1. Introduction

Nowadays, concrete is recognized as one of the basic constructional materials. However, strict requirements to its performance properties, in particular durability, are not met in all cases allowing big concerns to occur. With worsening of the ecological situation and larger volumes of use of off-standard materials as concrete constituents, durability became a main criterion of concrete quality. One poorly studied and "hidden" reason explaining low durability of concrete in some cases is the so-called internal corrosion occurring in a cement paste/aggregate interfacial transition zone (ITZ) in concrete, where the cement paste with thicknesses of a few microns comes into interaction with the aggregate. The ITZ is considered as the strength-limiting phase in concrete.

2. Materials and testing techniques

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

28 days in normal conditions.

cement compositions were chosen:

Used as aggregates were:

• Crushed basalt rock

• Crushed perlite rock

• Expanded perlite

• Portland cement (OPC) + water

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

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

To study properties of the concrete made using real aggregates, the following

• Alkali-activated slag cement (GBFS + sodium metasilicate (Na2OSiO25H2O))

curing at t = 90 2°С for 8 h. The ITZ was studied at an age of 28 days.

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

• Alkali-activated slag cement (GBFS + sodium carbonate Na2CO3)

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

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.

/kg.

/kg

• Sodium metasilicate (Na2OSiO25H2O) with <sup>ρ</sup> = 1.25 g/cm<sup>3</sup>

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

• Sodium carbonate (Na2СО3) with ρ = 1.18 g/cm<sup>3</sup>

• Alkali-activated Portland cement (OPC + soluble silicate)

• Alkali-activated slag cement (GBFS + soluble silicate)

• Glassy waste product from basalt fiber production

• Soluble silicate Мs = 2.9. ρ = 1.3 g/cm3

and that of the metakaolin, 1800 m<sup>2</sup>

2.1 Test methods

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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 service [9–16].

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 alkalisusceptible aggregates.

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, mineral composition of aggregates, etc.

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 alkalinealkali-earth aluminosilicate hydrates, analogs to natural zeolites (CaO) 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 different aggregates on their properties.

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