Chemical and microstructural characterization of environmentally friendly alkali‐activated slag cement pastes and fine‐limestone aggregate concretes have been carried out after aging for 20 months and compared with 55‐day‐old cement pastes. Although the production of crystalline phases slowed, the cement paste continues to react. Si moves to more ordered positions, and formulae activated by Na2CO3 undergo recarbonation, producing both calcite and aragonite, of which the latter occurs mainly in the recarbonation of Ca‐rich building materials known to have high durability. The reaction continues little in samples activated by NaOH/waterglass after 55 days. Elemental analysis shows the Ca/Si ratio decreasing from the center of unreacted slag grains to the outer product (Op). Elemental analyses of the cementing phase in the cement paste as well as the cementing phase in concrete specimens were quite similar. Peak shifts observed in Fourier transform infrared spectroscopy imply greater durability due to increased polymerization of Si in C–S–H, which may contain Na. Over time, more Ca dissolves from the slag and enters the Op, which was observed to be homogeneous.

Two concretes—with Ca/Si ratios of 0.28 and 1.75 fabricated by cementing a fine‐limestone aggregate with a mixture of lime, diatomaceous earth, and water—were compared with ones made with pure lime and a hydraulic lime product, containing ∼11 wt% naturally occurring reactive silica. When the Ca/Si ratio in the cementing phase was 0.28, compressive strengths of ∼6.5 MPa after 30 days and ∼7 MPa after 180 days were achieved by curing the samples in closed containers or in 100% relative humidity. When allowed to dry, however, these samples lost roughly half their compressive strength in 7 days. Increasing the Ca/Si ratio to 1.75 solved the drying problems. The resulting compressive strengths, however, after 180 days, were reduced to 5 MPa. Characterization of the various cementing phases formed in the different samples by X‐ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis showed the formation of two, somewhat competing, cementing reactions: C–S–H gel formation which results in early strength gains, and the recarbonation of portlandite, which results in longer term strength enhancements.

Lime–diatomaceous earth cement pastes were fabricated at room temperature starting with a mixture of diatomaceous earth (DE), lime, and water. The cementing phase, associated with the pozzolanic reaction of the diatom skeletons with lime, was identified as calcium silicate hydrate (C–S–H) gel using X‐ray diffraction, scanning electron microscopy, Fourier‐transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The FTIR and NMR results revealed that the C–S–H gel is mainly composed of silica dreierketten in all lime pastes using three different types of DE.

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