Effect of limestone dosages on some properties of geopolymer from thermally activated halloysite

The effect of 0-60% limestone powder in thermally activated halloysite clay (TAH) based geopolymer pastes cured at 24 +/- 2 degrees C was investigated. Metakaolin source using halloysite which is an available mineral can be an alternative to fired kaolinite. The alkaline solutions used were mixtures (1:1 vol ratio) of sodium silicate and sodium hydroxide (NaOH) solution at various molarities (5 M, 8 M and 10 M). Fresh geopolymer pastes were tested for initial setting time. Hardened pastes were tested for linear shrinkage, water absorption, apparent density, compressive strength, X-Ray Diffraction (XRD), Fourier Transformed Infrared spectroscopy (FTIR) and optical microscopy. Limestone acted as setting retarder in geopolymer but, the increase in molarity of the sodium hydroxide solution accelerated setting performances of pastes. Shrinkage of hardened pastes increased with the increase of limestone addition but it was less than 1% in all samples. For up to 45% limestone powder in pastes, water absorption of test specimens decreased when the percentage of limestone increased while the density of pastes increased with the increased amount of limestone in pastes. Up to 45% of TAH in pastes can be substituted by limestone powder with a positive contribution to the improvement of the strength of hardened products, especially when 8 M or 10 M NaOH solution is used. Beneficial effect was also observed in the rapid development of strength at early age, the formation of reaction gels, the densification and the better packing of the particles in the matrixes of final products. (C) 2019 Elsevier Ltd. All rights reserved.