Initial Hardening Period Research Articles

Influence mechanisms of fly ash in magnesium ammonium phosphate cement

This research systematically investigated the effects of fly ash (FA) in magnesium ammonium phosphate cement (MAPC) on the fluidity, setting time, compressive strength, and water resistance. The microscopic measurements and analyses, e.g., X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectrometer (FTIR), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) were conducted to elaborate the influence mechanisms of FA in MAPC. The reaction potential test results indicate that the chemical effect of FA in the MAPC system is lower than glass powder (GP) and ground granulated blast furnace slag (GGBS). FA mainly plays a physical ball-bearing effect during the initial hardening period of the fresh MAPC mortar for fluidity improvement and setting time prolongation. Besides, FA behaves a micro-aggregate effect during the hardened stage, where FA particles can compact the MAPC. Another effect is that the FA particle acts as the crystal nucleus in MAPC to promote the generation/crystallization of hydration products, e.g., the transformation from the hydrous magnesium ammonium phosphate to crystalline struvite constantly. Such promotion enables enhancing the later-stage mechanical properties (7d, 28d) and water resistance of FA-MAPC. Typically, the group F30 (30% FA) had the fluidity, initial setting time, and compressive strength value of 18 min, 286 mm, and 81.0 MPa, respectively, which exhibited a high property as a repairing material. The current results are helpful to understand the influence mechanisms of FA in MAPC.

Fine-grained Concrete with Complex Modifiers for Construction in Extreme Climatic Conditions

It has been established that nanomodification and the use of complex modifiers of concrete mixtures lead to a decrease in the content of loosely bound moisture at an early age and indicate an acceleration of hydration processes in the initial period of hardening. The ascertained fact correlates to data on the dynamics of sample hardening and the results of structural surveys. Optimal formulations of modified fine-grained concrete with high strength properties, which are recommended for the production of building products and structures that operate in extreme climatic conditions, have been developed.

Chemical Composition and Hydraulic Properties of Incinerated Wastepaper Sludge

The hydraulic properties of ash from incinerated wastepaper sludge were investigated. It is shown that the phase composition of wastepaper sludge after heat treatment is similar to the classical fired carbonate-clay mixes. The necessity of using the combination of retarding admixtures with different action mechanism is shown. The strength in the initial hardening period is ensured by the formation of AFm-phases and ettringite, and subsequently by increasing the role of CSH.

The Effect of Fine Dispersed Slag Component on the Slag Portland Cement Properties

Cement compositions based on slag Portland cement and fine dispersed slag component are considered. It has been established that the introduction of 1-3% fine slag obtained as a result of ultrafine grinding in a jet mill into the slag Portland cement composition provides an increase in the strength and structural characteristics of cement stone from the first day of hardening, which is important for slag cements. To ensure the fine dispersed slag particles’ stabilization in the cement matrix, the cement compositions were sealed with water with the addition of a plasticizer on either sulfa-naphthalene formaldehyde or polycarboxylate substrates in an amount of 0.5% of the cement composite content. In the research course it was found that in the initial stages of hardening and grade age, the strength of the samples containing a fine dispersed slag component and a sulfa-naphthalene formaldehyde-based plasticizer increased by 20%. When the cement composition is mixed with water with a polycarboxylate-based plasticizer, an increase in strength in the initial period of hardening by 60% is noted, at the vintage age - by 30%. At the same time, the porosity of the samples based on slag Portland cement with the addition of fine dispersed slag and one of the plasticizers in the initial period was reduced by 16-21%, at the vintage age - by 33-35%.

Heat Emission Kinetics and Hydration Behavior of Modified Polymer–Cement Mixtures for 3D Printing Construction

Abstract—The process of heat emission is investigated during the hydration of the binder in polymer–cement mixtures and mixtures modified by phloroglucinol–furfural superplasticizer. The specific binder hydration enthalpy in the presence of a polyvinyl acetate dispersion and redispersible polymer powder is determined. It is established that the specific enthalpy of polyvinyl acetate polymer–cement systems is positive, which indicates the endothermic nature of the overall process. The dependence of the absolute heat emission values in the initial periods of material hardening on the binder hydration time in the presence of organic mixture components (polyvinyl acetate and phloroglucinol–furfural modifier) is studied. It is proved that, when modifying polymer–cement systems, there is a decrease in the heat emission peak, a reduction in the timing of the initial hydration period, and an increase in the duration of the second (induction) period. The heat emission kinetic curves of the modified polyvinyl acetate–cement mixtures are presented, and the hydration behavior of the studied systems is considered. It is established that, in the initial period of hardening, polyvinyl acetate increases and phloroglucinol–furfural superplasticizer reduces the heat emission rate, while when modifying the polymer–cement system with a polymer in the form of a polymer dispersion, this process is partially balanced.

Heat Liberation of Barium Cements as a Background of their Application in Mass Concrete Structures

The article deals with heat liberation of barium cements, which influences the crack formation in mass concrete structures. The results of heat release and physical testing of barium cements with a different content of BaO are presented in this paper. The strength of cement and its setting time will decrease significantly, if the content of BaO in clinker is high (about 47%). This cement is characterized by a low water requirement and heat liberation. Properties of cement with a low content of barium oxide (5.5%) are close to the properties of the standard Portland cement. In this case heat liberation is low in the initial period of hardening, but at the later stage it exceeds the exotherm of a reference sample. A small amount of BaO in cement clinker reduces the exothermic effect and increases the thermal crack resistance of concrete.

Experimental Study of Dipotassium Hydrogen Phosphate Influencing Properties of Magnesium Phosphate Cement

AbstractA magnesia-phosphate cement (MPC) paste based on magnesia and potassium dihydrogen phosphate (KH2PO4 or PDP) was prepared with the additions of dipotassium hydrogen phosphate (K2HPO4 or DHP) and borax as set-retarders. The resulting materials were characterized in terms of their setting characteristics, strength properties, hydration products, and microstructures. The replacement of PDP by DHP was found to increase the setting time and reduce the intensity of the exothermic reactions during the initial setting and hardening stages. When the replacement percentage did not reach 50%, substitution of dipotassium hydrogen phosphate for potassium dihydrogen phosphate in the MPC paste could strike a good balance between the speedy strength gains and delayed setting time. The setting time of the MPC system increased from 5 min (without DHP addition) to 180 min with PDP replaced by DHP completely. The addition of borax also led to an extension of the initial hardening period, as indicated by reductions in...

Properties of self-compacting concrete mixtures containing metakaolin and blast furnace slag

Rheological, mechanical and durability properties of self-compacting concrete (SCC) mixes produced using blended binders containing metakaolin and blast furnace slag are studied. The rheological properties of SCC mix with metakaolin are characterized by significant yield stress and relatively low viscosity, while the mix with blast furnace slag shows zero yield stress and higher viscosity. The compressive strength of SCC with metakaolin grows very fast during the initial hardening period and remains significantly higher, as compared with the mix with blast furnace slag, up to 90 days. Durability properties of the mix containing metakaolin are excellent. Water absorption coefficient and water penetration depths are very low. The freeze resistance tests show zero mass loss after 56 cycles in deicing salt solution.

THE FORMATION OF THE MODIFIED CONCRETE STRUCTURE FOR PIPE COLLECTORS

In the article main points of developed technology of vibroimpact-pulse compaction of concrete mix for producing the pipes of sewage collectors are stated. It is shown that using the offered technology ensures the intensive growth of concrete strength at an initial period of hardening, the reduction of duration of heat treatment or its absolute avoidance, the manufacture of products with the stability and longevity of structures.

Cyclic deformation behaviour of superduplex stainless steels

The cyclic hardening–softening response, the cyclic stress–strain curve (CSSC) and the substructure evolution within the constitutive phases of a high nitrogen superduplex stainless steel SAF 2507 have been studied and the results compared with reference to low and medium nitrogen duplex stainless steels (DSSs), particularly AISI 329 and SAF 2205 grades, respectively. All three materials were manufactured in the form of hot rolled and solution annealed cylindrical bars with the same diameter. It is shown that the previously reported description of the CSSC of a DSS in terms of three regimes, each associated to distinct controlling cyclic deformation mechanisms, applies well in the case of the superduplex steel too. The main differences between the cyclic deformation behaviour of the DSSs studied are: (i) the CSSC of the SAF 2507 steel is placed at higher stress levels, mainly as a consequence of nitrogen alloying; (ii) the initial hardening period is shorter whereas the softening stage is more prolonged; and (iii) increasing nitrogen content promotes a more planar glide deformation character in austenite.

The effect of retarders on the microstructure and mechanical properties of magnesia–phosphate cement mortar

Magnesia–phosphate cement (MPC) mortars based on magnesia and ammonium dihydrogen orthophosphate (ADP) were prepared with additions of sodium triphosphate (STP), boric acid, and borax as set-retarders. The resulting materials were characterised in terms of their setting characteristics, mechanical properties, and microstructures. The retarders were found to increase the setting time and to reduce the intensity of the exothermic reactions during the initial setting and hardening stages. The effectiveness of STP as a retarder was limited by its solubility in the saturated ammonium phosphate solution, determined as 340 g dm −3, which meant that the maximum setting time obtained was approximately 15 min. In contrast, successive additions of boric acid and borax led to a cumulative increase in setting time to a maximum of 1 h for the compositions investigated. The use of retarders also led to an extension of the initial hardening period, as indicated by reductions in the early values of Young's modulus and flexural strength. The microstructural characteristics of the materials produced were very dependent on the type and level of additive employed; for STP, the binding phase was relatively fine grained (<0.5 μm), while for boric acid relatively large-grained (>10 μm) structures were observed as a result of a reduction in the supersaturation of the aqueous solution. Well-aged mortar specimens exhibited a high incidence of microcracks on the fracture surfaces, which were related to the occurrence of a fracture process zone ahead of the main crack tip during fracture.

Kinetics of the hydration reactions in the cement paste with mechanochemically modified cement 29Si magic-angle-spinning NMR study

A comparative 29Si solid state NMR study of kinetics of the hydration reactions in cement pastes based on rapid-hardening ordinary Portland cement (SH) and on the mechanochemically modified cement (MSH) is presented. The mechanical activation of a cement/silica fume blend in a vibrating mill accelerates the hydration reactions by 15–20%, especially during the initial period of hardening. Variations in relative intensities of 29Si resonances assigned to the hydration products in SH/MSH blends suggest different structures of hydrated SH nets. This can be correlated with a pronounced increase of the MSH-cement binding capacity reported earlier.

Hard-to-cook defect in black beans: hardening rates, water imbibition and multiple mechanism hypothesis

The existence of a slower initial hardening period during storage of dry black beans, independent of cooking time and heat pretreatment, was corroborated. Water vapor production during storage for 1 year at 25 and 35°C did not change appreciably the moisture in dry beans stored at 10 and 12% moisture content. Further support to the hypothesis that water in soaked hard beans is held differently from that in soft beans was based on differences in the rate of drying and microstructural evidence. A dual contribution to the hardening mechanism, one reversible and another irreversible, was postulated based on soaking experiments with monovalent salts and chelating agents. Cell separation was not changed by salt treatment although the cell wall morphology was modified.

STUDIES ON COLD WEATHER CONCRETING BY THE USE OF SPECIAL SUPER HIGH EARLY STRENGTH CEMENT

The contents of the study were first to clarify the strength characteristics of the special super high early strength cement at different temperature, secondly, to clarify what effect freezing exerts on the increase of strenght when the concrete is subjected to freezing at the initial period of hardening, and thirdly, to clarify the resistivity when the concrete at the initial period of hardening or after sufficiently hardened is subjected to the repeated rapid freezing and thawing. It was clarified by this study (1) that the concrete using special super high early strength cement showed the very high manifestation of strength even at low temperature and short age, (2) that when the concrete was subjected to freezing at the initial period of hardening, the lowest compressive strength just before freezing, with which the increase of strength was able to be expected by continuing freezing, was about 50kg/cm2 similarly to ordinary concrete.

AGING PHENOMENA IN MAGNESIUM RICH ALUMINIUM-ZINC-

To study the mechanism of age-hardening of magnesium rich aluminium-zinc-magnesium alloys, the following experiments were performed. AZI alloy (containing 9.98%Al, 0.75%Zn and bal. Mg), AZ2 alloy (containing 2.35%Al 5.72%Zn and bal. Mg) and AZ3 alloy (containing 0.24%Al, 7.79%Zn and bal. Mg) were prepared and the precipitation constituents of these alloys are, respectively, (Al3Mg4 solid solution), T (Al2Zn3 Mg3 solid solution) and ζ (Zn Mg solid solution). They were solution heat-treated at 425°, 360° and 330°, respectively, then quenched into cold water. With these specimens, (1) changes in hardness, electrical resistance and microscopic structures during aging at room temperature, 100°, 150°, 200° and 250° were studied and also (2) changes in hardness, electrical resistance, thermal expansion and microscopic structures during heating from room temperature to 300° at the heating velocity of 1°C per min., were measured.The results were as follow: (1) In natural aging, AZ2 and AZ3 were hardened to some extent but in the case of AZ1 its hardness increase was less. (2) As to the hardness when quenched, that of AZ1 was the highest (Rockwell-F 72)and those of AZ2 and AZ3 were Rockwell hardness F61. The increments of hardness of these alloys by aging were almost equal, but softening tendency caused by over-aging was considerably remarkable at AZ2, and AZ3 and, at AZ, not so marked. (3) At every aging temperature, experimented, the incubation period of hardening of AZ2, whose precipitation constituent is ternary intermetallic compound (T) phase, was the shortest and that of AZ1 the longest. The hardening velocity of initial hardening period of AZ2 was the highest and that of AZ1 the lowest. In the case of tempering by ascending temperature, AZ3 began hardeningfirst and AZ1 last, (4) These three alloys harden by precipitation of the second phase from super-saturated solid solution, but the effect given by precipitation to crystal lattices depends on the degree of supper-saturation of the solid solution, and the hardening takes place in two stages and this fact is distinct in lower temperature aging. The principal hardening of AZ1 was the first hardening stage and those of AZ2 and AZ3 were the second one. (5) The of precipitation of AZ1 alloy belongs to discontinuous type, that is, at first, precipitation takes place along grain boundaries and, after this stage finished, at the remaining parts of grains, but those of AZ2 and AZ3 are continuous type (general precipitation).As one of the factors which causes such different types of precipitation, I propose the difference of diffusion velocity of solute atoms in the super-saturated solid solution.