Sodium sulfate is known as one of the most destructive salts causing weathering. Many experiments on accelerated weathering tests show that major deterioration effects by weathering are caused by drying and wetting cycles of porous materials saturated with salt solution. In this study we have performed accelerated weathering tests with sodium sulfate in common building materials (fired-clay brick, Indiana limestone and Cordova limestone) measuring the concentration in the materials simultaneously with their expansion. The concentration of sodium sulfate solution is measured non-destructively using Nuclear Magnetic Resonance, while the expansion of the sample caused by crystal growth is measured with a fiber optic displacement sensor. The simultaneous measurement of solution concentration within a material and expansion allow assessment of crystallization pathways most responsible for damage during weathering, i.e., cycles of wetting and drying. It was shown by direct observation, that with rewetting of the partially dried samples, the present thenardite experiences a rapid partial transformation to decahydrate. Simultaneously with this transformation a rapid expansion of the sample was measured in situ.