Multi-Technique Characterization of a Fine Fraction of CDW and Assessment of Reactivity in a CDW/Lime System

Abstract

<jats:p>This study analysed the fine particle (&lt;5 mm) waste generated during siliceous or calcareous (depending on the composition of the original aggregate) concrete waste crushing. In the absence of industrial applications, such waste is amassed in open-air stockpiles on construction and demolition wastes (CDW) management plant grounds. The aim pursued was to find an outlet for that material in the cement industry. The starting waste, sourced from six Spanish management facilities, was characterised for its chemical and mineralogical composition, physical properties and pozzolanicity. The mineralogical phases in the CDW/lime system and their variations during the pozzolanic reaction were likewise identified. The findings showed that the fine waste consisted primarily in quartz, calcite, micas and feldspars, with smaller fractions of kaolinite and cement anhydrous phases. No portland cement hydration phases were identified. All six types analysed exhibited medium to low pozzolanicity, with the highest values recorded for the siliceous waste. Ettringite, C–S–H gels and calcium aluminate hydrates (C4AH13, C4AcH12) were identified during the pozzolanic reaction in CDW/lime system. Therefore, this type of waste can be reused as supplementary cementitious material with low-medium pozzolanic activity.</jats:p>

Behaviour and Properties of Eco-Cement Pastes Elaborated with Recycled Concrete Powder from Construction and Demolition Wastes

“…The remaining signals observed in the diffractograms are analogous for the three types of pastes. Based on them, the crystalline phases portlandite, ettringite and monocarbolauminate could be identified as hydration products, which is in agreement with the results obtained in the study of the pozzolanic reaction in a pozzolan/lime system in previous works [ 22 ]. In addition, traces of anhydrous phases from cement can be detected at 12.2° (C 4 AF) and 31.8–33.0° (C 2 S/C 3 S).…”

“…On the other hand, the 27 Al NMR-MAS spectra in both samples showed the presence of aluminium in tetrahedral and octahedral coordination geometries, as indicated by the signals found around −50 and 0 ppm, respectively ( Figure 2 ). While the amount of Al(VI) was similar in both samples, Al(IV) is more abundant in HsT, which is in agreement with the larger proportion of Al 2 O 3 detected by XRF in this waste [ 22 ].…”

“…The cause behind the moderate reactivity of HsT is that even though the waste is rich in silica, most of it is found as quartz in the waste, which is considered mostly inert for the timeframes contemplated in this study. Previous works identified small amounts of C-S-H gels and ettringite as reaction products, as well as hydrated calcium aluminates in the case of HcG waste [ 22 ].…”

“…According to the 27 Al NMR MAS ( Figure 7 ), all the pastes show signals reflecting the presence of aluminium in both tetrahedral (IV) and octahedral (VI) coordination. The Al (IV) signals can be attributed to aluminium incorporation into C-S-H gels, with the exception of the peak observed at 56 ppm in HsT pastes, which can be assigned to non-reactive tecto- or phyllosilicates from the waste [ 22 ]. On the other hand, two overlapping signals, centred at 13 and 9.3 ppm, can be distinguished in the Al (VI) region of the spectra, the former being more intense.…”

“…Recent research [ 22 , 23 , 24 , 25 ] has explored the viability of recycling the fine C&DW fraction as a supplementary cementitious material (SCM), which arises from the potential cementitious reactivity concealed in the cement paste profusely found in this fraction of C&DW. This provides an industrial outlet for this kind of waste and the consequent decrease in the exploitation of natural resources.…”