Carbonation of Alkali-Activated Materials: A Review.

This paper presents a literature review on the effects of accelerated carbonation on alkali-activated materials. It attempts to provide a greater understanding of the influence of CO2 curing on the chemical and physical properties of various types of alkali-activated binders used in pastes, mortars, and concrete. Several aspects related to changes in chemistry and mineralogy have been carefully identified and discussed, including depth of CO2 interaction, sequestration, reactions with calcium-based phases (e.g., calcium hydroxide and calcium silicate hydrates and calcium aluminosilicate hydrates), as well as other aspects related to the chemical composition of alkali-activated materials. Emphasis has also been given to physical alterations such as volumetric changes, density, porosity, and other microstructural properties caused by induced carbonation. Moreover, this paper reviews the influence of the accelerated carbonation curing method on the strength development of alkali-activated materials, which has been awarded little attention considering its potential. This curing technique was found to contribute to the strength development mainly through decalcification of the Ca phases existing in the alkali-activated precursor, leading to the formation of CaCO3, which leads to microstructural densification. Interestingly, this curing method seems to have much to offer in terms of mechanical performance, making it an attractive curing solution that can compensate for the loss in performance caused by less efficient alkali-activated binders replacing Portland cement. Optimising the application of such CO2-based curing methods for each of the potential alkali-activated binders is recommended for future studies for maximum microstructural improvement, and thus mechanical enhancement, to make some of the "low-performing binders" adequate Portland cement substitutes.

Optimising the Performance of CO2-Cured Alkali-Activated Aluminosilicate Industrial By-Products as Precursors.

Three industrial aluminosilicate wastes were studied as precursors to produce alkali-activated concrete: (i) electric arc furnace slag, (ii) municipal solid waste incineration bottom ashes, and (iii) waste glass rejects. These were characterized via X-ray diffraction and fluorescence, laser particle size distribution, thermogravimetric, and Fourier-transform infrared analyses. Distinctive combinations of anhydrous sodium hydroxide and sodium silicate solution were tried by varying the Na2O/binder ratio (8%, 10%, 12%, 14%) and SiO2/Na2O ratio (0, 0.5, 1.0, 1.5) to find the optimum solution for maximized mechanical performance. Specimens were produced and subjected to a three-step curing process: (1) 24 h thermal curing (70 °C), (2) followed by 21 days of dry curing in a climatic chamber (~21 °C, 65% RH), and (3) ending with a 7-day carbonation curing stage (5 ± 0.2% CO2; 65 ± 10% RH). Compressive and flexural strength tests were performed, to ascertain the mix with the best mechanical performance. The precursors showed reasonable bonding capabilities, thus suggesting some reactivity when alkali-activated due to the presence of amorphous phases. Mixes with slag and glass showed compressive strengths of almost 40 MPa. Most mixes required a higher Na2O/binder ratio for maximized performance, even though, contrary to expectations, the opposite was observed for the SiO2/Na2O ratio.

Incorporation of Alkali-Activated Municipal Solid Waste Incinerator Bottom Ash in Mortar and Concrete: A Critical Review.

In the light of one of the most common waste management issues in urban areas, namely the elimination of municipal solid waste (MSW; about 486 kg of the waste per capita were generated in the EU in 2017), this study discusses one technique as an outlet in the construction industry for the by-product of the waste's incineration in energy recovery facilities (i.e., MSW incinerator bottom ash-MIBA). There have been some investigations on the use of MIBA as partial replacement of cement to be used in cementitious composites, such as concrete and mortars. However, the waste's incorporation ratio is limited since further products of hydration may not be produced after a given replacement level and can lead to an unsustainable decline in performance. In order to maximize the incorporation of MIBA, some research studies have been conducted on the alkali activation of the waste as precursor. Thus, this study presents an extensive literature review of the most relevant investigations on the matter to understand the material's applicability in construction. It analyses the performance of the alkali-activated MIBA as paste, mortar, and concrete from different perspectives. This literature review was made using search engines of several databases. In each database, the same search options were repeated using combinations of various representative keywords. Furthermore, several boundaries were made to find the most relevant studies for further inspection. The main findings of this review have shown that the chemical composition and reactivity of MIBA vary considerably, which may compromise performance comparison, standardization and commercialization. There are several factors that affect the performance of the material that need to be considered, e.g., type and content of precursor, alkaline activator, curing temperature and time, liquid to solid ratio, among others. MIBA-based alkali-activated materials (AAM) can be produced with a very wide range of compressive strength (0.3-160 MPa). The main factor affecting the performance of this precursor is the existence of metallic aluminum (Al), which leads to damaging expansive reactions and an increase in porosity due to hydrogen gas generation stemming from the reaction with the alkaline activator. Several approaches have been proposed to eliminate this issue. The most effective solution was found to be the removal of Al by means of eddy current electromagnetic separation.

Use of recycled aggregates from construction and demolition waste in geotechnical applications: A literature review.

The use of recycled aggregates (RA) in construction constitutes a significant step towards a more sustainable society and also creates a new market opportunity to be exploited. In recent years, several case-studies have emerged in which RA were used in Geotechnical applications, such as filling materials and in unbound pavement layers. This paper presents a review of the most important physical properties of different types of RA and their comparison with natural aggregates (NA), and how these properties affect their hydraulic and mechanical behaviour when compacted. Specifically, the effects of compaction on grading size distribution curves and density are analysed, as well as the consequences of particle crushing on the resilient modulus, CBR and permeability. The paper also contains an analysis of the influence of incorporating different RA types on the performance of unbound road pavement layers as compared with those built with NA by means of the International Roughness Index and deflection values. The results collected from the literature indicate that the performance of most RA is comparable to that of NA and can be used in unbound pavement layers or in other applications requiring compaction.