodifying additive in the cement binder accelerates the setting time of the compositions.

Also, there is a decrease in capillary porosity by 11–52% and total porosity by 2–10%, an increase in gel porosity by 5–23% and contractional porosity by 4–23% compared with the control sample (without additives). Thus, it can be said that the introduction of a nanostructuring additive helps to increase the durability of the composition using cement.

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0 •        • I        I i        I i i I I i         к I I I         I I        I I i i        i i        i I i i        i i i i

0   6   12 18 24 30 36 42 48 54 60 66 72 78 84 90

Curing time, day

Fig. 1. Strength set of compounds with a nanostructuring additive: 1 – sample without additive (control); 2 – specific surface area of the additive S_sp = 1.03 m²/g; 3 – S_sp = 0.69 m²/g; 4 – S_sp = 0.31 m²/g; 5 – S_sp = 0.1 m²/g

18 I

0，. 一一一 I 一一一，I 一一 ■，一

О 2     4     6     8    10    12    14    16    18   20   22    24

Time, hour

Fig. 2. Changing the water absorption of compounds: 1 – control sample; 2 – nanomodifying additive with specific surface area S_sp = 1.03 m²/g; 3 – nanomodifying additive with S_sp = 0.69 m²/g; 4 – nanomodifying additive with S_sp = 0.31 m²/g; 5 – nanomodifying additive with S_sp = 0.1 m²/g

In order to reduce cement consumption and increase the mobility of the mixture, a fine filler sand is introduced. Sand with the lowest intergranular voidness should be used in the composition being developed. In this regard, sands with a mixed grain composition were studied (Table 2). Table 2 shows the granulometric compositions of the sands of the Ukhtinsky and Chaadaevsky deposits.

Thus, the analysis of the data in Table 2–3 showed that for further development of the composition of the

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3D construction mixture, sand from the Ukhtinskoye deposit should be used.

To find the optimal ratio between the three main sand fractions (0.16–0.315mm; 0.315–0.63mm; 0.63– 1.25mm), the value of the bulk density of fine aggregate was estimated using the method of mathematical description of physical adsorption (Table 4).

It has been found that the use of sand with a fraction ratio of 0.63–0.315:0.315–0.16 (mm) of 80% to 20% leads to an increase in the value of the coefficient of angularity (K u), which contributes to the strong adhesion of sand particles to the cement matrix.

In the process of selecting the composition of the concrete mix for 3D printing of small architectural forms, their technological properties were taken into account. Four compositions of a concrete mix for 3D printing with different amounts of nanostructuring additive, sand, and cement were studied. Table 5 shows the basic

requirements for mixtures and the results of the tests performed.

Thus, in the course of the conducted research, it was found that despite the mobility of compositions 3 and 4 cannot be used in 3D printing technology of small architectural forms, since the long-term setting speed of the mixture does not make it possible to apply the mixture in layers. Therefore, formulations 1 and 2 will be used for further research. In the future, a plasticizer will be introduced into the composition to increase the mobility of the mixture.

CONCLUSION

As a result of the study, it was found that the addition of a nanostructuring additive to cement compositions increases their density, shortens the setting time and reduces capillary porosity. To optimize the composition

Table 1. The change in the properties of the composition from the percentage of the nanostructuring additive

	The amount of nanomodified additive
	–	10%	20%	30%
The normal density of the cement paste, %	28	34	41	43
Setting time	Beginning 2 h 30min End of 5 h	Beginning 50 min End 1 h 40 min	Beginning 40 min End 1 h 30 min	Beginning 20 min End 1 h 15 min
Total porosity, %	41.1	40.3	38.3	36.9
Capillary porosity, %	18.7	16.7	12	9
Gel porosity, %	15.5	16.3	18.1	19.5
Contractional porosity, %	7.0	7.3	8.2	8.8

Table 2. Granulometric composition of fine aggregate

The content of fractions, %	Sand fraction, mm
	More 2.5	2.5–1.25	1.25–0.63	0.63–0.315	0.315–0.16	Less 0.16
The sand of the Ukhta deposit	1.0	2.1	9.8	39.5	44.0	3.8
Sand of the Chapaevsky deposit	–	0.3	12.1	58.1	26.6	2.9

Table 3. Density and intergranular voidness of fine filler

Fraction size, mm	Chaadaevsky sand		Ukhtinsky sand
	Bulk density, kg/m3	Intergranular voidness, %	Bulk density, kg/m3	Intergranular voidness, %
0.315–0.16	1418.2	46.5	1460.1	44.9
0.63–0.315	1529.9	42.3	1522.3	42.6
1.25–0.63	1599.3	39.6	1530.3	42.3

CONSTRUCTION MATERIALS SCIENCE of the 3D construction mix, it is recommended to use sand from the Ukhtinskoye deposit with a fraction ratio of 0.63–0.315:0.315–0.16 (mm) 80%:20%, which provides better mobility and adhesion to cement.

Formulations with long setting times are not suitable for 3D construction printing, so further research will be aimed at reducing the setting time of the formulations and increasing their mobility.

Table 4. The coefficient of angularity for the Ukhta sand deposit

Name of the material	Sf according to the BET method, m2/kg	Sт , m2/kg	К u
Sand without sifting	800	776.35	1.03
Sand fractions of 0.16 – 0.315; 0.315 – 0.63; 0.63 – 1.25 (mm) in a ratio of 70%:20%:10%	789	750.4	1.05
Sand fractions 0.63–0.315:0.315–0.16 (mm) in a ratio of 70%:30%	756	652.3	1.09
Sand fractions 0.63–0.315:0.315–0.16 (mm) in a ratio of 75%:25%	720	621.7	1.12
Sand fractions 0,63–0,315:0,315–0,16 (mm) in the ratio 80%:20%	700	582.39	1.20

Table 5. Technical requirements for a concrete mix solution for 3D printing

Requirements	The value of the indicator	Composition 1	Composition 2	Composition 3	Composition 4
Mobility, mm	no more 7	4.4	3.8	6.7	5.4
Delamination, %	no more 10	2.5	1.9	7.8	8.8
Water retention capacity	no less 97	97.72	98.86	96.92	9.25
Plastic strength, Pa	no less 80	81.3	86.9	31.1	75.8
Setting speed of the mixture, min	no more 20 min after extrusion	Beginning 12 min, end – 30 min	Beginning 15 min, end – 20 min	Beginning 20 min, end – 67 min	Beginning 45 min, end – 90 min