Effects of co-calcining kaolinite-rich clay blends with alkali and alkali earth metal hydroxides.

The aim of this paper is to investigate the effects of co-calcining kaolinite and montmorillonite clay blends with alkali and alkali earth metal (AAEM) hydroxides as a route to enhance the reactivity of the clay blends. NaOH, KOH, and Mg(OH) 2 were added in different dosages (0–20%) to the clay blend and calcined together at 750 °C. When NaOH and KOH added at a dosage <10%, this co-calcination promoted amorphization, increased non-bridging oxygen per tetrahedron (NBO/T), increased dissolution of Si, and therefore improved reactivity. At a higher dosage, these additives decreased reactivities. These clay blends were utilized as a precursor in a seawater-cured lime-pozzolana binder system. The evolution of the microstructures was monitored using XRD, FTIR, 29Si solid-state NMR, 27Al solid-state NMR, ICP-OES, and SEM-EDS for up to 90 days. The maximum compressive strength came from the 5% Mg(OH) 2 batch which was 22.1 MPa at 90 days. The 5% dosages of all the AAEM hydroxides showed better performance than that of the control batch. The mineralogy showed the presence of hydrocalumite, ettringite, gehlenite, hydrogarnet, and calcium aluminum silicate hydrate (C-A-S-H) in seawater cured paste samples. The SEM-EDS revealed the presence of plate and needle like C-A-S-H in the seawater cured samples. • Co-calcination with NaOH & KOH at 5% dosage increased the amorphous content. • Co-calcination with NaOH & KOH reduced polymerization of the aluminosilicate. • Co-calcination with Mg(OH) 2 did not alter the aluminosilicate network. • Co-calcination with NaOH & KOH changed Al coordinate in aluminosilicate network. • The maximum compressive strength was from the 5% Mg(OH) 2 sample. [ABSTRACT FROM AUTHOR]