Financing entity: Programa i-coop 2020: Proyecto COOPA20454
Duration: 2021 - 2022
Principal investigator(s): Mª Teresa Blanco Varela e Inés García Lodeiro
Budget
Official web

Abstract

Portland cement (OPC) manufacture is an energetic and raw material consuming intensive process that contributes to air pollution, (CO₂, SO₂, NOx), accounting for 7 % of anthropogenic CO2 (Nidheesh and Suresh Kumar, 2019), (0.87t CO2/t clinker) (Hanein et al. 2018).

The use of alkali activation technology not only allows developing new Eco-Cementitious material with low CO₂ emissions, but also contemplates the valorization of by-products, avoiding the overexploitation of the natural resources. Alkali cements usually consist of two main components: a precursor (secondary by-products, such as Coal-Fly Ash (FA) or Blast furnace slags (BFS), but also raw materials such as aluminosilicates clays (C) and feldspars (F) and an alkaline salt as activator. The combination of these materials allows developing compact cementitious binders with excellent engineering properties.

This project proposes the use of yellow clays as raw material (precursor) to produce a new alkali activated cement, with a much more lower CO₂ footprint that ordinary Portland cement (OPC). Yellow clays comes from Moroccan phosphate mines, as a waste rock lithology and are stored in large dumps within the mine site. Previous studies from the Moroccan group in alkali activation, using yellow clays (a by-product from phosphate mines ) as a precursor, reveal that are mainly composed by montmorillonite, dolomite and quartz (as major phases). The thermal activation of the yellow clay (mandatory to increase its reactivity) allow the amorphization of the the montmorillonite structure, and new phases such as periclase (MgO) and gehlenite (Ca₂Al₂SiO₇) are formed. The toxicity characteristic leaching procedure (TCLP) showed no existence of contaminant but its natural radioactivity has not been determined yet.

The suitability of the yellow clay to be alkali activated has been demonstrated, furthermore, the 27Al – 29Si MAS-NMR and the XRD analysis of the activated materials confirmed the formation of two cementitious gels; C-A-S-H gel (a Calcium aluminosilicate hydrate) and N-A-S-H gel (Sodium aluminosilicate hydrate) ( Mabroum et al 2020). However the formation of periclase (MgO) (due to the calcination process) can produce some mechanical and durability issues. MgO is a hydraulic phase that can induce expansion problems when is in contact with water, after the setting and hardening of the concretes. By other hand the mechanical strengths are considerably reduced with the increasing of the waterglass content (activator). For a proper development of new cements, using the yelow clay (YC) as a precursor, additional studies are required in order to i) to optimize the activation conditions, ii) to determine the natural radioactivity (naturally occurring radioactive materials (NORM)) and iii) to improve the mechanical strengths and the durability.

This is collaborative research between the Moroccan and Spanish teams and it will allow the training of researchers including youth enrolled in doctoral programs in the University of Cadi Ayyad (UCA) ( Marrakech ) under a collaborative management between the two teams (CSIC and UCA).

Key words: Geopolymer; alkali activation; waste valorization; yellow clays; phosphate sludges