The contamination of cement binders with zinc represents a significant problem due to the negative effects on cement hydration. Zinc compounds cause a drastic increase in setting time due to ...prolonging the induction period. The currently accepted mechanism behind this effect is the formation of Ca(Zn(OH)
3
)
2
·2 H
2
O hydrates on the surface of cement grains that deplete Ca
2+
ions from the pore solution and creates a diffusion barrier. Hydration accelerators are commonly employed in the concrete industry to counter long setting time caused by low temperature and contamination by heavy metals. These compounds influence both hydration kinetics and composition of hydration products. The influence of various compounds on the mechanism of cement hydration can be studied using calorimetric methods such as isoperibolic and isothermal calorimetry. The hydration of the material was stopped, and the properties of hydrated cement pastes were studied using differential thermal analysis, X-ray diffraction and scanning electron microscopy. Some setting accelerators have been found to significantly decrease setting time of zinc-contaminated cement. Out of the most used compounds in commercial accelerators, the efficiency of thiocyanates is yet to be determined. The results show that thiocyanates induce a visible change in hydration mechanism of cement to various degrees depending on concentration and on the presence of the specific cation. Alkali thiocyanates drastically retard the hydration of zinc-contaminated OPC. With further retarded hydration of cement, the mechanical properties were negatively impacted. Calcium thiocyanate on the other hand effectively accelerates setting and positively impacts compressive strength at low doses. Main difference between the influence of alkali and calcium thiocyanates on setting is the change in ettringite content. Alkali salt promotes AFm phases at the expense of ettringite content while calcium salt promotes formation of ettringite at early stages of cement hydration.
The influence on the bond between the steel fiber and the matrix of the anticorrosive treatments of steel used for concrete reinforcement is not yet fully understood. The topic of steel fiber ...treatment was not also studied clearly in terms of brass removal before. This paper deals with how the brass on the surface of steel fibers behaves in the UHPC matrix and how it affects its properties. The steel fibers were firstly modified with a number of surface treatments to remove brass on their surface. Some of the treatments have never been tried before for this purpose. Secondly, the surface of the fibers was analyzed by SEM, EDS, XRF, and stereomicroscopy. Lastly, the properties of the composites were analyzed. It was found out that the majority of brass on the surface of the fibers could be removed by mixture of NH
and H
O
with a ratio of 3:1 (
/
). It was also found out that the surface treatment slightly affects the mechanical properties, but it does that only by mechanical interlocking between the fiber and the matrix. No dissolution of the surface treatment was observed under the given conditions. According to the results, steel fibers without surface treatment should be used in UHPC if available.
•Lignosulfonate efficient for NaOH activation regardless of the time of addition of paste components.•Calorimetry of alkali-activated slag with low silicate modulus of activator (0; 0.10; 0.25, ...0.50).•Effect of lignosulfonate on rheology greatly related to its retarding effect on hydration.•The use and evaluation of strain amplitude sweep tests after different time discussed.•Hump on loss modulus curve for the pastes with significant precipitation of products.
One of the crucial issues of alkali-activated materials is the application of conventional plasticizers. Lignosulfonate plasticizer (LS) works well for sodium hydroxide-activated slag but loses its efficiency in sodium waterglass-activated slag because of competitive adsorption with silicates. Therefore, the effects of the timing of LS and activator addition on the structural build-up and breakdown of AAS pastes with low-silicate moduli activator (0 to 0.50) were investigated, particularly by means of non-traditional strain sweep tests, which are thoroughly discussed. The rheological tests were correlated with the Vicat needle test and isothermal calorimetry. The results showed the high efficiency of LS in NaOH-activated slag paste over time, regardless of the order of addition of the components of the paste. It was accompanied by a considerable retardation effect of LS on the hydration of AAS. The beneficial effect of LS on the rheological parameters and hydration kinetics decreased rapidly with an increase of the silicate modulus of the activator. The delayed addition of the concentrated silicate-containing activators completely counteracted the slightly beneficial effect of LS and even accelerated the initial structural build-up.
There are many highly variable approaches to mix proportioning of alkali-activated materials (AAMs), which considerably contribute to their great complexity and often incomparability of the obtained ...results. Therefore, this paper suggests one specific approach, based on the molarity of the activator and the volume fraction of the precursor. First, an inherently simple calculation procedure is introduced to obtain the weights of the raw materials required to prepare a desired volume of AAM paste and possibly mortar or concrete. Second, the rationale for this approach is demonstrated using calorimetric experiments conducted on alkali-activated slag pastes, activated with sodium and potassium hydroxides, waterglasses, and carbonates in wide ranges of molarities. Third, the applicability and impact of the presented approach are discussed in-depth, considering practical applications. The overall outcome is a complex set of information emphasizing the importance of molarity, precursor volume fraction, and space-filling ability of the reaction products for different activators.
•Mix designing of AAM based on activator molarity and precursor volume fraction•Space-filling ability of products for each activator also considered•Reaction kinetics of AAS mapped over the concentrations of six common activators•Molarity of 2 to 4 M Na+ or K+ recommended for slag activation•Impact of the presented approach and data discussed in a wide practical and scientific context
•Increasing glycol dose had minor effect on strength while significant on shrinkage.•Longer molecules reduced drying shrinkage of AAS more than short molecules.•Shrinkage was reduced thanks to ...surface tension decrease and porosity changes.•Ethylene glycol reduced both 2nd and 3rd peaks of AAS calorimetric curve.•3rd AAS calorimetric peak reduction increased with increasing molecular weight.
The aim of this study was to explore the influence of ethylene glycol in a wide range of polymerization degree, i.e. from monomer (EG) up to polyethylene glycol (PEG) with molecular weight of about 35,000, on properties of alkali-activated slag mortars. Changes in molecular weight and dose of all tested glycols had only minor effect on compressive strength, while drying shrinkage was significantly affected by both these factors. EG had negligible impact on drying shrinkage, but other tested glycols reduced it significantly with PEG2000 and PEG10000 being the most effective. Such different shrinkage behavior was attributed to the changes in surface tension and pore structure. All tested glycols reduced the total heat released during the hydration and interestingly modified the heat flow depending on their molecular weight.
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.
•Amino alcohol surfactants reduced the surface tension of AAS pore solutions.•Surfactants had a minor effect on autogenous shrinkage.•Drying shrinkage was reduced mainly thanks to hydration ...retardation caused by surfactants.•Decrease in surface tension does not necessarily lead to decrease in shrinkage.•Adsorption of surfactants on slag particles were observed.
One of the most important technological problems associated with alkali-activated materials is high shrinkage. In this study, shrinkage reducing admixtures (SRAs) based on amino alcohols were used in alkali-activated slag (AAS) as strong surfactants that should, in terms of capillary pressure theory, decrease shrinkage via the decrease in surface tension. Although the surface tension of the pore solution was reduced by SRAs, autogenous shrinkage was not affected in the long run, while drying shrinkage was noticeably reduced and simultaneous weight changes were dramatically increased. The expected retardation effect of SRAs on hydration was confirmed using isothermal calorimetry, strength development, mercury intrusion porosimetry and scanning electron microscopy. The obtained results suggest that the observed effect of SRAs on drying shrinkage was caused by coarser pore structure rather than by a decrease in surface tension of the pore solution. Since the decrease in surface tension does not necessarily lead to decrease in shrinkage, the application of capillary pressure theory in AAS can sometimes be an issue.
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.
The kinetics of early hydration reactions of Dyckerhoff G-oil cement and its blends using conduction calorimeter and thermogravimetric analysis are reported in the present paper. The coupling effect ...of temperature (25, 40, 50, 60, 80 °C) and material composition (100, 95, 90, 85, 80, 75, 70, 65% Dyckerhoff) with different substitution levels by blast slag furnace, metakaolin and limestone were analyzed on the light of hydration products and activation energy of different reactions. Besides the initial peak in the first 10 min of hydration, two or more other particular peaks characterizing the sequence of successive hydration reactions of the cement are observed, the intensities and the time position of which depend on the temperatures and on the composition of the materials. The apparent activation energies calculated from the hydration peaks characterizing the different reactions decrease with the increase in the substitution levels, but that of the second peaks (31.95 to 33.31 kJ mol
−1
) is higher than that of the former (27.88 to 31.33 kJ mol
−1
) at the same level of substitution. Thermogravimetric analysis of the samples after calorimetric measurement shows that the main hydration products are C–S–H, C–A–S–H, CH, and calcium carbonate. Their quantity depends on the material composition of the material and the temperatures of hydration.
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