The hardening behaviour of cementitious materials

Data from tests on samples of lime-pulverised fuel ash (PFA), lime–gypsum–PFA and cement–PFA mixtures have been analysed. In the case of the lime–PFA mixture, three phases of strength gain were identified, as follows. Phase 1—a rapid initial gain in strength, followed by an induction phase during which little or no strength gain was recorded. After an induction period this was followed by: phase 2—a further period of rapid strength gain; and phase 3—a period of slow gain in strength. A strengthening model based on the interlacing of fibrous growth forms is considered for the phase 3 stage of strengthening which is validated by scanning electron microscope work. It is shown that additional strength resulting from the additions of gypsum at 20°C is due to the phase 3 strengthening stage.