No abstract

Concretes containing mixed recycled aggregate (RA) have a larger number of coarse aggregate/paste interfacial transition zones (ITZs) than conventional concretes, due to the various component materials present in recycled aggregate. This study investigated the properties of various RA/paste ITZs in concrete using nanoindentation and scanning electron microscopy (SEM) and analysed the possible impact of the properties of the ITZs on the macromechanical performance of recycled concrete. It was found that the elastic modulus of the ITZ varies with the type of constituent materials present in recycled aggregate, with ITZs associated with organic components (e.g. wood, plastic and asphalt) exhibiting lower minimum elastic modulus values. The impact of ITZ properties on macro-mechanical properties of concrete depends on the relative content of different constituent materials present in the recycled aggregate and the micro-mechanical properties of the ITZs involved.

Depending on its composition and properties, construction and demolition waste (C&DW) may be used today as recycled aggregate to manufacture more eco‐efficient concrete, for drainage or as a sub‐base in roads and on occasion as a decorative or esthetic element in pedestrian pathways in parks and landscaped grounds. In Spain, 54% of C&DW is ceramic‐based (CB‐C&DW). Since the use of such waste as recycled aggregate is not envisaged in Spanish legislation, it is presently stockpiled in landfills, an environmentally, technically, and economically detrimental procedure. The CB‐C&DW recycled at 12 Spanish waste management plants was assessed to determine the feasibility of its use as an alternative to pozzolans such as silica fume and fly ash presently added to cement during manufacture. The proportion of ceramic‐based material contained in this recycled waste varied from plant to plant. The effect of the ceramic‐based material content on the chemical and mineralogical composition, morphology, and pozzolanic activity of CB‐C&DW was explored in a more exhaustive study of two types of waste, one with 20 and the other with 100% ceramic‐based material content. In light of its chemical and mineralogical composition, morphology and lime fixation capacity, this type of C&DW was found to be apt for use as a pozzolan, and hence as a valid alternative for manufacturing more eco‐efficient cements.

Imperative to the design of new cements that bear different types of waste as additions is a parallel study of the mechanical strength and durability of the new materials to ensure their performance will be satisfactory throughout their service life. This study explored the effect of adding 10 % or 20 % granite quarry dust to cement on properties such as transport (total and capillary water absorption and electrical resistivity), dimensional stability (drying shrinkage and expansion), the alkali-silica reaction, heat of hydration and colour. No alkali-silica reaction was observed in the new materials and expansion and contraction were less intense than in conventional cement. The water absorption and capillary absorption coefficients rose less in the additioned cements than the replacement ratio, whilst their higher resistivity values afforded greater corrosion protection than found in the reference. The inclusion of this waste also prompted a rise in lightness and a decline in peak heat of hydration. The multivariate analysis of variance (MANOVA) conducted showed that the factors time and replacement ratio affected the properties significantly, whereas the combined effect of the two was statistically significant or otherwise depending on the property analysed. The findings showed that the partial replacement of cement with quarry dust is not detrimental to product durability and the recycled material qualifies as a strength class 42.5, type II/A. Adittionally, the binders bearing 20 % granite quarry dust meet the requirements to qualify as low heat cements (CEM II/A LH).

The phenomena involved in portland cement hydration and interactions with nanosilica are very complex and not yet fully understood. In addition, few papers have currently proposed to investigate the microstructure and mechanical properties of ternary mixtures using portland cement, colloidal nanosilica, and highly reactive mineral additions. This article investigates, for the first time, the behavior of different highly reactive mineral additions (silica fume and metakaolin) when partially replaced by colloidal nanosilica in the microstructure and hydration of cementitious materials. For the study of the cementitious material microstructures, a Langavant calorimeter, compressive strength, Xray diffraction, thermogravimetry, infrared spectroscopy, and mercury intrusion porosimetry were used. The pastes with a 1% substitution of highly reactive mineral additions by nanosilica showed higher compressive strength and more refined porosity than the pastes with only silica fume or metakaolin. The results show that nanosilica appears to have better synergism with metakaolin than with silica fume.