2. Materials and testing techniques

microns comes into interaction with the aggregate. The ITZ is considered as the

The ITZ is formed in the process of redistribution of the substances of the cement and aggregate and a result of the reaction "alkali-silicic acid." Depending upon a composition of the formed hydration products, this reaction can be either a destructive (negative effect) one or a constructive (positive effect) one [1–5]. It is believed [6–9] that destructive corrosion of concrete can occur as a result of chemical interaction of alkalis Na2O+K2O of the cements with amorphous silica present in particles of aggregates. Not only amorphous silica but also other substances of aggregate constituents—microcrystalline quartz, micas, clay minerals—can enter into interaction with alkalis. This changes a phase composition of the hydration products in the ITZ, resulting in either its weakening or occurrence of critical deformations of expansion. The problem is that these processes are very slow and signs of corrosion can appear after months and in most cases after years of

An alkali can come into concrete in case of cements with the increased contents of alkaline oxides (Na2О + K2O) (over 0.6%). The higher quantities of alkalis in concrete can be attributed to the wider use of chemical, mineral, and organic additives and admixtures containing them. Alkalis can also come from outside, for example, with seawater, de-icing chemicals, etc. [17, 36–38]. Also, in recent years more and more widely spread are the alkali-activated cements [18–20], in which the alkali contents (1.5–5.5% by mass) are much higher than the values permissible for Portland cements (no more than NaO + K2O = 0.6% by mass). This can in the future initiate an alkali-silica reaction (ASR) in case of alkali-

The results of first observations of cases of severe concrete damage as a result of alkali-aggregate reaction were reported by E.A. Stephenson as long ago as in 1916. In 1940 Stanton [21] observed the alkali-aggregate in the concrete used for diverting dam in California. In the recent years, this problem attracted attention of many researchers, which not only studied and continue to study a mechanism of corrosion process but showed main factors which could affect the alkali-silica reaction [1, 39–40]. Among these factors are the higher alkali contents of cement, the higher cement content of concrete, quantity of alkali-susceptible aggregates, temperature and humidity, permeability of concrete, ingress of alkalis from outside,

All these allowed to develop measures on how to prevent or weaken the alkalisilica reaction. Among such measures are the use of cements containing additives with latent hydraulic activity or pozzolanic materials, such as granulated blastfurnace slag (GBFS), fly ash, microsilica, volcanic glass, and metakaolin. So, Malek and Roy [22] studied a role of Al2O3 and made a conclusion on its positive role in transformation of the ASR from a destructive one into a constructive one. As a result of the so-called "constructive" reaction, the insoluble alkaline and alkaline-

Na2OAl2O3nSiO2mH2O, can be formed in the ITZ. These conclusions were further supported by numerous researchers [23–30], which provided practical solutions on struggle with the ASR through addition in the cement composition of Al2O3 containing additives. However, this oxide is contained in aggregates as well [31–35]. For this reason, the structure formation processes in the ITZ flow with participation of not only substances of cement components, but of substances of aggregate constituents as well, which always contain finely dispersed clay particles.

The purpose of this research was to study the influence of the processes flowing in the interfacial transition zone in the alkali-activated cement concretes made with

alkali-earth aluminosilicate hydrates, analogs to natural zeolites (CaO)

strength-limiting phase in concrete.

Compressive Strength of Concrete

service [9–16].

susceptible aggregates.

mineral composition of aggregates, etc.

different aggregates on their properties.

4

In order to study the structure formation processes in the interfacial transition zone, an artificial (model) granular aggregate was used: the granules were prepared from the following mix—clay loam, 75% by mass; granulated blast-furnace slag, 25% by mass; and alkaline component (Na2CO3 solution), 15% by mass, without any firing. The preparation process was as follows: all constituents were mixed, and granules 10–20 mm in size were formed, which then were allowed to harden for 28 days in normal conditions.

The concrete cube specimens (10 10 10 cm) from the alkali-activated cement (granulated blast-furnace slag, 92% by mass, and Na2CO3, 8% by mass) and these granules to be used as aggregate were prepared and were subjected to steam curing at t = 90 2°С for 8 h. The ITZ was studied at an age of 28 days.

To study properties of the concrete made using real aggregates, the following cement compositions were chosen:


Used as aggregates were:


Chemical composition of the constituent materials is given in Tables 1–3. The alkaline activators were added in a form of solution; those were:


Specific surface area of the granulated blast-furnace slag was 300–330 m2 /kg and that of the metakaolin, 1800 m<sup>2</sup> /kg.

Grain sizes of the aggregates were within ranges of 3–5 mm.

#### 2.1 Test methods

Thin sections were cut directly from the beam specimens of the composition "cement-aggregate" taken as 1:2 which were used to study the interfacial transition zone.


Strength determination was done on beam specimens (4 4 16 cm) prepared from the concrete mixture "cement-aggregate" taken as 1:2 by mass, except for the specimens made using the expanded perlite as aggregate. Since mean densities of crushed basalt, glassy waste product from basalt fiber production, perlite rock, and cement were more or less close to each other, that of the expanded perlite was different—it was by 10 times lower. For this reason, in order to maintain in all specimens under study an equal volume of cement matrix, the proportions between the cement and expanded perlite in the concrete were taken as 1:0.2 by mass. After preparation, the specimens were kept for 2 days in normal condition and then in a thermostat at temperatures of 20 and 65°С and relative humidity (RH)

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

Autogenous deformations were measured using a device with a dial indicator with a scale 0.01 mm. The basic measurements were taken at an age of 2 days. A state of the interfacial transition zones, as was earlier mentioned, is determined, first of all, by the composition and properties of the hydration products as well as by the interface bond strength due to a mutual penetration of the substances of various constituents of the concrete mixture [31, 35]. However, because of small dimensions of the subjects to be studied, the examination of the hydration products in the interfacial transition zones is difficult. For this reason, a spectral imaging in

The presence and distribution of chemical elements that were supposed to pre-

sent in the composition of the hydration products, occurring in the interfacial transition zone, was determined using this examination technique [41]. The elemental (Na, Al, Si, and Ca) distribution was done using the X-ray images and their concentrations—by an intensity of the characteristic lines. The concentration curves of elemental distribution were plotted in accordance with the results of qualitative analysis. A width of the section under study was 200 μm. The measurement of microhardness was done with the use of a microhardness measuring appa-

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

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

The structure of the ITZ in the steam-cured alkali-activated cement concrete at

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

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

of about 100%.

X-ray microanalysis was applied.

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

ratus with a diamond pyramid.

3. Results and conclusions

3.8 MPa and after 3 years—4.5 MPa.

7

border between the cement stone and aggregate.

cement-alkali-susceptible aggregate"

Table 1.

A chemical composition of the constituent materials.


#### Table 2.

Characterization of the soluble silicate.


#### Table 3.

Chemical composition of the aggregates.

The ITZ was studied with the help of a scanning electron microscope.

A hardness and elemental distribution in the ITZ were studied as well.

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

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

Strength determination was done on beam specimens (4 4 16 cm) prepared from the concrete mixture "cement-aggregate" taken as 1:2 by mass, except for the specimens made using the expanded perlite as aggregate. Since mean densities of crushed basalt, glassy waste product from basalt fiber production, perlite rock, and cement were more or less close to each other, that of the expanded perlite was different—it was by 10 times lower. For this reason, in order to maintain in all specimens under study an equal volume of cement matrix, the proportions between the cement and expanded perlite in the concrete were taken as 1:0.2 by mass. After preparation, the specimens were kept for 2 days in normal condition and then in a thermostat at temperatures of 20 and 65°С and relative humidity (RH) of about 100%.

Autogenous deformations were measured using a device with a dial indicator with a scale 0.01 mm. The basic measurements were taken at an age of 2 days.

A state of the interfacial transition zones, as was earlier mentioned, is determined, first of all, by the composition and properties of the hydration products as well as by the interface bond strength due to a mutual penetration of the substances of various constituents of the concrete mixture [31, 35]. However, because of small dimensions of the subjects to be studied, the examination of the hydration products in the interfacial transition zones is difficult. For this reason, a spectral imaging in X-ray microanalysis was applied.

The presence and distribution of chemical elements that were supposed to present in the composition of the hydration products, occurring in the interfacial transition zone, was determined using this examination technique [41]. The elemental (Na, Al, Si, and Ca) distribution was done using the X-ray images and their concentrations—by an intensity of the characteristic lines. The concentration curves of elemental distribution were plotted in accordance with the results of qualitative analysis. A width of the section under study was 200 μm. The measurement of microhardness was done with the use of a microhardness measuring apparatus with a diamond pyramid.
