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During the early stages of hydration, cement paste develops a structure which will ultimately lead to setting through a depercolation process. This structuring process is reversible until setting, meaning that it can be destroyed by imposing a mechanical shear stress but it will rebuild with approximately the same kinetics. The driving force for this process lies in the attractive forces acting among the cement particles in the course of hydration. A wealth of organic admixtures is used to modify the interparticle forces and consequently altering the fresh state properties of cement paste, and we will concentrate in the following on superplasticizers and retarders. Both these classes of admixtures modify the microstructure of cement paste and yield different mechanical properties and different kinetics of the structuring process. The evolution of mechanical properties is followed by rheological measurements in the oscillatory mode, which enables to determine the storage and loss modulus of cement paste. These properties can be interpreted along a modeling scheme referring to a heterogeneous composite material, similarly to what has been done with nanoindentation and ultrasonic measurements. It is shown that the difference between superplasticizers and retarders lies only in a different grading of the same basic interaction with the hydrating surfaces, and in fact it is possible to devise molecular structures yielding a dispersing retardant or a slump-retaining superplasticizer, i.e., the intermediate member between the two families. In a similar way, it is possible to tailor the interaction of the organic molecules with the hydrating surfaces in order to develop more robust admixtures with regards to variations of the interstitial solution chemistry.

The retention performance of concrete is highly influenced by external parameters such as cement type, water content, materials, temperature and so on which sometimes appears to be a problem for ready-mixed concrete producers. The influences of the molecular structure and functional groups of PCE superplasticizers on the retention performance in terms of external changes were studied. In the course of our investigations we focussed on the slump-loss-controlling agent with release compound (SLCA) and a common PCE with shorter polyethylene oxide chains. It was found that the retention ratio of the SLCA was much less affected by changes of temperature and mixing time, but the retention ratio of both superplasticizers was influenced by change of w/c. Superplasticizer SLCA with an additional release compound showed a much higher adsorption increase over time than the common selected PCE. The importance of the chemical structure of polycarboxylated superplasticizer to the retention stability could be related to the adsorption mechanism. At the same time a lower retardation effect on the setting time could be observed for the SLCA superplasticizer.

The examination of superplasticizer compatibility with cement and its influence on rheology of concrete has essential significance from the workability control point of view. Usually, the performance of superplasticizer is tested using cement paste. However, the effects of superplasticizer on rheological properties of cement paste and concrete may be considerably different depending on the material in use. Simultaneously analysis of so far executed studies indicate that mortar can be considered to be concrete without coarse aggregate and its testing has attractions for the study of superplasticizer and other ingredients effects at small scale. The main objectives of this study were to investigate the influence of superplasticizer on rheology of mortar and of concrete and to develop empirical formula making possible of forecasting rheological properties of concrete basing on rheological measurements made on mortars. The influence of superplasticizer type and content on rheology of mortars and concretes that have different mortar volumes was investigated, cement type and w/c influence was also considered. Rheology results have been evaluated according to the Bingham model. Rheological parameters were measured using BT2 rheometer. The results show that superplasticizer has generally similar influence on the rheological parameters of mortar and concrete. It was demonstrated, that superplasticizer effect on rheological properties of concrete could be predicted with a high degree of certainty from on rheological measurements made on mortars. The regression models of influence of rheological parameters of mortars on rheological parameters of fresh concretes made of different volumes of analogical mortars were developed. These models may be applied for designing and developing workability of concrete.

A new class of polycarboxylate superplasticizer with a grafted pendant group of polyethylene oxide has been developed and extensively used in the construction field due to its outstanding water reducing ratio, high workability retention at small dosages and excellent compatibly with higher dosage rates of industrial by-products. Nevertheless, a drawback of this new generation superplasticizer is their effects in delaying the rate of cement hydration, especially when they are used at higher dosages and in low temperatures environment. These aspects limit their industrial application because of the slow mechanical strength development at the early age. In order to overcome this drawback, a new polycarboxylate superplaticizer (SSP) has been developed by means of optimizing the comb copolymer structure so as to meet ultra-early strength development requirements of construction projects. The results of mortar or concrete tests clearly indicate that SSP has the capacity to considerably accelerate the development of early age mechanical strength in conditions of low curing temperature, in comparison with the ordinary superplasticizers. The compressive strength of concrete containing SSP can reach 500% and 480% at an early age of 24 hours and 30 hours, respectively, when compared with ordinary polycarboxylate polymers at the same dosages. Engineering experimental results proved that the SSP can meet the requirements of concrete admixtures for high-speed railway construction projects.

Alkali-free flash setting accelerators are widely used in the area of underground construction as they allow a very rapid hardening of cementitious systems. These chemicals are commonly intermixed with an air flow into a fluid concrete which is rapidly projected on the rock wall to form a consolidating shield of variable thickness. The typical accelerator dosages range between 6 and 8 % by cement mass. Nevertheless, sometimes, due to concrete fluidity loss, low air flow pressure, or poor homogenisation of the concrete/accelerator system, a very high local accelerator concentration could result in the sprayed mixture. Practical experiences point out that these conditions could cause significant strength loss of the sprayed layer, thus compromising the safety of the tunnelling operations. In order to elucidate the influence of high accelerator concentrations on hydrating cement, setting time and mechanical strength development of cement paste samples, admixed with several dosages of different alkali-free accelerators, were measured. Furthermore morphological (ESEM-FEG) and crystal-chemical (TG/DSC, XRD) analyses were also performed to clarify the reasons responsible for these reduced mechanical strengths.