Phase formation behavior of alkali-activated calcined clays: effects of the reactive phase and NaOH concentration

The relevance of calcined clays as sources with a great potential for non-clinker alkali-activated materials has significantly increased in recent decades. This article reports on a study to investigate how NaOH with concentrations in the 5–14 M range affects the mechanical properties, amorphous-crystalline phase transformations, and reaction product composition of alkali-activated calcined kaolinite/montmorillonite cements with reactive phases of 60% and 33%, and with the SiO2/Al2O3 ratios of 1.54 and 1.74. The hardened pastes based on both calcined clays exhibited the highest compressive strength up to 9.1 MPa following their activation with 8 M NaOH. The higher amorphous fraction in the calcined clay provided gradual zeolitisation/crystallisation at Na2O/SiO2 and Na2O/Al2O3 molar ratios in the 0.36–0.63 and 0.56–0.98 ranges, respectively, accompanied by the deterioration of the mechanical characteristics of the samples. The zeolite content in the alkali-activated calcined clay cements with an amorphous fraction of 33% peaked for NaOH at 11 M, and the corresponding molar ratios of Na2O/SiO2 and Na2O/Al2O3 were 0.85 and 1.46, respectively. The main reaction products in the studied systems determined by XRD, TG/DSC, and FTIR spectroscopy analyses were N-A-S-H and zeolite A. The properties and reaction products obtained favor the use of proposed alkali-activated cements for controlled low-strength materials

Calcined low-grade clays as sources for zeolite containing material

The continuous development and expansion of a raw material base in response to increasing environmental and technical requirements for most consumable commodities are crucial for the sustainable development of resource-and energy-intensive materials and technologies. As the sources for alkali-activated cements and zeolites, recent studies have reported the suitability of applying calcined clays with various chemical compositions, percentages, and constituent clay minerals and secondary minerals. In this article, the results of a feasibility study on the alkali activation of low-grade clays with 7.26 % montmorillonite/chlorite and 23.14 % kaolinite/ montmorillonite minerals are reported. The 6M NaOH activation of clays thermally treated at 700 °C results in the formation of a hardened paste with a compressive strength of up to 5 MPa, with an N-(A)-S-H binder gel and zeolite Na2 Al2 Si2.5 O9 ∙6.2H2 O being the main reaction products

Calcined low-grade multimineral clays as supplementary cementitious materials: a feasibility study

© 2020, © 2020 The Korean Society of Mineral and Energy Resources Engineers (KSMER). The relevance of calcined clays as sources with great potential for low-clinker blended Portland cements has significantly increased in recent decades. This article reports on a study to evaluate the potential of the two low-grade clays containing 9.37% of montmorillonite/chlorite and 12.26% of montmorillonite/kaolinite/chlorite accompanied with 29.73% and 32.79% of calcite, respectively, as supplementary cementitious materials. The clays after thermal treatment at 800°C demonstrated poor pozzolanic activity, slight increase in the C-(S)-H and decline in the CH contents, and decrease in the mechanical properties of the blended Portland cement pastes. The results showed that the investigated clays fall below the range of the clays suitable for pozzolan production

Role of Clay Minerals Content and Calcite in Alkali Activation of Low-Grade Multimineral Clays

© 2020 American Society of Civil Engineers. The increased contribution of science-intensive clean materials and technologies to the total industrial production is one of the essential prerequisites for the future of sustainable industrial development. Chemical activation based on a nonfired or low-temperature approach for the production of cementitious materials from glassy aluminosilicates, including calcined clays, is an intensively developing and promising clean technology. This study investigated the potential of calcined clays as precursors of alkali-activated cements (AACs) by considering three types of low-grade multimineral clays (with a clay mineral content of <20%). The alkali activation of the calcined clays was analyzed based on the type and content of clay minerals, presence of calcite, calcination temperature, type of alkali activator, and curing conditions. The results indicated that clays containing 20% of 21 layer lattice clay minerals are not suitable as precursors of AACs, while those containing 9%-12% of 11 and/or 21 layer lattice minerals combined with 29%-32% calcite lead to hardened pastes displaying compressive strengths of up to 12.4 MPa after 2 days and 20 MPa after 28 days and after steam curing. The reaction products of 6M NaOH-activated systems determined by XRD, TG/DSC, and FTIR spectroscopy analyses were X-ray amorphous binder gel, calcite (CaCO3), clinozoisite (Ca2Al3(SiO4)(Si2O7)(O,OH)), and gaylussite (CaNa2(CO3)(H2O)5)