Interaction of cement and admixtures and its effect on rheological properties: Fid-Bau Portal

Interaction of cement and admixtures and its effect on rheological properties

Kubens, Stefan

Cement samples from several countries and producers were examined on their rheological properties. An aim of the study was to see if fluctuations due to different cement production dates can be seen in mortar mixes. The focus was on mixes in particular where dispersing admixtures were used. The admixtures used were based on melamine and polycarboxylate. An impact of different cemcnt production dates on rheological properties was observed. The effect is moderate in mixes without any dispersing admixturcs but far more prominent in mixes containing dispersing admixtures. The melamine-based admixture caused the highest fluctuations, whereas the fluctuations with the PCi (polycarboxylate) based admixture were less. Another aim of the study was to identify the cement constituents which lead to fluctuations in mortar mixes. Thc ingredients which seemingly exert the largest influence on yield value are not the same for blank mixes as for those containing dispersing admixtures. In blank mixes the increase in yield value is seen to go together with increasing surface area of the cement used, whereas this effect is not observed for dispersing admixtures From the received cement production dates, no influencc of (3CaO)(Al2O3) on yield value was observed in blank mixes and mixes containing melamine. In mixes containing polycarboxylate an increase in (3CaO)(Al2O3) resulted in increase in yield value. When synthetic (3CaO)(Al2O3) was added to blank mixes, an influence on rheology was observed. For alkali content, a similar tendency was observed; no clear trend in blank mixes or those containing melamine, but an increase in yield value of PCi mixes with increasing alkalis. When alkali sulphates and hydroxides (NaSO4, K2S04, NaOH and KOH) were added to blank mixes the yield value decreased, whereas it increased when alkali sulphates and hydroxides were added to PCi and melamin mixes.Another aim of the study was to quantify the effect of cement delivery on production properties of Eco and standard SCC (self compacting concrete). The utilization of high yield cement in Eco and standard SCC resulted in low slump flows and high yield stresses, whereas the utilization of low yield cement resulted in high slump flows and low yield stresses. The vaxiations in concrete were less prominent as in mortar. This might be explained by the lower cement content in Eco-SCC and the higher inner shear rate in concrete. Adsorption measurements indicate that cements differing in rheological properties also differ in polymer consumption. Cement giving a low initial yield stress had a bigger reservoir of available polymers after ten minutes of hydration than cement giving a high initial yield stress. Pore solution analysis on cement with differing rheological properties showed clear differences in ion concentration. The addition of silica fume resulted in an improvement of rheological properties, meaning the effect of cement production date was considerabiy reduced. While some grinding aids had no influence on rheology or even improved flow properties, other compounds had a big influence on rheology and caused sticky mixes.