Влияние золей различного состава на изменение структуры и свойств цементного камня

M achine - readable information on CC- licenses (HTML- code ) in metadata of the paper

The effect of different composition sols on change of structure and properties of cement stone by Urkhanova L.A., Savelyeva M.A. is licensed under a ...

т oday there is an increasing interest attracted by use sol-gel technology in cement compositions and concrete for increase its main structural and technical properties: frost resistance, durability, water resistance, reduction shrinkage strains [1]. So far there was the significant amount of additives on the basis of silica acid sol, iron hydroxide sol, aluminum hydroxide sol which used for obtaining high-strength concrete due to structural change of material. The great contribution to studying of this direction was made by such scientists as M.M. Sychev, P.G. Komokhov,

L.B. Svatovskaya, N.P. Lukutsova and etc. which received generally one-component sols and applied them for modification of cement and concrete [2–17]. Authors [18–21] received the sols included in its composition at the same time silica acid sol, iron hydroxide sol, aluminum hydroxide sol. They developed methods of receiving sols through exchange reactions between salts of multivalent metals.

The analysis of the periodic system of D.I. Mendeleev showed that to date there are a few studies focused on the production of sols based on chemical elements of VI group. The VI group contains chemical compounds which form sols of different composition and stability at hydrolysis or as a result exchange reactions. Authors of article conducted researches of cement stone modification by sols of different composition. Among them chemical compounds of sulfur and chromium are of interest because they are available and rather cheap.

The sulfur sol is a nanodispersed colloidal system which receiving is possible by two methods: redox reaction and solvent substitution method. By the last method the fat solution of sulfur is added gradually to liquid which well mixes up with solvent, for example, with acetone or ethanol, but it doesn’t mix up with water.

Receiving of sulfur sol occurs at interaction of the strong acids with sodium thiosulfate by method of redox reaction. Free thiosulfuric acid breaks down with formation of elemental sulfur:

2HCl + Na₂S₂O 3 ^ 2NaCl + H 2 S 2 O 3 . (1)

H₂S₂O 3 ^ S + H₂O + SO₂ T . (2)

The formation of hydroxide of chromium (III) sol occurs as a result complete ion-exchange reaction – the irreversible hydrolysis proceeded at mixture of solutions of two salts reinforced hydrolysis of each other. Sedimentation with emission of carbon dioxide gas leads to formation turbid-green hydroxide of chromium (III) sol:

2Cr [( (NO)] 3 ) 3 + 3Na₂CO 3 + 3H 2 O ^ 2Cr(OH)₃ ^ +3CO 2 T + 6NaNO 3 . (3)

The barium chromate sol was received as a result of ion-exchange reaction; it is a dispersed system in which the yellow fine-crystalline sediment

precipitates. This sediment is soluble in the strong acids and insoluble in acetic acid:

2BaCl 2 + K₂Cr₂O 7 + 2CH 3 COONa + H₂O ^ 2BaCrO 4 ^ + 2KCl +    (4)

+ 2CH3COOH + 2NaCl.

The main characteristics of aqueous solutions of sols were defined by physical and chemical methods of researches with use of conductometer, areometer, rotational viscometer and calculated methods (tab. 1). Dynamic viscosity of objects in time was investigated for definition of rheology type of aqueous solutions of sols: the Newtonian – value of viscosity does not change in time, the non-Newtonian – value of viscosity changes in time: increases – rheopectic, decreases – thixotropic. The studied objects № 2 and

The main characteristics of aqueous solutions of sols

Table 1

Type of colloidal additive	Hydrogen ion exponent, рН	Density of sol by calculation, p s, g/cm3	Density of sol by areometer, P s, g/cm3	Electrical conductivity, µS	Dynamic viscosity, sPz	Content of colloidal particles in the material precipitated from the sol, % (calculation)
Sulfur sol S, received by solvent substitution method (№ 1)	5–6	0,9875	0,99	202	1,18±0,118	4,25 x 10-4
Sulfur sol S, received by redox reaction (№ 2)	1	0,976	0,98	>1000	1,53±0,015	1,06 x 10-3
Chromium hydroxide sol Cr(OH)3, received by ion-exchange reaction (№ 3)	10–11	1,0945	1,084	>1000	5,75±0,058	2,74 x 10-3
Barium chromate sol BaCrO4, received by ionexchange reaction (№ 4)	5	1,0065	1,006	>1000	1,72±0,017	8,5 x 10-4

№ 3 are non-Newtonian liquids. Object № 2 exhibits properties of rheopec-tic systems, object № 3 exhibits thixotropic properties. Exemplars № 1 and № 4, most likely, fall into to the Newtonian liquids which are not changing the properties at the application of loading. Difference in rheological behavior of the studied objects is explained by their chemical composition.

Analyzing the received characteristics, it is possible to conclude:

– the density of chromium hydroxide sol and barium chromate sol exceed the value of density of water. It means that colloidal solution contain nanoparticles. Density of sulfur sols is less than 1 g/cm3 because sulfur particles in the colloidal solution are easier than water;

– high electrical conductivity of sols demonstrated active surface of nanoparticles that probably promotes formation of additional structural component.

The optimum dosage of sols of various composition and methods of receiving were evaluated the compressing strength of specimens 2 x 2 x 2 cm of plastic forming with W/C = 0,26. For comparison there were tested specimens on the basis of cement without additives – control specimens. The analysis of results of researches on optimization of the colloid additives dosages and kinetics of hardening of cement stone with their application showed that the optimum content of additives are: 0,8% for sulfur sol received by solvent substitution method and for chromium hydroxide sol; 1% – for the sulfur sol received as a result of redox reaction; 0,1% – barium chromate sol (fig. 1, tab. 2).

The specimens with sulfur sol at the age of 7 days received by solvent substitution method were exposed to drying at t = 150oC during 2 hours for imitation of cement stone hardening at the age of 28 days (tab. 2).

The analysis of received results showed that the optimum content of barium chromate sol is 0,1%. It is equal 8,5 x 10^-4% of dry matter equivalent(tab. 1, tab. 2).

At the same time it should be noted that effect of additives begins in early periods of hardening (1, 3 days) with increase of compressive strength from 14% to 45%, therefore, all studied additives can be included in group of hardening-accelerating admixes of cement stone. The most effective additive is chromium hydroxide sol Cr(OH)3 at the dosage of 0,8%. Increase of strength of cement stone at the age of 3 days is 45%.

Drying of cement specimens with modifying additives at t = 150oC during 2 hours accelerates process of crystallization of gel which becomes

Fig. 1. The effect of type and dosage of sols on compressive strength of cement stone: a– sulfur sol (solvent substitution method); b – sulfur sol (redox reaction); c – chromium hydroxide sol; d – barium chromate sol.

additional structural component of cement matrix. It allows reducing time of hardening of PC.

The results of researches showed that sulfur sol received by redox reaction is the most effective additive. The optimum dosage of sulfur sol is 1% of cement mass. It is equal to 1,06 x 10^-3% of dry matter equivalent. Increase of strength of cement stone at the age of 28 days is 79%. Hence, strength of cement stone increases at the microdosage of different composition sols.

The use of sols as modifying additives leads to change of cement stone structure (fig. 2).

Table 2

Data		Time of hardening ( τ ), days
		1	3	7	28
Control specimens	Compressive strength Rc, MPa	31,4	46,5	55,5	66
Specimens with 0,8% of the sulfur sol (solvent substitution method)	Compressive strength Rc, MPa	38,3	55,1	74,5	79
	Rc at the age of 7 days in conditions of imitation of hardening at the age of 28 days(t = 150oC, τ =2 hours), Rc, MPa			82,3 > 79 (Rc 28 days)
Additive effect	Rc/Rc cont.	1,22	1,18	1,34	1,2
Specimens with 1% of the sulfur sol (redox reaction)	Compressive strength Rc, MPa	45	56,6	72,5	118
Additive effect	Rc/Rc cont.	1,4	1,22	1,3	1,79
Specimens with 0,8% of the chromium hydroxide sol	Compressive strength Rc, MPa	39,3	67,4	70,5	75
Additive effect	Rc/Rc cont.	1,25	1,45	1,27	1,14
Specimens with 0,1% of the barium chromate sol	Compressive strength Rc, MPa	40,8	58,1	74,1	92,1
Additive effect	Rc/Rc cont.	1,3	1,25	1,33	1,4

The kinetics of strength setting of cement stone at the optimum content of different composition sols

The structure of control specimen at the age of 3 days has significant amount of pores, there is a portlandit in the form of large smooth surfaces, the crystals of ettringite in the form of long and short needles are visible (fig. 2, a). The microstructure of cement stone with sulfur sols received by solvent substitution method and method of redox reaction is more compact (fig. 2, b, c). The crystals of ettringite have form of needle. They are formed both near the surface of grains of tricalcium aluminate hydrate, and in intergrain space. The process of hardening and compaction of cement stone with sulfur sols is caused by formation of calcium aluminate sulfate hydrateat early stages of hardening [22]. Besides, compounds of sulfur disappear from liquid phase completely at hydration of tricalcium

Fig. 2. The microstructure of cement stone (CS):

a – CS without additives (control specimen); b – CS with sulfur sol (solvent substitution method); c – CS with sulfur sol (redox reaction);

d – CS with chromium hydroxide sol; e – CS with barium chromate sol.

silicate and tetracalcium aluminaferrite in suspension at ratio of solid and fluid phases 1:3. The reduction reactions proceed on surface of solid phase that leads to formation of crystal sulfur and the difficult compounds of calcium containing sulfur [23–24].

The microstructure of cement stone with the additives contained chromium is also much more compact than microstructure of control specimen

(fig. 2, d, e). Sols of barium chromate and chromium hydroxide activate hydration of cement stone and change of phase formation, influencing hydrogen ion concentration, i.e. they are acting on the hard acid principle. The ability of some substances to be an acceptor of electron pair and respectively proton donor or hard acid is reason of increasing of hydrogen ion concentration. Eventually this process in cement pastes is leveled due to hydration of silicates [25].

Thus, the use of sols on the basis sulfur and chromium compounds leads to compaction of cement stone structure that is why strength characteristics are improved. With that strength of cement increases at the microdosage of additives. The presented methods of receiving of different composition sols are available, simply in realization and rather cheap. It is possible to use these technologies in practice.

D ear colleagues !

T he reference to this paper has the following citation format :