Abstract Alkali-activated Slag Research Articles

The analysis of cracking risk by shrinkage restraint of an alkali-activated slag mortar

Alkali-activated slag (AAS) binders show in general larger autogeneous shrinkage strains than ordinary Portland cement (OPC) based binders. However, AAS can be a relevant alternative to OPC, if, for example low hydration heat release and fine pores, are required. This study proposes an evaluation of the advantage of using AAS materials in small-sized or massive structures with regard to cracking risk by autogeneous shrinkage and thermal strains. A cracking risk index is calculated; this risk is defined as the ratio between stress generated by full restraint and tensile strength. All required experimental data were investigated in an OPC and AAS mortar, these are: heat release, autogeneous shrinkage, Young’s modulus, tensile strength and basic creep evolutions. The material parameters of a rate-dependent model developed in 1D were then identified. Numerical simulations were then performed for different thicknesses in full-restraint conditions. These show that, as expected, basic creep is a very important material parameter to assess. Indeed, basic creep enables the significant reduction of the generated stresses. Besides, it is found that the more the structure is large (and sensitive to cracking by risk by thermal strain), the more the AAS material is becoming appropriate compared to the OPC material.

Differences in Electrical Properties of Portland Cement and Alkali-Activated Slag Mortars

Alkali-activated slag is known as a building material for more than sixty years and is considered an alternative to Portland cement based binders. Compared to Portland cement it exhibits some superior properties such as higher resistance against chemical attack and exposure to elevated temperatures. Aluminosilicate binders are generally electrical insulators; however, electrical properties of building materials gain the importance in the new field of applications such as self-sensing or self-heating materials. This paper brings a comparison of the electrical properties, especially resistance and capacitance, between Portland cement and alkali-activated slag mortars. The measurements revealed that alkali-activated slag shows enhanced conducting properties due to the presence of mobile hydrated sodium ions and metallic iron microparticles.

Electrical Properties of Steel Fibre Reinforced Alkali-Activated Slag Composite

Alkali-activated slag is an alternative binder to the ordinary Portland cement. In order to improve its tensile properties steel fibres as dispersed reinforcement can be used. Since steel is very good conductor it changes the electrical properties of alkali-activated slag composite that can have a potential to be used as self-sensing material then. In this study up to 20% of steel fibres by mass of the slag was added to alkali-activated slag mortar and the mechanical properties, electrical resistance, capacitance and microstructure of the composites were investigated. The results showed that the best improvement of both the mechanical and electrical properties can be observed for the composite with 15% of steel fibres.

Multiple Phase Identification in Alkali Activated Slag by SEM-EDS

Alkali-activated slag is studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray microanalysis. Attention is focused on delineating the phases induced by the alkali activation, as these phases are important in determining the mechanical properties of the material. The starting material, slag, is found to be a heterogeneous material with at least two phases. Upon alkali activation the material becomes more heterogeneous, now exhibiting at least four phases with significant different chemical composition. Furthermore, the alkali activation is found to modify the phase rich in Ca in the unactivated slag more than the other. Alkali activation of the slag produced mostly an amorphous material with some crystalline phases such as hydrotalcite and calcite, also some nanocrystalline structures were detected by TEM.

The Influence of Activator Composition on the Strength, Shrinkage and Chloride Migration Resistance of Alkali-Activated Slag Mortars

Alkali-activated slag materials are known for their high chloride penetration resistance. This makes them potentially applicable as repair systems for damaged steel-reinforced concrete structures, which are exposed to chloride attack. For this purpose, the influence of the activator composition, i.e. the SiO2and Na2O concentration of the alkaline solution on a) the compressive strength, b) shrinkage and mass change and c) the resistance against chloride penetration of four alkali-activated slag mortars (AASM) were studied. An ordinary Portland cement-based mortar was used as the reference sample. Increasing SiO2and Na2O concentrations increased the strength, shrinkage and mass loss of the AASMs. The resistance of the mortars against chloride penetration was evaluated using the non-steady-state migration coefficient Dnssmobtained from NT BUILD 492. The results indicate that the Dnssmis related to differences in the pore solution of the AASMs rather than to differences in their microstructure. An upcoming study of the authors is going to evaluate this hypothesis by the accelerated chloride penetration (diffusion) test.

Effect of Curing Regime on the Properties of Alkali-Activated Slag/Limestone Mortar Subjected to High Temperatures

Alkali-activated slag has a higher resistance against exposure to high temperature compared to ordinary Portland cement concrete. Nevertheless, its mechanical properties are also deteriorated due to partial decomposition of the C-A-S-H matrix. This paper investigates the effect of various curing regimes applied to the samples which were exposed to temperature 200–1200 °C on the mechanical properties and microstructure of the alkali-activated slag/limestone blends. The amount of limestone was 15% from the mass of slag. On set of specimens was treated on air at laboratory conditions, the second set was immersed in water and the last was treated stored in humid chamber with 95% relative humidity for the period of 60 days. The results showed that water curing has a strong positive influence on the compressive as well as flexural strength of the specimens exposed to temperatures up to 600 °C. Specimens which were exposed to 800 and 1000 °C showed the best performance after treatment in humid chamber, and after exposure to 1200 °C the best compressive strength exhibited the air cured samples.

Influence of Chemical Admixtures on Properties of Alkali-Activated Slag-Based Mortars

Alkali-activated slag (AAS) based binders can have similar or even better properties in terms of mechanical performance, durability and chemical resistance than ordinary Portland cement (PC), but AAS binders also show some drawbacks, especially high shrinkage and poor rheology. Improvement of these features in the case of PC binder is well established, but a lot of conventional admixtures like superplasticizers do not work in AAS. The influence of shrinkage reducing admixture (SRA) and airentraining admixture (AEA) on mechanical properties, workability and shrinkage was investigated in this study. These admixtures were used in the different dosages in the range of 0–2.0% (by mass of slag). With increasing content of both admixtures compressive and flexural strength decreased. Similar effect was observed for workability. On the other hand, promising results in respect of shrinkage reduction were obtained particularly for SRA, where its 1.0% dose reduced drying shrinkage of the reference mortar by more than 70%. Only slight shrinkage reduction was observed for 0.50% of AEA.

Effect of Polymer Admixtures on Mechanical Properties of Alkali-Activated Slag Mortars

Alkali-activated slag (AAS) binders have lower environmental impact due to its production process, but also have disadvantages as an increased shrinkage followed by formation of microcracks. The effect of polymer admixtures based on vinyl acetate, ethylene and acrylic acid ester, methyl metacrylate and different types of polyethylene glycol (PEG) and polypropylene glycol (PPG) on properties of alkali-activated slag concrete was studied. Admixtures used for mortars were tested to improve shrinkage, workability and compressive strength, flexural strength. The analysis also showed the effect of the admixtures on microstructure of the alkali-activated slag pastes and mortars.

Effect of Carbon Nanotubes and Hydroxypropyl Methylcellulose on the Mechanical Properties and Cracking Tendency of Alkali-Activated Slag

Alkali-activated slag (AAS) is a material which has great potential for use in building industry. However, its major disadvantage is an increased shrinkage followed by formation of microcracks. The aim of this work was to investigate the influence of carbon nanotubes and hydroxypropyl methylcellulose on the mechanical properties and cracking tendency of alkali-activated slag material. The results showed that both admixtures have a positive effect on compressive as well as flexural strength. Impact-echo measurement revealed that both admixtures reduce cracking tendency and can retain water in the binder.

Effect of Water-Soluble Carbon Nanotubes on the Mechanical Properties and early Hydration of Alkali-Activated Slag

Alkali-activated slag (AAS) is a material which has great potential for use in building industry. The aim of this work was to gain new superior properties by the addition of carbon nanotubes (CNTs). This material can act as a microreinforcement improving mechanical properties of cementitious materials. The effect of 0–1 wt.% addition of CNTs on the mechanical properties, hydration characteristics and microstructure of AAS binder was determined. The addition of CNTs delays the setting of the binder and a partial deterioration of strength parameters was observed.

Effect of Spontaneous Drying on Mechanical Parameters of Alkali-Activated Slag Mortar

Alkali-activated slag is a material which has great potential for use in industrial applications. The main drawback of this material is the high level of autogenous and especially drying shrinkage that affects it, causing the deterioration of its mechanical properties. Mechanical and mechanical fracture parameter values show that an alkali-activated slag composite which is saturated with water is more brittle and exhibits lower strength in comparison with the same material after being surface-dried. These results are essential for the adjustment of testing conditions in order to obtain accurate and reproducible results.