Abstract Four different mortar mixes with different alkali contents were prepared to analyze the distribution and influence of soluble alkalies on the cement paste microstructure around air voids: a reference mix and three mixes with additions of Na 2 SO 4 , K 2 SO 4 and NaOH respectively. Two methods were used for this study: scanning electron microscopy and electron microprobe analyses. The results obtained with the microprobe clearly indicate an important concentration of sodium and potassium around the air voids, which fact can probably explain the positive influence of alkalies on air void stability (described in previous publications). However, the microstructure of the paste around the air voids as examined under the electron microscope did not appear to be significantly influenced by this concentration of alkalies.
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