Surplus water extraction: A simple method to determine the alkali metal concentration in the pore solution of cement paste

Determining the composition of the pore solution of hardened cementitious materials remains a challenge due to the difficulty of accessing it. In this paper, a simple method called “Surplus Water Extraction” (SWE) is presented and compared to the more traditional high-pressure “Pore Water Expression” (PWE). SWE involves casting a cement paste with a w/b = 1.00, leading to the formation of a supernatant liquid layer on top of the solid part. This supernatant liquid can easily be sampled and analysed to determine the Na + , K + and OH - concentrations. Investigations were carried out on various binary and ternary cementitious binders, complemented by an in-depth characterisation of binary systems with a siliceous fly ash. Even though there seems to be a slight gradient between the supernatant liquid and the pore solution in the solid part of SWE samples, the results show a strong linear correlation between SWE and PWE results for [Na + +K + ]. While the [Na + +K + ] in SWE did not change significantly between 4 and 26 weeks of curing at 38 °C, the K-to-Na atomic ratio decreased over time and upon increasing the FA content, likely due to the release of Na from fly ash. SEM-EDS measurements suggested that there was no significant effect of increasing the w/b from 0.50 to 1.00 on the chemical composition of C-(A)-S-H, in particular the Si/Ca and Al/Ca. Thus, SWE appears as a promising method to estimate the [Na + ] and [K + ] in the pore solution of cement pastes with w/b = 0.50.