Citation: Stel’makh, S.A.; Shcherban’,

E.M.; Beskopylny, A.N.; Mailyan,

L.R.; Meskhi, B.; Beskopylny, N.;

Dotsenko, N.; Kotenko, M. Influence

of Recipe Factors on the Structure

and Properties of Non-Autoclaved

Aerated Concrete of Increased

Strength. Appl. Sci. 2022, 12, 6984.

https://doi.org/10.3390/app12146984

Academic Editor: Panagiotis G.

Asteris

Received: 1 July 2022

Accepted: 8 July 2022

Published: 10 July 2022

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applied sciences

Article

Influence of Recipe Factors on the Structure and Properties ofNon-Autoclaved Aerated Concrete of Increased StrengthSergey A. Stel’makh 1 , Evgenii M. Shcherban’ 1 , Alexey N. Beskopylny 2,* , Levon R. Mailyan 3,Besarion Meskhi 4, Nikita Beskopylny 5, Natal’ya Dotsenko 6 and Maria Kotenko 6

1 Department of Engineering Geology, Bases and Foundations, Don State Technical University,344003 Rostov-on-Don, Russia; sergej.stelmax@mail.ru (S.A.S.); au-geen@mail.ru (E.M.S.)

2 Department of Transport Systems, Faculty of Roads and Transport Systems, Don State Technical University,344003 Rostov-on-Don, Russia

3 Department of Roads, Don State Technical University, 344003 Rostov-on-Don, Russia; lrm@aaanet.ru4 Department of Life Safety and Environmental Protection, Faculty of Life Safety and Environmental

Engineering, Don State Technical University, 344003 Rostov-on-Don, Russia; spu-02@donstu.ru5 Department Hardware and Software Engineering, Don State Technical University,

344003 Rostov-on-Don, Russia; beskna@yandex.ru6 Department of Unique Buildings and Constructions Engineering, Don State Technical University,

344003 Rostov-on-Don, Russia; natalya_1998_dotsenko@mail.ru (N.D.);maria.kotencko2016@yandex.ru (M.K.)

* Correspondence: besk-an@yandex.ru; Tel.: +7-8632738454

Abstract: At present, the load-bearing enclosing structures of buildings and structures are designedand built considering the increasing requirements for energy efficiency and energy saving of suchstructures. This is due to the need for a thrifty attitude to the energy consumed and the need to strivefor the greening of construction and increase the energy efficiency of buildings and structures. Inthis regard, one of the most effective and proven building materials is cellular concrete. The purposeof this study was to study the influence of some prescription factors on the structure formation andproperties of non-autoclaved aerated concrete with improved characteristics. Standard test methodswere used, as well as SEM analysis of the structure of aerated concrete. Non-autoclaved aeratedconcrete with the replacement of part of the cement with microsilica in an amount from 4% to 16%MS showed higher strength characteristics compared to aerated concrete, where part of the cementwas replaced by the addition of granulated blast-furnace slag and a complex additive. The maximumvalue of compressive strength was recorded for aerated concrete with 16% MS addition. The largestincrease in the coefficients of constructive quality was observed in compositions of aerated concretewith the addition of silica fume from 11% to 46% compared with the control composition. Theaddition of microsilica makes it possible to achieve an improvement in the thermal conductivitycharacteristics of non-autoclaved aerated concrete (up to 10%). Replacing part of the cement withslag and complex additives does not have a significant effect on thermal conductivity. The obtaineddependencies were confirmed by the analysis of the structure formation of the studied aeratedconcrete at the micro level. An improvement in the microstructure of aerated concrete with theaddition of microsilica in comparison with samples of the control composition has been proven.

Keywords: aerated concrete; compressive strength; thermal conductivity; microsilica; granulatedblast furnace slag

1. Introduction

At present, the load-bearing enclosing structures of buildings and structures aredesigned and built considering the increasing requirements for energy efficiency andenergy saving of such structures. This is due to the need for a thrifty attitude to the energyconsumed and the need to strive for the greening of construction and increase the energy

Appl. Sci. 2022, 12, 6984. https://doi.org/10.3390/app12146984 https://www.mdpi.com/journal/applsci

Appl. Sci. 2022, 12, 6984 2 of 21

efficiency of buildings and structures. In this regard, one of the most effective and provenbuilding materials is cellular concrete. In turn, cellular concrete is classified according tovarious criteria and can be of various types, the main ones of which are foam concrete andaerated concrete.

These concretes differ from each other in the way of pore formation. At the sametime, if foam concrete has a simple manufacturing technology, but other things being equal,somewhat worse characteristics, then aerated concrete has several advantages in termsof physical, mechanical, and other operational characteristics, but the technology for itsproduction is much more complicated. In particular, not only the technological process ofobtaining aerated concrete is complex, but also the technological line itself, which oftenconsists of more expensive equipment. In this regard, the most high-quality, but at the sametime the most expensive, type of cellular concrete is the so-called gas silicate or autoclavedaerated concrete [1–17].

In the production of this concrete, special equipment is used—autoclaves—whichmake it possible to create hardening conditions for concrete in an environment of saturatedsteam under pressure. Therefore, “from the point of view of building science and engi-neering”, non-autoclaved aerated concrete seems to be the most interesting and promisingmaterial among clean concrete [18–21], and one of the main directions in building materialsscience is the search for ways to approximate the characteristics of non-autoclaved aeratedconcrete to the characteristics of its autoclaved analogue.

Due to its energy-saving properties, aerated concrete has found wide application aspartition walls and load-bearing walls [20,22,23].

Aerated concrete is a relatively homogeneous material in its “structure comparedto conventional heavy concrete, which contains a significant amount of coarse-grainedaggregate phase”. If the heavy concrete characteristics depend directly on coarse and fineaggregates, then the “properties of aerated concrete mainly depend on the compositionand microstructure” [24].

Its properties depend on the parameters of the microstructure of aerated concrete. Ingeneral, the structure of aerated concrete is characterized by its solid microporous matrixand macropores. The reason for the formation of macropores is an increase in volume dueto the effect of aeration; micropores are located in the walls between macropores [25–28].For example, the porosity of aerated concrete averages 70–80%, and about 45–55% of thisporosity is the porosity caused by the chemical reaction of aluminum powder in an alkalisolution [29,30]. The “porous microstructure of the concrete matrix is mainly representedby a weakly crystallized C–S–H phase and tobermorite 1.1 nm in size” [31,32].

In industrial practice, the most common blowing agent in the production of aeratedconcrete is aluminum powder. This type of blowing agent makes it possible to obtain poresof various shapes and sizes in aerated concrete. In some studies, instead of aluminumpowder, industrial waste was used as a gas-forming additive [33,34], which is aluminumwaste or bottom ash from the combustion of municipal solid waste.

Moreover, today, the use of agricultural waste and waste from the fuel and en-ergy complex in the technology of both heavy concrete and cellular concrete is quitepopular [31,35–56]. Table 1 presents a detailed analysis of waste types and their influenceon the characteristics of the resulting cellular concrete.

After the literature review, a lack of studies of structure formation and characteristicsof non-autoclaved aerated concrete, products, and structures made of them was revealed.

In this regard, the purpose of our article was to study the influence of several compo-sition factors on the structure formation and properties of non-autoclaved aerated concretewith improved characteristics.

Appl. Sci. 2022, 12, 6984 3 of 21

Table 1. Overview of the use of various types of waste in the technology of cellular concrete.

Reference Number Waste Type Purpose Influence of Applied Waste on the Characteristics ofCellular Concrete

[35] Graphite slag As a binderreplacement

The use of graphite slag instead of a part of the bindermakes it possible to obtain aerated concrete without loss

of strength characteristics.

[36,39,42] Rice husk ash As a binderreplacement

The addition of rice husk ash (RHA) leads to an increasein water demand, which negatively affects the strength of

aerated concrete.

[37] Copper tailings, blastfurnace slag

As a binderreplacement

The compressive strength decreases as the percentage ofadditive content increases. The optimal value of the

additive content, which provides the best values for thestrength and durability of aerated concrete, is its dosage of

5–10%.

[38] Iron tails As a binderreplacement

Copper tailings and blast-furnace slag can be used as asubstitute for lime to produce aerated concrete products

by obtaining compositions of CaO and MgO. For example,a raw material composition of 30% copper tailings, 35%

blast-furnace slag, 10% cement and 5% gypsum makes itpossible to obtain aerated concrete with a compressive

strength of 4 MPa.

[40] Foundry sand As a replacement forfine aggregate

In general, an increase in the mass ratio of this additiveinstead of a part of cement adversely affects the strengthcharacteristics, however, tailings of a certain size up to 80microns at a dosage of 27–34% make it possible to obtain

aerated concrete with the appropriate characteristics.

[31] Flue gas cleaningwaste

As a binderreplacement

Replacing ordinary sand with molding sand in an amountof 20% is the most optimal and provides an increase in

compressive strength up to 20%, tensile strength inbending up to 14%.

[41] Recycled AACPowder

As a replacement forfine aggregate

The addition of flue gas cleaning waste to replace part ofthe lime provides an increase in the strength

characteristics of aerated concrete. Furthermore, theaddition of waste leads to the formation of densely

packed lamellar tobermorite crystals with a size of 1.1 nm.

[43] Quarry dust, lime flyash

As a binderreplacement

The use of finely dispersed powder from recycledautoclaved aerated concrete contributes to an increase in

compressive strength up to 16%. So, the most optimaldosage of this type of waste instead of part of the sand is

its addition in the amount of 30%.

[18,48]Microsilica,

blast-furnace slag,metakaolin

As a binderreplacement

The addition of fly ash and “quarry dust has a significanteffect on the density of hardened aerated

concrete samples”.

[48] Microsilica, ricehusk ash

As a replacement forfine aggregate

The addition of microsilica (MS) is the most effective andprovides the greatest increase in strength characteristics incomparison with samples of aerated concrete, where slag

(GBFS) and metakaolin (MK) were used as a binderreplacement. At the same time, the GBFS additive proved

to be more effective than MK.

[44] Granite dust As a replacement forfine aggregate

Aerated concrete with 2.5% RHA and 7.5% MS shows thehighest mechanical properties compared to samples

containing quartz sand.

Appl. Sci. 2022, 12, 6984 4 of 21

Table 1. Cont.

Reference Number Waste Type Purpose Influence of Applied Waste on the Characteristics ofCellular Concrete

[45] Perlite waste As a replacement forfine aggregate

The addition of granite dust particles increases thecompressive strength and density of the hardened aeratedconcrete and reduces water absorption and porosity. Themaximum increase in compressive strength was observed

at the level of 42% for a mixture containing 20%replacement of fine aggregates with granite dust.

[46]

Residues fromoil-based drillingcuttings pyrolysis,

fly ash

As a binderreplacement

The results of the study show that the use of expandedperlite waste in aerated concrete leads to a decrease in itsdensity, and the thermal conductivity and compressivestrength of the samples decrease as the percentage of

expanded perlite in aerated concrete increases.

The objectives of the study were:

- analysis and review of literature concerning the issues of improving the quality ofcellular concrete and ways to control its properties and structure formation due toprescription and technological factors;

- after a detailed analysis, the next task was drawing up a program of experimentalstudies, preparing and planning the experiment, and processing data;

- at the end of the experimental and research part, the development of analyticalconclusions with the formulation of new provisions in the development of the theoryof hardening, structure formation, and the formation of properties of non-autoclavedcellular concrete with improved structure and characteristics.

The scientific novelty of this research is the developed theoretical concepts of structureformation at the micro level and the influence of microstructure on the formation of theproperties of non-autoclaved aerated concrete with improved characteristics based onnano-modifiers of various origins. The dependence of the processes of structure formationat the microlevel and the properties of non-autoclaved aerated concrete with improvedcharacteristics on prescription factors has been established. In particular, such prescriptionfactors were the nanomodification of aerated concrete in a qualitative aspect and theestablished rational dosage of such nanomodifiers in a quantitative aspect.

The practical significance of the study is the improvement of building enclosingstructures made of cellular concrete with improved characteristics, which in turn will leadto an increase in the energy efficiency of buildings and structures for various purposes.

2. Materials and Methods2.1. Materials

In this work, the following raw materials were used for the manufacture of non-autoclaved aerated concrete: Portland cement grade CEM I 42.5 N produced by JSCNovoroscement (Novorossiysk, Russia); river sand, alluvial sand from the Aksai quarry(Aksai, Russia); ground blast-furnace slag (GBFS) produced by NLMK JSC (Lipetsk, Russia);MK-85 microsilica (MS) produced by NPP ZIPO (Lipetsk, Russia); PAP-1 aluminum powderproduced by SKIF LLC (St. Petersburg, Russia); molding gypsum G-6 B III produced byMagma LLC (Moscow, Russia); and lump quicklime produced by Roskhimprom LLC(Rostov-on-Don, Russia). Characteristics of raw materials are shown in Tables 2–7.

Appl. Sci. 2022, 12, 6984 5 of 21

Table 2. Physical and mechanical characteristics and mineralogical composition of cement.

Property Value

Physical and mechanical characteristics

Specific surface, cm2/g 3548

Normal density, % 26.5

Density, kg/m3 3165

Setting time, hour:min-start 2:10

-the end 3:25

Compressive strength at the age of 28 days, MPa 58.7

Mineralogical composition

C3S 68

C2S 13

C3A 7

C4AF 12

Table 3. Physical characteristics of sand.

Property Value

Size modulus 1.39Content of dust and clay particles, % 0.47

True grain density, kg/m3 2697Bulk density, kg/m3 1438Voidness of sand, % 45

Table 4. Chemical composition of GBFS [52] and MS.

SubstanceContent, wt. %

GBFS MS

SiO2 26.56 80.8CaO 56.35 1.5MgO 6.42 0.3Al2O3 5.92 2.3Na2O 1.07K2O 0.29TiO2 2.37MnO 0.18

S 0.32Fe2O3 0.33 3.1

SrO 0.14P2O5 <0.01ZrO2 0.03

Co3O4, V2O5, Cr2O3, Nd2O3, WO3, Ta2O5, Nb2O5, Sc2O3,Y2O3, BaO, CuO <0.01

R2O 0.7SO3 2.4LOI 8.9

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Table 5. Characteristics of aluminum powder PAP-1.

Property Value

AppearanceEasy-to-smear, silver-gray product. Aluminumparticles in the powder are plate-shaped and

covered with a thin oxide and fatty film.

Average particle thickness, µm 0.25–0.50

Average linear size of particles, microns 20–30

Bulk density of powder, g/cm3 0.23

Content of active aluminum, % 92

Table 6. Characteristics of gypsum G-6 B III.

Property Value

Bulk density, kg/m3 658gypsum mortar pH 7Water absorption, % 38

Fineness of grinding, residue on a sieve with a mesh size of 0.2 mm, % 0.9Setting time, min

Start 7the end 18

Table 7. Characteristics of lime.

Property Value

Content of active CaO and MgO, % 88.5The content of unquenched grains, % 11

Extinguishing speed, min 20Hydration water content, % 3

2.2. Methods

The aerated concrete was prepared in the following sequence: cement, sand, lime,gypsum, and additives were mixed in full in a dry state for 30 s, then 50% of the waternecessary for mixing was introduced into the resulting homogeneous mixture of drycomponents and mixed for 1.5 min, then aluminum powder was introduced into theresulting mixture along with the rest of the mixing water. The resulting mixture was keptat rest for 1 min. After holding the aerated concrete mixture, its mixing continued for 2 minuntil a homogeneous mixture without lumps was obtained.

The samples were cast in one layer and compacted manually using a steel rod. Thecrust (surplus) was removed after 4 h of exposure with a scraper. Next, the samples werewrapped in a film and kept indoors at a temperature of 24 ± 2 ◦C for 24 h. After holdingfor a day, the samples were removed from the formwork and placed in a normal hardeningchamber, in which they were kept for 27 days.

To determine the proportions of the mixture that will provide the best “physicaland mechanical properties of non-autoclaved aerated concrete, with partial replacementof part of the cement with ground blast furnace slag (GBFS)” and microsilica (MS), thefollowing replacement levels were chosen: 4%, 8%, 12%, 16%, and 20% of these mineralcomponents by weight of cement. All samples were prepared at a ratio of water and bindercomponent W/B = 0.5, lime was introduced in an amount of 10%, gypsum in an amount of5%, and aluminum powder in an amount of 1% by weight of the binder. Table 8 shows theformulation of the experimental compositions.

Appl. Sci. 2022, 12, 6984 7 of 21

Table 8. Proportions of experimental compositions of mixtures.

Composition Number The Content of Microsilica, IntroducedInstead of Part of the Cement, %

Content of Ground Blast-Furnace SlagIntroduced Instead of Part of Cement, %

C 0 01A 4 02A 8 03A 12 04A 16 05A 20 01B 0 42B 0 83B 0 124B 0 165B 0 20

1AB 2 22AB 4 43AB 6 64AB 8 85AB 10 10

A total of 96 sample cubes were made for testing the average density and compressivestrength and 80 sample plates for testing the thermal conductivity (Figure 1).

Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 23

A total of 96 sample cubes were made for testing the average density and compres-sive strength and 80 sample plates for testing the thermal conductivity (Figure 1).

Figure 1. Experimental research program.

The average density of the manufactured samples of non-autoclaved aerated con-crete was determined according to GOST 12730.1 “Concretes. Methods of determination of density”. The samples were dried to constant weight for 24 h in a laboratory oven at a temperature of 110 ± 5 °C. The weight of the samples was determined by weighing with an error of no more than 0.1%. The density of samples of non-autoclaved aerated concrete sample ρw, kg/m3, was calculated with an error of up to 1 kg/m3 according to the formula:

1000,wmV

ρ = ⋅ (1)

where m is the mass of the sample, g; V is the sample volume, cm3. The compressive strength was determined according to GOST 10180 “Concretes.

Methods for strength determination using reference specimens”. After installing the prototype on the support plates of the press, its upper plate was

combined with the face of the prototype, such that their planes completely adjoined one to the other. The prototype “was loaded to failure at a constant load rate increase (0.6 ± 0.2) MPa/s” (Figure 2).

Figure 1. Experimental research program.

The average density of the manufactured samples of non-autoclaved aerated concretewas determined according to GOST 12730.1 “Concretes. Methods of determination ofdensity”. The samples were dried to constant weight for 24 h in a laboratory oven at atemperature of 110 ± 5 ◦C. The weight of the samples was determined by weighing withan error of no more than 0.1%. The density of samples of non-autoclaved aerated concretesample ρw, kg/m3, was calculated with an error of up to 1 kg/m3 according to the formula:

ρw =mV

· 1000, (1)

where m is the mass of the sample, g; V is the sample volume, cm3.The compressive strength was determined according to GOST 10180 “Concretes.

Methods for strength determination using reference specimens”.After installing the prototype on the support plates of the press, its upper plate

was combined with the face of the prototype, such that their planes completely adjoinedone to the other. The prototype “was loaded to failure at a constant load rate increase(0.6 ± 0.2) MPa/s” (Figure 2).

Appl. Sci. 2022, 12, 6984 8 of 21

Appl. Sci. 2022, 12, x FOR PEER REVIEW 8 of 23

(a) (b)

Figure 2. Aerated concrete sample: (a) before compression test; (b) after compression test.

The value of the coefficient of constructive quality (CCQ) was determined by the for-mula:

,bRb

RCCQ

ρ= (2)

where Rb is “the compressive strength, MPa; ρ is the density of concrete”, g/cm3. The thermal conductivity of samples of non-autoclaved aerated concrete was deter-

mined on the device ITP-MG4 LLC SKB Stroypribor (Chelyabinsk, Russia) in accordance with the requirements of GOST 7076 “Building materials and products. Method of deter-mination of steady-state thermal conductivity and thermal resistance” on sample plates 100 × 100 × 20 mm in size [57]. “The principle of operation of the device is based on creating a stationary heat flux passing through a flat sample of a certain thickness and directed perpendicular to the front faces of the sample, measuring the sample thickness, heat flux density and temperature of opposite front faces” (Figure 3).

The structure of aerated concrete samples was studied using a scanning electron mi-croscope “ZEISS CrossBeam 340 double-beam scanning electron/ion microscope equipped with an Oxford Instruments X-Max 80 X-ray microanalyzer Carl Zeiss Micros-copy GmbH (Factory), (Jena, Germany)” [52–55].

Figure 2. Aerated concrete sample: (a) before compression test; (b) after compression test.

The value of the coefficient of constructive quality (CCQ) was determined by the formula:

CCQRb =Rbρ

, (2)

where Rb is “the compressive strength, MPa; ρ is the density of concrete”, g/cm3.The thermal conductivity of samples of non-autoclaved aerated concrete was deter-

mined on the device ITP-MG4 LLC SKB Stroypribor (Chelyabinsk, Russia) in accordancewith the requirements of GOST 7076 “Building materials and products. Method of deter-mination of steady-state thermal conductivity and thermal resistance” on sample plates100 × 100 × 20 mm in size [57]. “The principle of operation of the device is based on creat-ing a stationary heat flux passing through a flat sample of a certain thickness and directedperpendicular to the front faces of the sample, measuring the sample thickness, heat fluxdensity and temperature of opposite front faces” (Figure 3).

Appl. Sci. 2022, 12, x FOR PEER REVIEW 9 of 23

Figure 3. Measurement of thermal conductivity of aerated concrete samples.

The study also used: - technological equipment: laboratory drying oven ShS-80-01 SPU JSC “Smolensk

SKTB SPU” (Smolensk, Russia); - testing equipment: hydraulic press IP 6010-100-1 “PKTs ZIM” (Armavir, Russia); - measuring instruments: metal measuring ruler 500 mm JSC “Stavropol Tool Plant”

(Stavropol, Russia); laboratory scales HT-5000 NPP Gosmetr (St. Petersburg, Russia); caliper ShTs-I-250-0.05 OOO NPP Chelyabinsk Tool Plant (Chelyabinsk, Russia) [52–59].

3. Results 3.1. Density

The results of determining the density of experimental samples of non-autoclaved aerated concrete are shown in Figure 4.

Figure 3. Measurement of thermal conductivity of aerated concrete samples.

The structure of aerated concrete samples was studied using a scanning electron mi-croscope “ZEISS CrossBeam 340 double-beam scanning electron/ion microscope equipped

Appl. Sci. 2022, 12, 6984 9 of 21

with an Oxford Instruments X-Max 80 X-ray microanalyzer Carl Zeiss Microscopy GmbH(Factory), (Jena, Germany)” [52–55].

The study also used:

- technological equipment: laboratory drying oven ShS-80-01 SPU JSC “Smolensk SKTBSPU” (Smolensk, Russia);

- testing equipment: hydraulic press IP 6010-100-1 “PKTs ZIM” (Armavir, Russia);- measuring instruments: metal measuring ruler 500 mm JSC “Stavropol Tool Plant”

(Stavropol, Russia); laboratory scales HT-5000 NPP Gosmetr (St. Petersburg, Russia);caliper ShTs-I-250-0.05 OOO NPP Chelyabinsk Tool Plant (Chelyabinsk, Russia) [52–59].

3. Results3.1. Density

The results of determining the density of experimental samples of non-autoclavedaerated concrete are shown in Figure 4.

Appl. Sci. 2022, 12, x FOR PEER REVIEW 10 of 24

Figure 4. Statistical density values of non-autoclaved aerated concrete.

Average density values for non-autoclaved aerated concrete range from 1770 kg/m3 to 1782 kg/m3. As can be seen from Figure 4, there are no significant changes in density for samples of non-autoclaved aerated concrete.

3.2. Compressive Strength The results of determining the compressive strength of experimental samples of non-

autoclaved aerated concrete are shown in Figure 5.

Figure 4. Statistical density values of non-autoclaved aerated concrete.

Average density values for non-autoclaved aerated concrete range from 1770 kg/m3

to 1782 kg/m3. As can be seen from Figure 4, there are no significant changes in density forsamples of non-autoclaved aerated concrete.

3.2. Compressive Strength

The results of determining the compressive strength of experimental samples of non-autoclaved aerated concrete are shown in Figure 5.

The maximum compressive strength was recorded for prototypes of non-autoclavedaerated concrete of type 4A composition, where part of the cement 16% MS was replaced,and amounted to 13.2 MPa. The minimum value of compressive strength was recorded forsamples of composition type 5B, where part of the cement was replaced with 20% GBFS andamounted to 5.5 MPa. In general, the compressive strength values of type A compoundsare higher than those of type B and AB compounds (Table 9).

The dependences of the change in the compressive strength of compositions of typesA, B, and AB are shown in Figure 6.

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Table 9. Experimental values of compressive strength of non-autoclaved aerated concrete.

Compositionnumber C 1A 2A 3A 4A 5A 1B 2B 3B 4B 5B 1AB 2AB 3AB 4AB 5AB

Compressivestrength, MPa 9.0 10.1 11.4 12.1 13.2 10.0 8.5 8.4 7.2 6.3 5.5 9.1 8.5 7.4 7.2 6.7

Appl. Sci. 2022, 12, x FOR PEER REVIEW 11 of 23

Figure 5. Statistical values of compressive strength of non-autoclaved aerated concrete.

The maximum compressive strength was recorded for prototypes of non-autoclaved aerated concrete of type 4A composition, where part of the cement 16% MS was replaced, and amounted to 13.2 MPa. The minimum value of compressive strength was recorded for samples of composition type 5B, where part of the cement was replaced with 20% GBFS and amounted to 5.5 MPa. In general, the compressive strength values of type A compounds are higher than those of type B and AB compounds (Table 9).

Table 9. Experimental values of compressive strength of non-autoclaved aerated concrete.

Composition number

C 1А 2А 3А 4А 5А 1B 2B 3B 4B 5B 1AB 2AB 3AB 4AB 5AB

Compressive strength, MPa

9.0 10.1 11.4 12.1 13.2 10.0 8.5 8.4 7.2 6.3 5.5 9.1 8.5 7.4 7.2 6.7

The dependences of the change in the compressive strength of compositions of types A, B, and AB are shown in Figure 6.

Figure 5. Statistical values of compressive strength of non-autoclaved aerated concrete.Appl. Sci. 2022, 12, x FOR PEER REVIEW 12 of 23

Figure 6. Change in compressive strength of non-autoclaved aerated concrete samples depending on the percentage of binder replacement by various types of additives.

Figure 6 shows that the addition of microsilica has the most positive effect on the compressive strength of non-autoclaved aerated concrete. Statistical processing of the ob-tained experimental data made it possible to determine regression curves describing the effect of additives x on compressive strength bR .

× × × =− −2 3 2 = 9.11 0.0135 + 0.0597 0.00282 , 0.936bMSR x x x R (3)

× − =− × 2 2 = 9.01 0.0758 0.00525 , 0.98bGBFSR x x R (4)

2 2 = 9.21 0.0995 0.00156 , 0.93bMS GBFSR x x R+ ×− − × = (5)

The significance of the regression coefficients was tested using Fisher’s and Student’s tests. In the above equations, all coefficients are significant. The coefficient of determina-tion is quite high from 0.93 to 0.98. It can also be seen that if the additive x is equal to zero, the value of the compression strength is approximately the same with an error of ± 5%.

Therefore, the MS additive, introduced instead of part of the cement in an amount of 4%, provides an increase in strength by 12% compared to the control composition, and replacing part of the cement with an MS additive in an amount of 8%, 12%, 16%, and 20% provides an increase in strength by 27%, 35%, 47%, and 11% respectively. The decrease in strength gained when replacing cement with 20% MS indicates that a further increase will lead to a loss in strength.

This increase in the strength of non-autoclaved aerated concrete when replacing part of the MS cement in an amount of 4–16% can be explained by the fact that the addition of MS, due to its pozzolanic activity, provides the formation of additional C-S-H and creates the effect of a denser packing of particles [43,60].

Figure 6. Change in compressive strength of non-autoclaved aerated concrete samples depending onthe percentage of binder replacement by various types of additives.

Appl. Sci. 2022, 12, 6984 11 of 21

Figure 6 shows that the addition of microsilica has the most positive effect on thecompressive strength of non-autoclaved aerated concrete. Statistical processing of theobtained experimental data made it possible to determine regression curves describing theeffect of additives x on compressive strength Rb.

RbMS= 9.11 − 0.0135 × x+0.0597 × x2 − 0.00282 × x3, R2 = 0.936 (3)

Rb GBFS= 9.01 − 0.0758 × x − 0.00525 × x2, R2 = 0.98 (4)

Rb MS+GBFS= 9.21 − 0.0995 × x − 0.00156 × x2, R2 = 0.93 (5)

The significance of the regression coefficients was tested using Fisher’s and Student’stests. In the above equations, all coefficients are significant. The coefficient of determinationis quite high from 0.93 to 0.98. It can also be seen that if the additive x is equal to zero, thevalue of the compression strength is approximately the same with an error of ±5%.

Therefore, the MS additive, introduced instead of part of the cement in an amountof 4%, provides an increase in strength by 12% compared to the control composition, andreplacing part of the cement with an MS additive in an amount of 8%, 12%, 16%, and 20%provides an increase in strength by 27%, 35%, 47%, and 11% respectively. The decrease instrength gained when replacing cement with 20% MS indicates that a further increase willlead to a loss in strength.

This increase in the strength of non-autoclaved aerated concrete when replacing partof the MS cement in an amount of 4–16% can be explained by the fact that the addition ofMS, due to its pozzolanic activity, provides the formation of additional C-S-H and createsthe effect of a denser packing of particles [43,60].

Replacing part of the cement with the addition of ground blast-furnace slag leadsto a decrease in the strength of non-autoclaved aerated concrete in comparison with thestrength value of the control composition. Thus, when replacing part of the cement with 4%GBFS, the compressive strength decreases by 5%, with the amount of GBFS 8%, the strengthdecreases by 6%, with the amount of GBFS 12%, the strength decreases by 19%, and thereplacement of cement with GBFS in the amount of 16% and 20% leads to a reductionin compressive strength of up to 29% and 39%, respectively. First, the drop in strengthcharacteristics is due to the fact that when using GBFS in concrete, it reduces “the degree ofhydration of the cement due to the lack of cement gel”.

As for the use of the complex additive MS + GBFS (1:1) as a replacement for part of thecement, it also, as in the case of GBFS, leads to a loss in compressive strength; however, thepercentage of strength reduction is noticeably less. In the case of using MS + GBFS (1:1) inan amount of 4%, a slight increase in strength is observed in an amount of 2%. In the caseof replacements of 8%, 12%, 16%, and 20% by weight of cement, the drop in compressivestrength was 6%, 17%, 20%, and 25%, respectively.

3.3. Coefficient of Construction Quality

The values of the calculated coefficients of structural quality of non-autoclaved aeratedconcrete are presented in Table 10.

The dependences of the change in the coefficients of structural quality of compositionsof types A, B, and AB are shown in Figure 7.

The dependences of the change in the coefficients of constructive quality for non-autoclaved aerated concrete with additives MS and GBFS and with the complex additiveMS:GBFS have a similar character as the dependence of the change in compressive strengthof compositions of types A, B, and AB.

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Table 10. Experimental values of the coefficients of structural quality of non-autoclaved aerated concrete.

Composition Number CCQ, MPa × cm3/g

C 5.061A 5.692A 6.413A 6.804A 7.395A 5.621B 4.802B 4.723B 4.084B 3.565B 3.07

1AB 5.142AB 4.773AB 4.194AB 4.035AB 3.79

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Figure 7. Change in the coefficient of structural quality of non-autoclaved aerated concrete depend-ing on the percentage of binder replacement by various types of additives.

The dependences of the change in the coefficients of constructive quality for non-autoclaved aerated concrete with additives MS and GBFS and with the complex additive MS:GBFS have a similar character as the dependence of the change in compressive strength of compositions of types A, B, and AB.

The regression equations for the design quality factors are given in Equations (6)–(8). The coefficient of determination is also quite high from 0.93 to 0.986.

× × − × =2 3 2CCQ = 5.12 + 0.000618 + 0.0323 0.00154 , 0.938MS x x x R (6)

× ×− =− 2 2CCQ = 5.18 0.0563 0.000848 , 0.93GBFS x x R (7)

+ × × =− 2 2CCQ = 5.06 0.0373 - 0.00325 , 0.986MS GBFS x x R (8)

Changes in the coefficients of structural quality of non-autoclaved aerated concrete as a percentage are presented in Table 11.

Figure 7. Change in the coefficient of structural quality of non-autoclaved aerated concrete dependingon the percentage of binder replacement by various types of additives.

The regression equations for the design quality factors are given in Equations (6)–(8).The coefficient of determination is also quite high from 0.93 to 0.986.

CCQMS= 5.12 + 0.000618 × x+0.0323 × x2 − 0.00154 × x3, R2 = 0.938 (6)

CCQGBFS= 5.18 − 0.0563 × x − 0.000848 × x2, R2 = 0.93 (7)

CCQMS+GBFS= 5.06 − 0.0373 × x − 0.00325 × x2, R2 = 0.986 (8)

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Changes in the coefficients of structural quality of non-autoclaved aerated concrete asa percentage are presented in Table 11.

Table 11. Change in the coefficients of structural quality of non-autoclaved aerated concrete depend-ing on the type of additive and its dosage.

AdditiveType

Change in % (∆) of the Coefficient of Constructive Quality at the FollowingDosages of Additives (%) Introduced instead of Part of the Binder

0 4 8 12 16 20

MS 0 +12 +27 +34 +46 +11GBFS 0 −5 −7 −19 −30 −39

MS:GBFS 0 +2 −6 −17 −20 −25

Figure 7 and Table 11 show that the best ratio between strength and density wasdemonstrated by aerated concrete samples with the addition of MS as a replacement forcement in the amount of 16%. The addition of GBFS had a negative effect on the ratiobetween strength and density of aerated concrete, i.e., reduced CCQ. While the complexadditive MS:GBFS in the amount of 4%:4% and 8%:8% has characteristics similar to thecontrol composition.

3.4. Thermal Conductivity

The results of determining the thermal conductivity of samples of non-autoclavedaerated concrete are presented in Table 12.

Table 12. Results of determining the thermal conductivity of samples of non-autoclaved aerated concrete.

Composition Number Dry Thermal Conductivity λ, W/(m·◦C)

C 0.3651A 0.3592A 0.3413A 0.3324A 0.3305A 0.3401B 0.3672B 0.3693B 0.3744B 0.3785B 0.382

1AB 0.3682AB 0.3713AB 0.3724AB 0.3805AB 0.383

The dependences of the change in the thermal conductivity coefficients of compositionsof type A, B, and AB are shown in Figure 8.

Figure 8 shows that the best values of thermal conductivity coefficients are observedin compositions of type A.

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Figure 8. Change in the thermal conductivity of non-autoclaved aerated concrete depending on the percentage of binder replacement by various types of additives.

Figure 8 shows that the best values of thermal conductivity coefficients are observed in compositions of type A.

Regression equations for thermal conductivity coefficients are given in Equations (9)–(11). The coefficient of determination is also quite high from 0.96 to 0.99.

λ −× × × × =− − 2 5 3 2 = 0.365 0.00118 0.000334 + 1.65 10 , 0.987MS x x x R (9)

λ −× × × =5 2 2 = 0.365 + 0.000477 + 2.00 10 , 0.99GBFS x x R (10)

λ −+ × × × =5 2 2 = 0.365 + 0.000460 + 2.23 10 , 0.96MS GBFS x x R (11)

Replacing cement with an MS additive in an amount of 16% provides the maximum reduction in thermal conductivity to 10%. Thus, in Table 13, the values of changes in the thermal conductivity coefficients of the experimental compositions are given in percent-age terms depending on the type and dosage of the additive.

Table 13. Change in the thermal conductivity coefficients of non-autoclaved aerated concrete de-pending on the type of additive and its dosage.

Additive Type Change in % (∆) of the Coefficient of Thermal Conductivity at the Following Dosages of

Additives (%) Introduced instead of Part of the Binder 0 4 8 12 16 20

MS 0 −2 −7 −9 −10 −7 GBFS 0 +1 +1 +2 +4 +5

MS:GBFS 0 +1 +2 +2 +4 +5

Figure 8. Change in the thermal conductivity of non-autoclaved aerated concrete depending on thepercentage of binder replacement by various types of additives.

Regression equations for thermal conductivity coefficients are given in Equations (9)–(11).The coefficient of determination is also quite high from 0.96 to 0.99.

λMS= 0.365 − 0.00118 × x − 0.000334 × x2+1.65 × 10−5 × x3, R2 = 0.987 (9)

λGBFS= 0.365 + 0.000477 × x+2.00 × 10−5 × x2, R2 = 0.99 (10)

λMS+GBFS= 0.365 + 0.000460 × x+2.23 × 10−5 × x2, R2 = 0.96 (11)

Replacing cement with an MS additive in an amount of 16% provides the maximumreduction in thermal conductivity to 10%. Thus, in Table 13, the values of changes in thethermal conductivity coefficients of the experimental compositions are given in percentageterms depending on the type and dosage of the additive.

Table 13. Change in the thermal conductivity coefficients of non-autoclaved aerated concrete depend-ing on the type of additive and its dosage.

AdditiveType

Change in % (∆) of the Coefficient of Thermal Conductivity at the FollowingDosages of Additives (%) Introduced instead of Part of the Binder

0 4 8 12 16 20

MS 0 −2 −7 −9 −10 −7GBFS 0 +1 +1 +2 +4 +5

MS:GBFS 0 +1 +2 +2 +4 +5

Table 13 shows that the most significant influence on the change in the thermalconductivity coefficient is exerted by the addition of silica fume at its content in the rangefrom 8% to 20%. The addition of ground blast-furnace slag and the MS:GBFS complex

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additive do not have a significant effect on the thermal conductivity coefficient and, inpercentage terms, have almost the same degree of influence on the change in this indicator.

3.5. Microstructural Analysis

To better understand the microstructural behavior of the AAC samples, SEM testswere performed on the NAC sample of the control composition and the AAC type 4Asample with the best compressive strength, structural quality factor, and the lowest thermalconductivity value (Figure 9).

Appl. Sci. 2022, 12, x FOR PEER REVIEW 18 of 24

(a)

(b)

Figure 9. Cont.

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(b)

(с)

(d)

Figure 9. SEM of non-autoclaved aerated concrete samples: (a) control composition at 300× magni-fication; (b) control formulation at 2000× magnification; (c) with the addition of microsilica in the amount of 16% with a 100-fold increase; (d) with the addition of microsilica in the amount of 16% with a 1000-fold increase.

As a rule, the microstructure of aerated concrete directly depends on the type and number of phases, the rate of hydration, the type of reaction products formed, and their distribution in the aerated concrete matrix [24]. All these features depend primarily on the

Figure 9. SEM of non-autoclaved aerated concrete samples: (a) control composition at 300× magni-fication; (b) control formulation at 2000× magnification; (c) with the addition of microsilica in theamount of 16% with a 100-fold increase; (d) with the addition of microsilica in the amount of 16%with a 1000-fold increase.

As a rule, the microstructure of aerated concrete directly depends on the type andnumber of phases, the rate of hydration, the type of reaction products formed, and theirdistribution in the aerated concrete matrix [24]. All these features depend primarily on the

Appl. Sci. 2022, 12, 6984 17 of 21

chemical composition of the raw materials used for the manufacture of aerated concretemixtures and the hardening mode. Thus, an increase in the compressive strength ofnon-autoclaved aerated concrete samples with the addition of microsilica, introducedinstead of part of the cement in an amount of 16%, can be explained by differences in themicrostructure of the matrix due to changes in the morphology of tobermorite (Figure 9).

In samples of the control composition (Figure 9a,b), tobermorite crystals are predom-inantly acicular in shape. As for the samples of aerated concrete with the addition ofmicrosilica, here, these crystals mainly have a lamellar shape (Figure 9c,d).

Thus, according to the results of microstructural analysis, it can be concluded thatthe microsilica additive, introduced instead of part of the cement, provides a densermicrostructure of non-autoclaved aerated concrete. The presence of a denser and morecompact microstructure of the aerated concrete matrix leads to an increase in strength. Thisstatement is consistent with some studies [1,2,31], where the formation of C–S–H phaseswith different microstructure and morphology was observed, which has a significant effecton the compressive strength of aerated concrete.

4. Discussion

The theoretical and experimental work carried out requires a detailed analyticalinterpretation. Let us divide the analysis of the obtained results into two stages. Thefirst stage will be a direct comparison of the results obtained and the construction of ananalytical Ishikawa diagram based on the intermediate conclusions made. Let us presentthe Ishikawa diagram as a dependence of the quality of the obtained non-autoclavedaerated concrete on various prescription factors. The Ishikawa diagram is shown below inFigure 10.

Analyzing Figure 10, we note that the following prescription factors turned out to bethe most important parameters—the type of powdered waste used and its dosage. At thesame time, microsilica waste showed 44% better results than the complex additive and53% better than the GBFS additive. In this regard, we have established rational recipeparameters both in qualitative and quantitative terms for non-autoclaved aerated concretewith improved strength.

Next, we compare our results with the results of other authors. The authors of [40–42]suggested using prescription factors to improve the physical and mechanical propertiesof aerated concrete; the increase in compressive strength ranged from 16% to 22%. In thepresent study, the increase in the strength of aerated concrete due to prescription factors incomparison with the control composition was up to 46%.

Finally, we analyzed not only the qualitative and quantitative patterns of improvingthe quality of aerated concrete, but also analyzed the fundamental essence of the process ofstructure formation of non-autoclaved aerated concrete, which proved the good compati-bility of the applied rational component, namely silica fume, for the best microstructureformation, which was confirmed at the micro level. That is, the particles proposed byus, which are modifiers of aerated concrete, act as crystallization centers, allowing you tocreate a dense packing of particles in interpore partitions, while simultaneously adjustingthe correct rational pore structure and creating concrete not only with improved strengthcharacteristics, but also with an improved structure.

In this regard, our study is scientifically new and practically significant, and deservesfurther development.

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chemical composition of the raw materials used for the manufacture of aerated concrete mixtures and the hardening mode. Thus, an increase in the compressive strength of non-autoclaved aerated concrete samples with the addition of microsilica, introduced instead of part of the cement in an amount of 16%, can be explained by differences in the micro-structure of the matrix due to changes in the morphology of tobermorite (Figure 9).

In samples of the control composition (Figure 9a,b), tobermorite crystals are predom-inantly acicular in shape. As for the samples of aerated concrete with the addition of mi-crosilica, here, these crystals mainly have a lamellar shape (Figure 9c,d).

Thus, according to the results of microstructural analysis, it can be concluded that the microsilica additive, introduced instead of part of the cement, provides a denser mi-crostructure of non-autoclaved aerated concrete. The presence of a denser and more com-pact microstructure of the aerated concrete matrix leads to an increase in strength. This statement is consistent with some studies [1,2,31], where the formation of C–S–H phases with different microstructure and morphology was observed, which has a significant ef-fect on the compressive strength of aerated concrete.

4. Discussion The theoretical and experimental work carried out requires a detailed analytical in-

terpretation. Let us divide the analysis of the obtained results into two stages. The first stage will be a direct comparison of the results obtained and the construction of an ana-lytical Ishikawa diagram based on the intermediate conclusions made. Let us present the Ishikawa diagram as a dependence of the quality of the obtained non-autoclaved aerated concrete on various prescription factors. The Ishikawa diagram is shown below in Figure 10.

Figure 10. Factors affecting the properties of aerated concrete: (1–3)—the main groups of factors. Figure 10. Factors affecting the properties of aerated concrete: (1–3)—the main groups of factors.

5. Conclusions

Based on the test results, the following conclusions can be drawn:

(1) The effect of formulation factors on the structure formation and properties of non-autoclaved aerated concrete with improved characteristics was studied.

(2) Non-autoclaved aerated concretes with replacement of part of the cement from 4%to 16% MS showed higher strength characteristics compared to aerated concreteswhere part of the cement was replaced with GBFS admixture and MS:GBFS complexadmixture. The maximum value of compressive strength was recorded for aeratedconcrete with 16% MS addition.

(3) Gains in design quality factors were observed only for formulations with MS additive,MS additive contributed to gains in CCQ in the range of 11–46% compared to thecontrol formulation.

(4) Addition of MS improved the thermal conductivity characteristics of non-autoclavedaerated concrete (up to 10%). Replacing part of the cement with GBFS and MS:GBFSadditives did not significantly affect the change in thermal conductivity values. Ingeneral, when replacing part of the cement with these additives in the range of 4–20%,small increases in thermal conductivity coefficients from 1% to 5% were observedcompared to the control composition.

(5) Theoretical ideas about structure formation at the microlevel and the influence ofmicrostructure on the formation of the properties of non-autoclaved aerated concretewith improved characteristics based on nanomodifiers of various origins have beendeveloped. Improvement of the microstructure of aerated concrete with the additionof MS in comparison with samples of the control composition has been proven.

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The practical significance of the study was to improve the building enclosing structuresmade of cellular concrete with improved characteristics, which in turn will lead to anincrease in the energy efficiency of buildings and structures for various purposes.

Author Contributions: Conceptualization, S.A.S., E.M.S., A.N.B. and N.D.; methodology, S.A.S.,E.M.S. and M.K.; software, S.A.S., E.M.S., A.N.B. and N.B.; validation, N.D., S.A.S., E.M.S. and A.N.B.;formal analysis, N.D., S.A.S. and E.M.S.; investigation, N.B., L.R.M., S.A.S., E.M.S., A.N.B. and B.M.;resources, B.M.; data curation, S.A.S., E.M.S. and M.K.; writing—original draft preparation, S.A.S.,E.M.S. and A.N.B.; writing—review and editing, S.A.S., E.M.S. and A.N.B.; visualization, S.A.S.,E.M.S., A.N.B. and N.B.; supervision, L.R.M. and B.M.; project administration, L.R.M. and B.M.;funding acquisition, A.N.B. and B.M. All authors have read and agreed to the published version ofthe manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The study did not report any data.

Acknowledgments: The authors would like to acknowledge the administration of Don State Techni-cal University for their resources and financial support.

Conflicts of Interest: The authors declare no conflict of interest.

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