Low-grade kaolinitic clays as SCMs for low-carbon cements and strength improvement by C-S-H gel nucleation seeding

This study seeks to push the limits of LC³-50 binders by utilising low-grade kaolinite clays and C-S-H gel nucleation seeding. Four commercially-available kaolinite clays (i.e. kaolinite contents ~20 wt%) have been studied by X-ray fluorescence, X-ray powder diffraction and thermal analysis. The clays were thermally activated and milled to D_v,50 = 11 ± 2 μm. The activated clays showed low pozzolanic performances, as determined by ASTM C1897-20 standard. The heat flows ranged 200–230 J/g calcined clay and the bound waters 4.3–5.7 %. The LC³-50 pastes were characterised by calorimetry, X-ray powder diffraction and the Rietveld method, thermal analysis, and porosimetry to evaluate pozzolanic reactivity and the seeding effect.

LC³ mortars, w/b = 0.40, have relatively low compressive strengths, especially at one day, but these values were increased by 64 % on average at one day by seeding. At 1 day, three LC³ mortars showed 11 MPa which increased to 18–20 MPa by seeding. A fourth mortar showed a much higher compressive strength, 16 MPa, that increased to 23 MPa, by seeding. A tested concrete, w/b = 0.50, also showed a high increase of 37 % at 1 day. The improvements at 28 days were maintained but quantitatively lower, 12 and 14 % for the mortars and the concrete, respectively. Insights into the pozzolanic reactions and C-S-H seeding are obtained from the quantitative study of the pastes. For instance, it is proven the pozzolanic reaction at 1 day from the portlandite consumption. Also shown is the enhanced formation of carboaluminates when the strength enhancing admixture is added.