The effect of gel-type contributions in lime-sand bricks, alkali-activated slags and CEMI/CEMIII pastes: Implications for next generation concretes

Lime-sand bricks of different ages were investigated using IR-spectroscopy, thermogravimetry, and X-ray diffraction/scattering. After subtraction of the dominant quartz contribution (80%), the IR spectra show the absorption peaks of the hydrothermally formed binder phases. The spectra also show the alteration of the binder during ageing under atmospheric conditions by the influence of CO2 forming carbonate and a condensed SiO2-gel (secondary gel). The alteration could also be proven in X-ray pattern, obtaining a separation between crystalline CSH and amorphous contributions in the freshly produced lime-sand brick, too. Here, the formation of CSHamorph could be understood as a precursor state (primary gel) to the crystallization of CSH phases. X-ray patterns of aged bodies of alkali-silicate solution activated slags (AAS), CEM-I/CEM-III pastes, and CEM-I concrete indicate that in all cases a similar amorphous CSH-type phase (CSHamorph) was formed, which is responsible for the hardening properties as the glue. The main X-ray peak of CSHamorph obtained using CuKα-radiation with a usual diffractometer is observed between 24◦ and 35◦ 2 Theta with maximum at about 29◦ 2 Theta, whereas it appears much more broadly distributed between 15◦ and 35◦ 2 Theta with maximum between 26◦ and 28◦ 2 Theta for a geopolymer body prepared using the reaction of alkali-silicate solution and metakaolin (AAMK). This is due to the network formed by siloxo and sialate units in the case of AAMK, given that any crystallization can be ruled out. The origin of increasing mechanical strength during the ageing of AAS mortars must be due to further crosslinking of the preformed siloxo chains. Thermal treatment up to 800◦C leads to a complete loss of any mechanical strength of the CEM pastes due to the destruction of crystalline CSH-phases, whereas geopolymer bodies maintain their strength. Implications for next generation concrete include that cement clinker could be completely replaced by using a using alkali silcate solution technology for gel formation. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Particles size and distribution on the improvement of the mechanical performance of high strength solid solution based inorganic polymer composites: A microstructural approach

This research reports on the influence of particle size and distribution on the physical, mechanical and microstructural features of solid solutions (feldspathic materials) based inorganic polymer composites (IPCs). Both granite and pegmatite were ground to different degree of finess making four different granulometry with particles of 63, 80, 125 and 200 μm. The respective mixes receive 15 wt% of metakaolin and were activated with a well designed alkaline solution. Matrices obtained showed high compressive and flexural strengths in the range 101.2–131.3 MPa, and 29–35.5 MPa, respectively. It was observed that the optimum mechanical performance of these matrices can be achieved through a mix-design of different grades of granulometry. This was suggested by mechanism combining reactivity and particles packing. In fact, although it can be expected that the finess of the combination of the particles size under 63 μm might present the better reactivity, it is showing that the combination of fine, medium and coarse particles is efficient in achieving denser and tougher microstructure. Lower cumulative pore volume (17 mL g−1) of the composites based on pegmatite, value not far from that of natural stones, resulted in a higher impact resistance of 3.03 J. It was concluded that designing the feldspathic rock-based composites with high strengths appear as sustainable, low energy consumption and environmentally-friendly materials for the structural construction