Search

Highlights: [•] The concrete industry is responsible for about 10% of the greenhouse gas emissions. [•] The production of concrete with recycled constituents can reduce such an impact. [•] The influence of the aggregate replacement on the hydration process is investigated. [•] The relationship between concrete strength and hydration degree is shown. [•] The influence of initial moisture conditions of aggregates is particularly addressed. [Copyright &y& Elsevier]

Geopolymers offer a potential alternative to ordinary Portland cement owing to their performance in mechanical and thermal properties, as well as environmental benefits stemming from a reduced carbon footprint. This paper endeavors to build upon prior atomistic computational work delving deeper into the intricate relationship between pH levels and the resulting material's properties, including pore size distribution, geopolymer nucleate cluster dimensions, total system energy, and monomer poly-condensation behavior. Coarse-grained Monte Carlo (CGMC) simulation inputs include tetrahedral geometry and binding energy parameters derived from DFT simulations for aluminate and silicate monomers. Elevated pH values may can alter reactivity and phase stability, or, in the structural concrete application, may passivate the embedded steel reinforcement. Thus, we examine the effects of pH values set at 11, 12, and 13 (based on silicate speciation chemistry), investigating their respective contributions to the nucleation of geopolymers. To simulate a larger system to obtain representative results, we propose the numerical implementation of an Octree cell. Finally, we further digitize the resulting expanded structure to ascertain pore size distribution, facilitating a comparative analysis. The novelty of this study is underscored by its expansion in both system size, more accurate monomer representation, and pH range when compared to previous CGMC simulation approaches. The results unveil a discernible correlation between the number of clusters and pores under specific pH levels. This links geopolymerization mechanisms under varying pH conditions to the resulting chemical properties and final structural state. [ABSTRACT FROM AUTHOR]

This paper proposes a conceptual formulation for predicting and controlling the compressive strength of Recycled Aggregate Concrete (RAC) mixtures, by explicitly taking into account the specific features of Recycled Concrete Aggregates (RCAs). In fact, since RCAs are significantly more porous than Natural Aggregates (NAs), the mix design rules commonly employed for ordinary structural concrete cannot be applied as such for RACs. Therefore, the formulation proposed herein is intended at generalising the aforementioned rules with the aim to take into account the higher porosity of RCAs. Although being a mainly conceptual methodology, the proposed formulation is supported by a wide set of experimental results: they unveil the influence of several aspects and parameters (such as source and processing procedures of RCAs, aggregate replacement ratio, water-to-cement ratio, water absorption capacity and initial moisture condition of aggregates) on the resulting compressive strength of RAC. Finally, the proposed mix design methodology demonstrates that the resulting compressive strength of RACs can be predicted by taking into account only one parameter (i.e., water absorption capacity) identifying the “quality” of RCAs. Further generalisations intended at controlling other physical and mechanical parameters of RAC are among the future development of this research. [ABSTRACT FROM AUTHOR]

As sustainability is becoming a fundamental requirement for all modern industrial activities, upcoming environmental research efforts should be intended at both enhancing the efficiency in the use of raw materials and reducing the carbon footprint. The present research activity has been launched at the Federal University of Rio de Janeiro (UFRJ, Brazil) for demonstrating the feasibility of “ecological concrete” for structural use, characterised by a significant replacement of natural aggregates with recycled ones, which are produced from processing Construction Demolition Waste (CDW). Particularly, this work explores alternative processing procedures and investigates their influence on the relevant physical and mechanical properties of the resulting aggregates and concrete mixtures. Therefore, particle size distribution, bulk density attached mortar content as well as the associated water absorption capacity of recycled aggregates were monitored to scrutinise the effect of such processing procedures. Subsequently, a series of concrete batches was produced for evaluating the influence that alternative processing procedures for recycled aggregates have on the main mechanical performance in a fresh and hardened state. Results reported in this paper show the feasibility of “autogenous cleaning”, which removes surface impurities and reduces particle heterogeneities, usually characterising the morphology of a recycled aggregate matrix. It shows that the considered cleaning procedure significantly reduces the gap between the performance of recycled aggregate concrete (RAC) and ordinary ones, both in terms of workability at the fresh state and strength at the hardened state of concrete mixtures. [ABSTRACT FROM AUTHOR]

This paper proposes a mesoscale approach for simulating moisture transport by capillary action in partly-saturated porous cementitious composites. The modeling approach explicitly accounts for moisture transport through a mesostructure composed of coarse aggregates, surrounding cementitious mortar and interfaces. These latter, namely interface transition zones (ITZs), allow to describe the interaction between aggregates and mortar, and may cause an alternative path for the internal moisture movements. Basic morphology effects of the ITZs are simulated using a ribbon approach. Random spatial distribution of cement particles are stacked in the meso-geometry. Aggregate particles are introduced as randomly perturbed polygons and the moisture transport is modeled as a diffusion problem and solved by means of the finite element method. The proposed constitutive models are based on a proper description of the permeability and pore size distribution which strongly affect the local moisture content. Numerical results at both macro- and mesoscale levels demonstrate the soundness and capability of the proposed approach. The integrated modeling results actually demonstrate the potential of the mesoscale approach and shows the role of the ITZs as an internal interconnected network. [ABSTRACT FROM AUTHOR]