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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