Gravel wash fines, a by-product of aggregate processing that contains mixed clay mineral phases, have demonstrated potential as an effective supplementary cementitious material upon calcination. Compositional analysis revealed that replacing >40% ordinary Portland cement (OPC) was hindered by a lack of portlandite (calcium hydroxide, CH), either due to cement dilution or being fully consumed by pozzolanic reactions, limiting the effectiveness of the calcined mixed clays as SCMs. Through the addition of CH at 0–8 wt% binder content, three calcined gravel wash fines (CGF), each having different dominant clay mineralogy, were shown to replace 40 wt% OPC in CGF-OPC blends with significantly increased calcium (aluminate) silicate hydrate and ettringite phase formation, as confirmed by infrared spectroscopy, thermogravimetry, electron microscopy and chemical analysis. Both kaolinitic/smectitic CGF and illitic CGF blends showed 28-day compressive strength improvements of up to 20% with 4 wt% CH added, while the strength of the smectitic CGF blend was enhanced by 42% at 28 days and 30% at 56 days with 8 wt% CH. Differences in strength development of the CGF-cement blends were attributed to differences in clay mineralogy: smectitic CGF A, a 2:1 clay, formed high-density C-S-H phases due to its higher silica availability, whereas CGF B (kaolinite 1:1–smectite 2:1) and CGF C (illite 2:1–kaolinite 1:1) incorporated both alumina and silica, leading to formation of low density C-(A)-S-H. These findings highlight that targeted CH supplementation can be an effective strategy for mixed mineral calcined clay SCMs for high cement replacement level, lower embodied carbon cementitious systems. • External CH added to 40 wt% mixed clay CGF mortars to offset portlandite deficit. • Smectite-rich CGF-A formed high density C-S-H cement product with added CH. • Kaolin-smectitic CGF-B and illite-kaolinitic CGF-C both formed C-(A)-S-H. • CGF-A gained 40% strength at 8% CH; CGF-B and C gained 20% with 4% added CH. • OPC replacement beyond 40% feasible without strength loss using external CH .

A mock-up test on decimetric scale in an innovative column test device and small-scale element tests were carried out on a German Ca/Mg bentonite, which is investigated as a suitable material for engineered barrier systems in nuclear waste disposal. Key hydro-mechanical parameters such as relative humidity, water content and swelling pressure were monitored and spatially resolved over the 30 cm height of the column in the mock-up test. The test was hydrated from the bottom and ran for 370 days. To compare the material behaviour at different scales, swelling pressure tests and water retention measurements were performed at the element-test scale. The results of the column test showed a heterogeneous evolution of swelling pressure and hydration state variables along the column height. Simultaneous measurement of vertical and radial swelling pressure indicated swelling anisotropy. Transient measurement of relative humidity and water content revealed a redistribution of porosity within the sample, which was confirmed by sampling after completion of the test. Comparison with the results of the element test showed that the two test scales are comparable in similar hydraulic states. The column test is therefore suitable as a precursor to the field test and provides valuable insights into the spatially and temporally resolved hydration behaviour of a buffer material, which helps for the design of in-situ experiments, the validation of numerical models and supports the overall process understanding. • Hydro-mechanical behaviour of a Ca/Mg bentonite in laboratory scale mock-up test and small-scale element tests. • Time domain reflectometry measurement in mock-up test to evaluate water content upon swelling. • Heterogeneous hydration, porosity redistribution and anisotropic stress field were observed in mock-up test.