Hydration characteristics of calcium sulfoaluminate-fly ash mixed with different alkalies

Due to its notable low carbon footprint, calcium sulfoaluminate (CSA) cement has become a research hotspot for cement chemists in recent years. Relevant studies have shown that supplementary cementitious materials (SCMs) can greatly improve the properties of CSA cement pastes, but the utility of SCMs in CSA cement has not yet been fully exploited due to their low alkalinity. To stimulate the activity of pozzolanic materials, the effect of different alkalies on the hydration of CSA-fly ash (CSA-FA) was systematically investigated, and the results indicate that after calcium hydroxide (Ca(OH)2) is incorporated into the CSA-FA paste, it first forms a saturated solution and then rapidly hydrates to form ettringite. The initial pH value of the suspension is approximately 12.65, which then slumps to a level lower than that without Ca(OH)2. The initial pH value of the CSA-FA suspension mixed with 1 M sodium hydroxide (NaOH) solution is 13.0, which is then rapidly decreased to a stable value of approximately 11.3. Ca(OH)2 can promote the hydration of CSA to form ettringite and improve the compressive strength of the paste, and the larger the Ca(OH)2 content, the more obvious this effect. NaOH inhibits the hydration of CSA. With increasing NaOH concentration, the compressive strength of the paste first increases and then decreases, and the effect of inhibiting the hydration of CSA becomes stronger. The CSA-FA paste blended with 1 % Ca(OH)2 and 1 M NaOH shows collapsing and cracking behavior, for which a large number of well-developed flaky aluminum hydroxide crystals are formed, and strong diffraction peaks for unhydrated CSA are observed.