Radoslav Novotný

Some Issues of Shrinkage-Reducing Admixtures Application in Alkali-Activated Slag Systems

Significant drying shrinkage is one of the main limitations for the wider utilization of alkali-activated slag (AAS). Few previous works revealed that it is possible to reduce AAS drying shrinkage by the use of shrinkage-reducing admixtures (SRAs). However, these studies were mainly focused on SRA based on polypropylene glycol, while as it is shown in this paper, the behavior of SRA based on 2-methyl-2,4-pentanediol can be significantly different. While 0.25% and 0.50% had only a minor effect on the AAS properties, 1.0% of this SRA reduced the drying shrinkage of waterglass-activated slag mortar by more than 80%, but it greatly reduced early strengths simultaneously. This feature was further studied by isothermal calorimetry, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Calorimetric experiments showed that 1% of SRA modified the second peak of the pre-induction period and delayed the maximum of the main hydration peak by several days, which corresponds well with observed strength development as well as with the MIP and SEM results. These observations proved the certain incompatibility of SRA with the studied AAS system, because the drying shrinkage reduction was induced by the strong retardation of hydration, resulting in a coarsening of the pore structure rather than the proper function of the SRA.

Effect of Na3PO4 on the Hydration Process of Alkali-Activated Blast Furnace Slag

In recent years, the utilization of different non-traditional cements and composites has been increasing. Alkali-activated cementitious materials, especially those based on the alkali activation of blast furnace slag, have considerable potential for utilization in the building industry. However, alkali-slag cements exhibit very rapid setting times, which are too short in some circumstances, and these materials cannot be used for some applications. Therefore, it is necessary to find a suitable retarding admixture. It was shown that the sodium phosphate additive has a strong effect on the heat evolution during alkali activation and effectively retards the hydration reaction of alkali-activated blast furnace slag. The aim of the work is the suggestion of a reaction mechanism of retardation mainly based on Raman and X‑ray photoelectron spectroscopy.

Cement Kiln By-Pass Dust: An Effective Alkaline Activator for Pozzolanic Materials

Cement kiln by-pass dust (CKD) is a fine-grained by-product of Portland clinker manufacturing. Its chemical composition is not suitable for returning back into feedstock and, therefore, it has to be discharged. Such an increasing waste production contributes to the high environmental impact of the cement industry. A possible solution for the ecological processing of CKD is its incorporation into alkali-activated blast furnace slag binders. Thanks to high alkaline content, CKD serves as an effective accelerator for latent hydraulic substances which positively affect their mechanical properties. It was found out that CKD in combination with sodium carbonate creates sodium hydroxide in situ which together with sodium water glass content increases the dissolution of blast furnace slag particles and subsequently binder phase formation resulting in better flexural and compressive strength development compared to the sample without it. At the same time, the addition of CKD compensates the autogenous shrinkage of alkali-activated materials reducing the risk of material cracking. On the other hand, this type of inorganic admixture accelerates the hydration process causing rapid loss of workability.

The Preparation of Heat Insulating Moulded Refractories

The thesis deals with the study of influence of the foaming agent Schäumungsmittel W 53 addition on the specific weight and mechanical characteristics of mullite based heat insulating refractory castables. The effect of dosing and mixing time was observed in this work. The influence of partial substitution of mullite matrix by kaolin was also studied. Kaolin was added in dry matter or in the foamed state. Optimal dosage of the agent was 0,25 % and the optimal mixing time was 10 min. The addition of kaolin was the most effective in the foamed state.

Effect of Phosphates on the Hydration Process of Alkali Activated Materials

The study deals with the one of the important feature of alkali activated blast furnace slag which is very rapid initial setting. Therefore, the influence of the retarding agents such as phosphates was tested. It is shown that phosphates decreases the hydration heat evolution and retards the hydration reaction of alkali activated slag effectively. The mechanism of retardation is studied through the microcalorimeter and electron microscopy equipped with energy dispersive analyzer (SEM-EDS). The optimal dosage of suitable retarding admixture in the means of mechanical properties has been determined.

The Role of Temperature on Hydration of Binary System of Metakaolin/Portland Cement

The role of temperature of metakaolin/Portland cement binary system was studied by isothermal calorimetry. Sample with 50 % of metakaolin replacement were monitored at 30 °C, 40 °C, 50 °C and 60 °C. Structural and chemical characterization of hardened pastes was obtained by scanning electron and Raman microscopy. Paper deals with kinetics of main exothermal reactions during early hydration of the system. Activation energies were calculated for processes related to each exothermic peak. The nature of hydration products at different temperatures was revealed by microstructural studies.

Rheological and Calorimetric Characterization of the Role of CaCl2 on Portland Cement Early Hydration

Paper describes a new way how to characterize early hydration and setting of Portland cement pastes with various dosages of setting accelerator CaCl2. The aim was not to characterize the role of well-known setting accelerator CaCl2 but to correlate rheology and calorimetry in view of early hydration of Portland cement. Characterization was based on measurements on rotational rheometer, isothermal calorimeter and semiadiabatic calorimeter. The courses of calorimetric exotherms in time were correlated to the development of the phase angle obtained by rheological oscillation tests. Heat flow peaks can be successfully correlated to rheological processes described by change of storage and elastic moduli up to the time that corresponds to the start of the setting which was found to be the limit for rheological tests. The start of the setting on calorimetric curve belongs to the onset point of the main silicate peak during acceleration period of hydration. The value of phase angle decreases as the setting cement paste changes from liquid to solid-like. Times of low values of phase angle ~ 2.5 (°) can be related to times of onset points of main peak for every dosage of setting accelerator.