This article presents the results of a round-robin test performed by 13 international research groups in the framework of the activities of the RILEM Technical Committee 260 RSC "Recommendations for use of superabsorbent polymers in concrete construction". Two commercially available superabsorbent polymers (SAP) with different chemical compositions and gradings were tested in terms of their kinetics of absorption in different media; demineralized water, cement filtrate solution with particular cement distributed to every participant and local cement chosen by the participant. Two absorption test methods were 2 considered; the tea-bag method and the filtration method. The absorption capacity was evaluated as a function of time. The results showed correspondence in behaviour of the SAPs among all participants, but also between the two test methods, even though high scatter was observed at early minutes of testing after immersion. The tea-bag method proved to be more practical in terms of time dependent study, whereby the filtration method showed less variation in the absorption capacity after 24 hours. However, absorption followed by intrinsic, ion-mediated desorption of a respective SAP sample in the course of time was not found by the filtration method. This SAP-specific characteristic was only displayed by the teabag method. This demonstrates the practical applicability of both test methods, each one having their own strengths and weaknesses at distinct testing times.

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Conventionally, isothermal calorimetry and ASTM C186 heat of hydration results are reported on a per mass of cement (powder) basis, with typical units being J/g (cement) for example. Based on the recognition that it is the filling of porosity with hydration products that is chiefly responsible for strength development in cement-based materials, there may be merit in instead reporting these results on a per unit volume of (initial) water basis. This paper examines a database of well over 200 mortar mixtures to investigate the relationship between heat release and mortar cube compressive strength development. For reasonably low water-to-cementitious materials ratios (w/cm < 0.43), a single universal straight line relationship with some scatter is obtained. Based on numerous experimental data sets and accompanying theoretical computations, the effects of w/cm, sand volume fraction, cement chemical composition, sulfate content, cement fineness, the incorporation of a high range water reducing admixture, and curing conditions on this universal relationship are all considered. Fifty data points from the Cement and Concrete Reference Laboratory (CCRL) proficiency sample program are analyzed to develop a linear relationship between ASTM C109 mortar cube compressive strengths and ASTM C186 heats of hydration at 7 d and 28 d. The application of this relationship for virtual testing is also evaluated. In this case, computer simulations would be employed to predict the heat of hydration vs. time for a particular cement and the developed equations would be employed to convert this heat release to a strength prediction at the age(s) of interest. In general, it appears that these relationships can be used to predict mortar cube compressive strengths based on measured heats of hydration, within about ± 10 % of the experimentally measured strengths. A preliminary analysis of a single dataset for concretes with and without limestone replacement for cement indicates that the linear relationship between strength and heat release likely holds for concretes as well as mortars.

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