Impact of retarders by controlled addition on the setting, early hydration and microstructural development of different cements Available to Purchase

Without the use of retarders, different applications in modern concrete technology would not be possible. The effect of retarders is among other factors influenced by its adsorption on first formed hydration products. Due to this, it is possible to enhance the retarder's impact on cement hydration significantly by controlled addition. This study presents investigations on cement pastes with additions of different retarders in varying dosages and addition times. Effects on the hydration process, setting behaviour and microstructural phase formation were monitored. With delayed addition after 5 min, the retarding effect was enhanced independent of the cement and the retarder type. However, with delayed addition after 15 min, an additional increase of the retardation depends on the cement admixture combination. Comparing calorimetry, setting time and ultrasonic measurements, similar observations were obtained. The correlation between the initial set and the derivation of the ultrasonic pulse velocity yielded good results. With the addition of sodium gluconate, the calcium hydroxide (Ca(OH)₂) formation after 1 d of hydration was significantly influenced by the addition time. The addition time of ethylenediamine tetra(methylene phosphonic acid) (EDTMP) did not show such influences on the microstructural development. The calcium–silicate–hydrate (C–S–H) formation yielded similar results compared to the portlandite formation.