HARDENING AND PROPERTIES OF A MIXED BINDER WITH SODIUM CITRATE AND POLYCARBOXYLATE ETHER PLASTICIZER

The paper presents the results of studying the joint effect of sodium citrate and plasticizer based on polycarboxylate ethers on the rheological and technological properties of cement paste. During the experiment the amount of additives did not change, the sequence of their introduction varied. For cement paste of five compositions containing 5% sodium citrate and 0.5% superplasticizer, the cylinder spreading diameter, setting time, shear stress, viscosity were determined and structure formation in the first 15 minutes of curing was evaluated by environmental scanning electron microscopy. It was found that sodium citrate had a plasticizing effect (cylinder spreading diameter increased by 2.7 times, shear stress decreased by 29%, viscosity decreased by 79%). When the superplasticizer was added, the cylinder spreading diameter could be increased by 17-30%, the shear stress could be reduced by 46-73%, and the viscosity could be reduced by 52-84%. When sodium citrate was introduced, the initial setting time occurred earlier by 56% and the final setting time by 61%. For all compositions with sodium citrate a significant reduction in the induction period was observed, which is confirmed by the reduction of the time interval between the initial and final setting time by 74%. Sodium citrate accelerates the formation of AFt- and AFm- phases. Cement grains in the samples with sodium citrate and with sodium citrate and superplasticizer added simultaneously are surrounded by a shell consisting of poorly crystallized AFt-phases, the number of newly formed AFt- and AFm-phases. Cement grains in samples with sodium citrate and with sodium citrate and superplasticizer added simultaneously are surrounded by a shell consisting of poorly crystallized AFt-phases, the number of new formations significantly exceeds the composition without additives. The introduction of sodium citrate before or after the superplasticizer leads to slowdown of hydration.