The engineering properties of alkali activated materials (AAMs) mainly depend on the constituent materials and their mixture proportions. Despite many studies on the characterization of AAMs, guidelines for mixture design of AAMs and their applications in engineering practice are not available. Extensive experimental studies are still necessary for the investigation of the role of different constituents on the properties of AAMs. This paper focuses on the development of alkali-activated fly ash (FA) and ground granulated blast furnace slag (GBFS) paste mixtures in order to determine their suitability for making concretes. In particular, the influence of the GBFS/FA ratio and liquid-to-binder (l/b) ratio on the slump, setting, strength, and autogenous shrinkage of the alkali activated pastes is studied.It is shown that fresh properties largely depend on the type of precursor (GBFS or FA). The slump and setting time of GBFS-rich pastes was significantly reduced. These pastes also have higher compressive strength than FA-rich pastes. The study identifies important practical challenges for application of the studied mixtures, such as the behavior of their flexural strength and high amplitudes of autogenous shrinkage of GBFS-rich mixtures. Finally, the optimum GBFS/FA ratio for their future use in concretes is recommended.

In this paper, carbonation resistance of alkali-activated slag (AAS) pastes exposed to natural and accelerated conditions up to 1 year was investigated. Two aspects of carbonation mechanism were evaluated. The first was the potential carbonation of the main binding phases in finely powdered AAS pastes. The second was the reactivity and diffusivity of CO₂ within the bulk AAS paste. From Fourier transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectroscopy time-series measurements, it was found that powdered AAS was largely carbonated within 28 days with a CO₂ uptake of 14 wt%. The main carbonation products were calcium carbonates. Nevertheless, the bulk paste samples were highly resistant to carbonation, regardless of the exposure conditions. The findings showed that the pH value (initial pH > 12) and strength of the samples did not decrease under accelerated carbonation compared to those of the samples exposed under natural conditions. The mineralogy of the samples in these two carbonation exposures did not alter either, except for outdoor conditions. The gel pores were dominant in the pastes (pore size in range of 2–15 nm). The dense microstructure was the main barrier for CO₂ to diffuse and further react with binding phases.

The current understanding of the carbonation and the prediction of the carbonation rate of alkali-activated concretes is complicated inter alia by the wide range of binder chemistries used and testing conditions adopted. To overcome some of the limitations of individual studies and to identify general correlations between mix design parameters and carbonation resistance, the RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ Working Group 6 compiled and analysed carbonation data for alkali-activated concretes and mortars from the literature. For comparison purposes, data for blended Portland cement-based concretes with a high percentage of SCMs (≥ 66% of the binder) were also included in the database. The analysis indicates that water/CaO ratio and water/binder ratio exert an influence on the carbonation resistance of alkali-activated concretes; however, these parameters are not good indicators of the carbonation resistance when considered individually. A better indicator of the carbonation resistance of alkali-activated concretes under conditions approximating natural carbonation appears to be their water/(CaO + MgO_eq + Na₂O_eq + K₂O_eq) ratio, where the subscript ‘eq’ indicates an equivalent amount based on molar masses. Nevertheless, this ratio can serve as approximate indicator at best, as other parameters also affect the carbonation resistance of alkali-activated concretes. In addition, the analysis of the database points to peculiarities of accelerated tests using elevated CO₂ concentrations for low-Ca alkali-activated concretes, indicating that even at the relatively modest concentration of 1% CO₂, accelerated testing may lead to inaccurate predictions of the carbonation resistance under natural exposure conditions.

Understanding the role of curing conditions on the microstructure and phase chemistry of alkali-activated materials (AAMs) is essential for the evaluation of the long-term performance as well as the optimization of the processing methods for achieving more durable AAMs-based concretes. However, this information cannot be obtained with the common material characterization techniques as they often deliver limited information on the chemical domains and proportions of reaction products. This paper presents the use of PhAse Recognition and Characterization (PARC) software to overcome this obstacle for the first time. A single precursor (ground granulated blast-furnace slag (GBFS)) and a binary precursor (50% GBFS–50% fly ash) alkali-activated paste are investigated. The pastes are prepared and then cured in sealed and unsealed conditions for up to one year. The development of the microstructure and phase chemistry are investigated with PARC, and the obtained results are compared with independent bulk analytical techniques X-ray Powder Fluorescence and X-ray Powder Diffraction. PARC allowed the determination of the type of reaction products and GBFS and FA’s spatial distribution and degree of reaction at different curing ages and conditions. The results showed that the pastes react at different rates with the dominant reaction products of Mg-rich gel around GBFS particles, i.e., Ca-Mg-Na-Al-Si, and with Ca-Na-Al-Si gel, in the bulk paste. The microstructure evolution was significantly affected in the unsealed curing conditions due to the Na⁺ loss. The effect of the curing conditions was more pronounced in the binary system.

The utilization of locally available concrete waste for producing recycled concrete aggregates is recognized as one of the most sustainable ways of satisfying the growing demand for concrete production. However, the quality of concrete waste depends on its origin and it may significantly differ from one concrete structure to another. Knowing the chemical composition of the parent concrete is crucial for determining or verifying the origin of the raw materials. For this reason, pre-demolition concrete waste streams need to be characterized and classified. Therefore, a new non-destructive method for determining the cement and aggregate type in hardened concrete using handheld X-ray fluorescence (hXRF) analyser is presented in this paper. The method was tested on different raw powders and on concretes containing different types of cements including CEM I 42.5 N (Portland cement), CEM II/B-V 42.5 N (Portland-fly ash cement), CEM III/B 42.5 N (GGBFS cement). Combined desktop XRF and Energy-dispersive X-ray Spectroscopy (EDS) measurements were used for the purpose of validation. The results revealed that the curing of concrete affects the results: a dried concrete surface condition was optimal for measurements since it limits the impact of the concrete surface moisture and efflorescence on characteristic element oxides, such as CaO. The effective measurement duration was 30 s. A CEM III/B 42.5 N (GGBFS)-based concrete surface was distinguished from other concretes using Al₂O₃, MgO and Fe₂O₃as characteristic oxides. The inner layers of concrete were rich in SiO₂, the oxide characteristic for the aggregate composition tested in this study. This shows that hXRF is suitable for use in concrete, provided that the concrete surface is dried and the characteristic elements are defined to ensure a distinction between different cement and aggregate types. Direct adoption of such characterization, however, requires field testing across a wide range of concrete compositions and in situ conditions.