ISSN 0012�5008, Doklady Chemistry, 2009, Vol. 429, Part 2, pp. 324–327. © Pleiades Publishing, Ltd., 2009.Original Russian Text © S.A. Zubekhin, S.B. Klimov, V.I. Rakhovskii, E.I. Rau, B.E. Yudovich, 2009, published in Doklady Akademii Nauk, 2009, Vol. 429, No. 5, pp. 636–639.

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Nonautoclaved foam concretes manufactured bystirring a cement paste and a foam formed by an aque�ous solution of a foaming agent, thanks to their lowweights, low materials, power, and labor consump�tions, durability, and high efficiency, are promising foruse in low�floor house building. Here, we report thefirst comparison of the submicron structures of twofundamentally different types of nonautoclaved foamconcretes, one being an ordinary (monoliquid) foamconcrete and the other a new biliquid foam concrete.The use of biliquid foams generates nanostructuredelements in the foam concrete microstructure, therebyradically improving the performance macroparame�ters of foam concretes, namely strength, frost resis�tance, and crack resistance.

The idea of this work belongs to Rebinder [1] andPopov [2], who were the founders of the industry offoams and foam concretes in our country, as early as inthe 1930s. Working with high�quality Portlandcements and biliquid foams formed by two surfactants,one normally and the other weakly water soluble, thatgenerated double�walled and multiwalled shells onvesicles, Rebinder and Popov created the world’s firstproduction of nonautoclaved foam concrete blockswith densities of about 450 kg/m3 [3]. Rebinder andPopov claimed that, although monoliquid foams cameinto universal use after the Second World War becauseof then increasing costs of surfactants, foam concretesrequire the best fine�pulverized cements and commer�cial biliquid foams where both surfactants are watersoluble, but they had not enough time for solving thisproblem. In the 1990s, however, after the productionof low�water cements (LWCs) grades 900 and 1000,which hardened three times as rapidly as unblendedPortland cement grade 500 (free of mineral additives)(PC500 D0), it was timely to develop the aforemen�

tioned biliquid foams for a new foam concrete. Suchan aphron�type foam (the term embedded in [4]) wascreated [5], but the water contents of mature foamconcretes based on it and LWCs exceeded the permis�sible level (15%). A demand arose for the developmentof modified high�strength cement for manufacturingbulk�hydrophobic foam concrete with 6⎯13% watercontents in the 28�day age and water absorptions upto 7%.

The new cement was called Portland cement with adense contact zone (DCZ PC), the contact zone [6]meaning the pores around cement particles in thewater paste filled with hydration products. A new DCZPC foam concrete with an aphron�type foam wasreferred to as a submicrocrystalline (or nanostruc�tured) foam concrete. Precursors were manufacturedon pilot�scale setups; the submicrocrystalline foamconcrete sample for use in this work was manufacturedon a mobile large�scale setup.

The nanostructured foam concrete was comparedto PC500 D0 foam concrete containing a proteinfoaming agent and having improved quality of blend�ing the cement paste (hereafter, the reference foamconcrete). Standard parameters were compared: aver�age density D (kg/m3), contraction strength B (MPa),and frost resistance F measured after the nanostruc�tured and reference samples were stored for four andtwo years, respectively, dry storage under especiallysevere (for foam concretes) conditions (relative airhumidity: 40–55%; temperature: 18–22°C) followingthe 28th day of preliminary humid�air storage. Theresults obtained for the nanostructured and referencefoam concretes, respectively, were as follows: D = 300and 400 kg/m3, B = 1.0 and 0.25 MPa, and F = 35and �15.

Naturally, the advantages of the nanostructuredsample are on account of its submicron structure,which is the subject matter of this work.

One preliminary remark: high�resolution micro�structure examination with an optical microscope isinefficient because of low depths of field. Luminophor

The Role of Biliquid Foam in Generating Nanostructured Elementsof the Submicron Structure of Foam Concretes

S. A. Zubekhina, S. B. Klimovb, V. I. Rakhovskiib, E. I. Rauc, and B. E. Yudovicha

Presented by Academician G.A. Mesyats March 2, 2009

Received April 22, 2009

DOI: 10.1134/S0012500809120064

a Geostrom Ltd., Moscow, Russiab Nanotekh Ltd., Moscow, Russiac Moscow State University, Moscow, 119991 Russia

CHEMICALTECHNOLOGY

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THE ROLE OF BILIQUID FOAM IN GENERATING NANOSTRUCTURED ELEMENTS 325

impregnation followed by optical density measure�ments and ultrasonic analysis [7] implied that, thecloser the pore shapes to spherical and the higher thehomogeneity of their distribution (without clouds(pileups) and rains (pore chains corresponding to theslump of freshly placed foam concretes), the lower thestress concentrations around pores generated by exter�nal loads, the higher the strength, elasticity modulus,and frost resistance, and the lower the heat conductiv�ity of the concrete.

Our study was performed on a LEO scanning elec�tron microscope after evacuating air�dry samples at10–3 mmHg for more than 1 h, sputtering gold, andagain evacuating the test samples at 10–5 mmHg for3 h. Figure 1 displays micrographs of the reference andnanostructured foam concrete samples.

The results obtained on the reference foam con�crete sample (Fig. 1a) indicate that pores, althoughbeing rounded, most have distorted and irregularshapes; boundaries on the cleave are tortuous.According to Rebinder, boundaries of Marangoni�sta�bilized vesicles after mineralization should look likethis; Marangoni’s stabilization means the existence ofsurface tension at the vesicle–environment interfaces,which draws back small vesicles that have been sepa�rated from larger ones in the foam concrete in

response to mechanical impacts (beating up, pump�ing, and placement). Rebinder, as many others sincethen [4], thought that Marangoni’s effect stabilizesonly vesicles with sizes less than 1 μm. The surfaces oflarger pores should include entrances to small poresand be permeable. This is made clear by Fig. 1a. Atlarger pores, internal pore partitions have relativelysmaller thicknesses and densities. This agrees withpore coarsening in response to decreasing density ofordinary foam concretes observed by V.G. Khozin,V.V. Kondrat’ev, and N.N. Morozova (Kazan); thepore diameter increased to 1.5 mm for D = 300 kg/m3.Khozin et al.’s effect characterizes the impertinence ofmanufacturing monoliquid foam concretes withD lower than 400 kg/m3.

Figure 1b makes it clear that the cleave surface ofthe reference foam concrete in pores and internal porepartitions is formed of spherulites. These spherulitesare known to arise from the carbonization of hydrationproducts (calcium hydroxide and calcium silicatehydrates (hereafter, CSH) by atmospheric carbondioxide. Their complete carbonization in foam con�cretes with age of two years is legitimate. The degree ofcarbonization for foam concretes is 20–30% as earlyas by the 28th day of humid�air storage [8]. Carboniza�

100 µm 1 µm 100 µm 1 µm

A

B

C

(a) (b) (c) (d)

Micrographs of foam concrete cleaves for (a, b) the reference foam concrete sample ((a) general view and (b) a pore�surface frag�ment) and (c, d) the nanostructured concrete sample ((c) general view and (d) internal pore partition).

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tion reduces the strength of foam concrete, almostdoubles its shrinkage, and enhances crack formation.

Microstructurally, nanostructured foam concrete ischaracterized by a smooth surface and near�sphericalpore shapes (Fig. 1c). Pore�size distribution is close tounimodal (averaging about 200–250 μm); therefore,mechanical impacts have not separated vesicles, thefoam concrete has not changed its density duringtransportation, the material in building’s wallsremains uniform, and spherical pores serve as externalstress concentrators in the least degree, endowing thefoam concrete with the high strength and crack resis�tance. Pores are preferentially closed and chemicallystabilized. Pore walls are continuous, withoutentrances to small pores, as if being built of dense thinegg�like shells. Some pores enclose fragments of inter�nal pore partitions, which withstood cleave crackinguntil a considerable difference was reached betweenthe energy supply to the crack and the energy absorp�tion by the material and, after the critical value of thisdifference was surpassed, by an explosive motion ofthe crack with the ejection of fragments. Therefore,their existence is an indicator of an elevated strength ofinternal pore partitions. Khozin et al.’s effect does notappear, and the quest for decreasing the densities ofaphron�type foam concretes to D = 300 kg/m3 orlower is quite pertinent.

The submicron structure of the nanostructuredfoam concrete (Fig. 1d) does not signify carboniza�tion. In a cleave, the pore surface is of a columnarCSH layer 30–50 nm thick (denoted by A), this layerwas formed topotaxically on the cellular juvenile sur�face of a vesicle with cell sizes of 30–70 nm and gener�ates shells with smooth surfaces (Fig. 1c). Above theshell section, one can see the common matrix of sub�micron�sized fibrous CSH gel (denoted as phase B inFig. 1) and acicular pseudomorphoses to high�waterhydrates, these pseudomorphoses being morphologi�cally close to AFt phases (after H.F.W. Taylor) withincreased SiO2 impurity levels (denoted as C in Fig. 1).Only such phases grow together with CSH so that arenot separated by cracks even upon vacuum drying.This is indicative of an inverted phase crystallizationorder in the nanostructured foam concrete based onDCZ PC and aphron. First, shells A appear instanta�neously around newly formed vesicles because of therecovery into solution of the alkali cations of the foam�ing agent; then, phases B appear; and phases C areformed after the specified cations are bound. (In ordi�nary foam concretes, AFt phases are the first to crys�tallize without SiO2 impurities, followed by phases B,but do not grow together and, finally, decompose andare carbonized in contact with atmospheric air.)

Noteworthy, columnar CSH layers with more thanone order of magnitude larger sizes were observed ontricalcium silicate particles [9] and primary cementhydrates [10] in set paste samples.

We were the first to discover a third zone of colum�nar CSH crystallization on the surface of living vesi�cles owing to the chemical affinity between the phenolgroups of the organic salt additive glued into the dislo�cation zones of DCZ PC particles upon comilling withthe saturation of the newly appearing calcium dan�gling bonds (verified by Auger spectroscopy [11]) andpolyphenols in the outer shell of the biliquid foam inthe presence of sodium salts from conjugation reactioncofactors during the cooking of the two water�solublecomponents of the foaming agent (for the composi�tion, see [5]), thus increasing the silica solubility in theliquid phase of the cement–water system by two ordersof magnitude.

The above is the reason for the appearance of nano�structured elements in the submicron structure of thenew foam concrete, namely, CSH shells of vesicles,then on pores, these shells being decisive for the afore�mentioned radical improvement of the engineeringproperties of the material in comparison to ordinaryfoam concretes.

The above�described results are due to the occur�rence of a rapid, although three�step, process in a mix�ture of an aqueous cement paste (a supersaturatedCSH solution) and a foam, this process comprising thestages of

(i) the instantaneous interaction of surface and sol�ute cement hydrates (dominated by CSH) with thefoam by means of sodium naphthalinesulfonates (NS)molecules of the powdered surfactant grafted to theinitial cement particles during comilling; the latter,entering cement hydrates via their aromatic π elec�trons thanks to calcium cations they have extractedfrom the clinker, are immediately sorbed by similargroups in polyphenols of the outer shell of the biliquidfoam;

(ii) two�dimensional diffusion of aromatic NSgroups from cement hydrates accompanied by cal�cium cations with tail chains of composition –Ca–O–Si(OH)2–O– across the surface of the polyphenolfoam until complete filling;

(iii) controlled CSH precipitation from the liquidphase over the tail paling in the form of a nanocellularsystem, observable by electron microscopy, where cellsare grown as columnar nanocrystals being responsiblefor foam mineralization into poreless nanostructuredshells around pores, these nanostructured shells pro�viding all positive properties of nanostructured foamconcretes.

ACKNOWLEDGMENTS

The authors are sincerely grateful to R.A. Sennovfor collaboration.

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