3.1 The structure formation process in the ITZ "alkali-activated cement-alkali-susceptible aggregate"

#### 3.1.1 Concrete mixture "alkali-activated cement-artificial alkali-susceptible aggregate"

The study of the ITZ structure formation in the concrete mixture "artificial granular aggregate-GBFS-Na2CO3 solution" taken in the following proportions, 8:1.95:0.5 by mass calculated on Na2CO3 dry matter, showed that the highest values of microhardness were characteristic of the interfaces between the cement paste and granules. This can be attributed to strong adhesion of the cementation material to an activated matter of the clay loam-based loamy granules and formation of the hydration products which determine high-performance properties of the concrete. Bond strength in the interfacial transition zones of the steam-cured concrete is 3.8 MPa and after 3 years—4.5 MPa.

The structure of the ITZ in the steam-cured alkali-activated cement concrete at an age of 28 days was dense. The boundary of the interface in some regions discontinues, testifying to a mutual penetration of these regions and blurring the border between the cement stone and aggregate.

The study of the 3-year-old concrete showed that the ITZ in this case is itself a close interlacement of substances included in the cement paste and the granule and

The ITZ was studied with the help of a scanning electron microscope. A hardness and elemental distribution in the ITZ were studied as well.

Material Mass percentage of oxides Σ % Мо Ма

GBFS 39.0 5.9 47.7 5.56 0.29 1.48 0.1 0.5 100.53 1.19 0.151 ОРС 21.82 5.30 65.91 1.11 4.86 0.99 0.22 0.2 100.41 — — Metakaolin 55.05 35.40 3.01 0.92 4.27 0.28 — 0.07 99.00 — — Clay loam 92.10 3.50 5.29 — 0.87 — — 2.12 103.88 — —

Basalt Perlite

Content calculated on dry matter, g

> In 1 kg of solution

> > Perlite rock Expanded perlite

Mass percentage of oxides

SiO2 R2O3 Na2O CaO

SiO2 Al2O3 CaO MgO Fe2O3 SO3 Na2O+K2O LOI

retard the ASR.

Chemical composition of the aggregates.

Table 3.

6

Table 1.

g/cm3

Table 2.

Resulted density,

Compressive Strength of Concrete

Characterization of the soluble silicate.

A chemical composition of the constituent materials.

Silicate modulus

Oxides Mass percentage of oxides

Basalt rock Glassy waste product from

basalt fiber production

SiO2 50.200 50.050 72.820 76.730 Al2O3 14.000 15.350 12.500 13.160 Fe2O3 6.340 6.230 0.650 0.680 FeO 8.670 7.000 0.900 0.950 TiO2 1.620 2.680 0.110 0.110 MnO 0.240 0.300 0.030 0.030 CaO 8.350 9.210 1.070 1.120 MgO 6.600 5.580 0.170 0.180 P2O5 0.320 — 0.007 — K2O 0.710 0.770 4.510 4.740 Na2O 2.270 2.180 2.100 2.210 SO3 0.080 0.150 0.050 0.040 LOI 0.550 — 4.750 — Σ, % 99.950 99.500 99.667 99.950

In 1 l of solution

1.40 2.96 539.9 385.6 28.5 0.19 9.37 0.15

The metakaolin, taken in quantities 5–15% by mass, was chosen as an additive to

The presence of such interaction is supported by the statistical correlation analysis of the obtained data [42]. Analysis of the concentration curves suggested to draw a conclusion on a correlation dependence between the chemical elements in

The Influence of Interfacial Transition Zone on Strength of Alkali-Activated Concrete

The coefficients of correlation were calculated by methods of pair and multiple

Analysis of individual coefficients of multiple correlation, which characterize a bond between two elements in that time, a third element present at a constant level, showed that these coefficients differ in value from the coefficients of pair correla-

interdependence. So, for example, at constant Si, a local coefficient of correlation for Na and Al is equal to 0.26 and at constant Са—0.96, which is caused by great influence of silicon (Si) on the bond "Na–Al." Almost similar action is rendered by sodium on the bond "Al–Si." This can be seen from comparison of the corresponding local coefficients of correlation. Moreover, the bond between Al and Si is strongly affected by Са/RAl,Si,Ca = 0.114, in its turn closely connected with

It is worth mentioning that the coefficients of pair correlation for Na and Сa in the case under study are insignificant. However, this is not witnesses the absence of the bond between them, since the individual local coefficient of multiple correlation by alumina is 0.950. Evidently, this bond is considerably affected by aluminum Al,

An interrelation between all elements under study is observed and after inter-

The presence of such bond witnesses the appearance of the cement paste/granule interface in the alkali-activated cement concrete of chemical compounds of alkaline, alkali-earth, and mixed alkaline-alkali-earth aluminosilicate composition. Investigation of specimens of steam-cured alkali-activated cement concrete at the age of 3 years showed that also in this case the compounds containing uniformly/homogeneously interrelated elements, Na, Al, Si, and Ca (all correlation coefficients before the interface line and after it are significant), are present over the ITZ. Moreover, the coefficients of pair correlation for Na and Al/ζNa, Al = 0.682– 0.707/, Na and Si/ ζNa, Si = 0.796/, and especially for Na and Сa/ζ Na, Ca = 0.580– 0.620/ at simultaneous increase of common coefficients of multiple correlation significantly increased. A conclusion was made that the aluminum affects the Na-Ca bond and calcium the Na-Al bond. This conclusion is supported by a difference of individual coefficients of correlation between these elements from the coefficients of pair correlation. All this is an evidence that quantities of the compounds of mixed alkaline-alkali-earth alkaline aluminosilicate composition increased in the ITZ. The higher correlation between all elements in time is caused by growth of the hydration products in the ITZ and, hence, densification and strengthening of the

tion. This difference is a proof of the interaction of all elements and their

A comparative evaluation of the obtained data showed that in early ages of hardening of the steam-cured alkali-activated slag cement concrete with artificial granular aggregate in transition zone, the elements Na and Si, Al and Si, Al and Ca, Na and Al, and Са and Si are combined; their coefficients of pair correlation are significant. The closest bound in that case characterizes a coefficient of pair correlation for Si and Са, which amounts for 0.782 before interface line and 0.689 after the interface line. This can serve as evidence of the presence of calcium silicates in the ITZ. However, the values of the coefficients of multiple correlation for Na, S, and Са/RNa, Si, Ca/ and for Al, Si, and Са/RAl, Si, Ca/ for all interface exceed 0.6. That showed that Si and Ca so far as Al and Na could incorporate in such compounds as

the interface.

correlation.

bonded Si.

interface itself.

9

alumina silicates of sodium and calcium.

DOI: http://dx.doi.org/10.5772/intechopen.90929

which is much stronger attained by Na.

face line from the side of the granule.

#### Figure 1.

The concentration curves of elemental distribution in the cement paste/granular aggregate interfacial transition zones in the steam-cured alkali-activated cement concrete: (а) at age of 28 days; (b) at age of 3 years [42].

is characteristic of the presence of the clearly visible (expressed) new phases. The interface is not visible (absent).

A comparative analysis of the obtained concentration curves of elemental distribution (Figure 1) suggested to conclude that sodium, aluminum, silicon, calcium, and iron are present over the whole width of the ITZ in the steam-cured alkaliactivated cement concrete. Sodium and silicon were distributed homogeneously over the interface, iron is contained in small quantities (Figure 1a). The distribution of Ca and Al is inhomogeneous. So, the content of calcium is reducing in two times as far as closer to the granule, and that of aluminum—increasing in the same direct. Evidently, it is attributed to the fact that the hydration products in the cement paste are represented chiefly by low-basic calcium silicate hydrates and partially by the hydrogarnets, so far as the alkaline aluminosilicate hydrates are formed in the granules along with the abovementioned.

On the contrary to the investigated concentration curves of elemental (Ca and Al) distribution in the ITZ of the 3-year-old alkali-activated cement, concrete is characteristic of homogeneous uniform elemental (Ca and Al) distribution over the width of the interface (Figure 1b). The earlier observed aggregations of calcium near the cement paste and aluminum granules are absent. Calcium is homogeneously distributed over the interface, and quantity of aluminum approaching to the cement paste of the concrete greatly increased. After 3 years of hardening of the alkali-activated cement concrete, the curves of elemental distribution of all elements are characterized by high-frequency oscillations of concentrations, which can be an evidence of the increase of the interface bond strength and compaction the structure of the interface.

Evidently, change of the state of the ITZ in time can be attributed to physicalchemical processes occurring in the concrete. Above such phenomena as adsorption, diffusion, and others, a chemical interaction of the substances contained in various cements and aggregates took place, leading to formation of the hydration products of the alkaline, alkali-earth, and mixed alkaline-alkali-earth aluminosilicate composition—such compounds like aluminosilicates of sodium and calcium.

The Influence of Interfacial Transition Zone on Strength of Alkali-Activated Concrete DOI: http://dx.doi.org/10.5772/intechopen.90929

The presence of such interaction is supported by the statistical correlation analysis of the obtained data [42]. Analysis of the concentration curves suggested to draw a conclusion on a correlation dependence between the chemical elements in the interface.

The coefficients of correlation were calculated by methods of pair and multiple correlation.

A comparative evaluation of the obtained data showed that in early ages of hardening of the steam-cured alkali-activated slag cement concrete with artificial granular aggregate in transition zone, the elements Na and Si, Al and Si, Al and Ca, Na and Al, and Са and Si are combined; their coefficients of pair correlation are significant. The closest bound in that case characterizes a coefficient of pair correlation for Si and Са, which amounts for 0.782 before interface line and 0.689 after the interface line. This can serve as evidence of the presence of calcium silicates in the ITZ. However, the values of the coefficients of multiple correlation for Na, S, and Са/RNa, Si, Ca/ and for Al, Si, and Са/RAl, Si, Ca/ for all interface exceed 0.6. That showed that Si and Ca so far as Al and Na could incorporate in such compounds as alumina silicates of sodium and calcium.

Analysis of individual coefficients of multiple correlation, which characterize a bond between two elements in that time, a third element present at a constant level, showed that these coefficients differ in value from the coefficients of pair correlation. This difference is a proof of the interaction of all elements and their interdependence. So, for example, at constant Si, a local coefficient of correlation for Na and Al is equal to 0.26 and at constant Са—0.96, which is caused by great influence of silicon (Si) on the bond "Na–Al." Almost similar action is rendered by sodium on the bond "Al–Si." This can be seen from comparison of the corresponding local coefficients of correlation. Moreover, the bond between Al and Si is strongly affected by Са/RAl,Si,Ca = 0.114, in its turn closely connected with bonded Si.

It is worth mentioning that the coefficients of pair correlation for Na and Сa in the case under study are insignificant. However, this is not witnesses the absence of the bond between them, since the individual local coefficient of multiple correlation by alumina is 0.950. Evidently, this bond is considerably affected by aluminum Al, which is much stronger attained by Na.

An interrelation between all elements under study is observed and after interface line from the side of the granule.

The presence of such bond witnesses the appearance of the cement paste/granule interface in the alkali-activated cement concrete of chemical compounds of alkaline, alkali-earth, and mixed alkaline-alkali-earth aluminosilicate composition.

Investigation of specimens of steam-cured alkali-activated cement concrete at the age of 3 years showed that also in this case the compounds containing uniformly/homogeneously interrelated elements, Na, Al, Si, and Ca (all correlation coefficients before the interface line and after it are significant), are present over the ITZ. Moreover, the coefficients of pair correlation for Na and Al/ζNa, Al = 0.682– 0.707/, Na and Si/ ζNa, Si = 0.796/, and especially for Na and Сa/ζ Na, Ca = 0.580– 0.620/ at simultaneous increase of common coefficients of multiple correlation significantly increased. A conclusion was made that the aluminum affects the Na-Ca bond and calcium the Na-Al bond. This conclusion is supported by a difference of individual coefficients of correlation between these elements from the coefficients of pair correlation. All this is an evidence that quantities of the compounds of mixed alkaline-alkali-earth alkaline aluminosilicate composition increased in the ITZ. The higher correlation between all elements in time is caused by growth of the hydration products in the ITZ and, hence, densification and strengthening of the interface itself.

is characteristic of the presence of the clearly visible (expressed) new phases. The

The concentration curves of elemental distribution in the cement paste/granular aggregate interfacial transition zones in the steam-cured alkali-activated cement concrete: (а) at age of 28 days; (b) at age of 3 years [42].

A comparative analysis of the obtained concentration curves of elemental distribution (Figure 1) suggested to conclude that sodium, aluminum, silicon, calcium, and iron are present over the whole width of the ITZ in the steam-cured alkaliactivated cement concrete. Sodium and silicon were distributed homogeneously over the interface, iron is contained in small quantities (Figure 1a). The distribution of Ca and Al is inhomogeneous. So, the content of calcium is reducing in two times as far as closer to the granule, and that of aluminum—increasing in the same direct. Evidently, it is attributed to the fact that the hydration products in the cement paste are represented chiefly by low-basic calcium silicate hydrates and partially by the hydrogarnets, so far as the alkaline aluminosilicate hydrates are formed in the

On the contrary to the investigated concentration curves of elemental (Ca and Al) distribution in the ITZ of the 3-year-old alkali-activated cement, concrete is characteristic of homogeneous uniform elemental (Ca and Al) distribution over the width of the interface (Figure 1b). The earlier observed aggregations of calcium near the cement paste and aluminum granules are absent. Calcium is homogeneously distributed over the interface, and quantity of aluminum approaching to the cement paste of the concrete greatly increased. After 3 years of hardening of the alkali-activated cement concrete, the curves of elemental distribution of all elements are characterized by high-frequency oscillations of concentrations, which can be an evidence of the increase of the interface bond strength and compaction the

Evidently, change of the state of the ITZ in time can be attributed to physicalchemical processes occurring in the concrete. Above such phenomena as adsorption, diffusion, and others, a chemical interaction of the substances contained in various cements and aggregates took place, leading to formation of the hydration products of the alkaline, alkali-earth, and mixed alkaline-alkali-earth aluminosilicate composition—such compounds like aluminosilicates of sodium

interface is not visible (absent).

Compressive Strength of Concrete

Figure 1.

granules along with the abovementioned.

structure of the interface.

and calcium.

8

## Compressive Strength of Concrete

The alkali-activated cement concrete with the artificial granular aggregate had a dense structure and was characteristic of high bond strength of the granules with the cement paste. A dense structure of the alkali-activated cement concrete under study, as well as the composition of the hydration products, determine high physical-mechanical properties of such concrete, which, in their turn, determine performance properties.
