Contribution of mixture design to chemical and autogenous shrinkage of concrete at early ages

In this work, autogenous shrinkage at early ages (<24 h) was accurately measured by linear displacements on slabs simulating field constructions. The best correlation of the amount of chemical to autogenous shrinkage was found at the time of 4 h after the final setting time. It was possible to account for test arrangement artifacts, such as thermal dilation, to get a measure of pure autogenous shrinkage. Many material parameters, such as superplasticizer (SP) and aggregate amount, effected the magnitude of autogenous shrinkage in secondary ways. These consequential effects, such as amount of bleed water and time of setting, were accounted for in the slab measurements. Recommendations are given for reducing the likelihood of cracking due to early age chemical and autogenous shrinkage.     

Physico-mechanical behavior of self-cementing class C fly ash-clay mixtures

Self-cementing class C fly ashes are being increasingly used for soil stabilization of road bases and in other civil constructions. Because of their self-cementing capability in the presence of water, they can be used for clay subgrade improvement as cement surrogates, or as road subgrade material. However, for efficient and economic utilization of self-cementing class C fly ash, the physico-mechanical characteristics of these ashes must be determined extensively. This paper focuses upon the laboratory evaluation of the (1) stabilization characteristics of clay soils blended with self-cementing class C fly ash, and (2) residual self-cementation capabilities of ponded class C fly ash. Testing carried out by the authors and other researchers have indicated that curing time, curing condition, clay mineralogy, amount of fly ash and swelling potential in the soil-fly ash mix are the important variables that control stabilization characteristics. In this paper, the stabilization characteristics were evaluated in terms of the gain in the uniaxial compressive strength and stiffness, and swelling potential. To examine these effects, 12 set of mixtures of ideal clay soils with known percentages of kaolinite and montmorillonite, self-cementing class C fly ash and appropriate amount of water were compacted and cured. In the mixed samples, amount of montmorillonite varied from 0, 2, 4 and 6%, and the amount of self-cementing class C fly ash varied from 5, 10 and 20%. To investigate the effect of curing condition, three curing environments were used. For swelling test, the cured samples were inundated and allowed to swell at the seating pressure of about 2 kPa applied by the weight of the top porous stone and load plate using the one dimensional oedometer apparatus. In addition to the stabilization characteristics of clay soils-fly ash blend, the residual self-cementation capabilities of ponded class C fly ash were also investigated in terms of unconfined compression and CBR tests performed at 7 and 14 days of curing. Results obtained from these test were encouraging and compared favorably with the typical subgrade materials.     

Alkali activation of fly ashes. Part 1: Effect of curing conditions on the carbonation of the reaction products

This paper deals with the alkaline activation of fly ashes for the production of a novel cementitious material and with the effect of curing conditions on the nature of the reaction products. Curing procedures favouring carbonation process negatively affects the development of mechanical strength of this new alkaline cement. Carbonation of the system involves its pH modification and consequently the modification of the nature of the reaction products and the kinetics of reactions.     

Absorption of CO2 into aqueous solutions of MEA and AMP in a wetted wall column with film promoter

The process for removing CO₂ from a gas mixture can be achieved by absorption into amines through of different conventional techniques. But, the report of mass transfer parameters that can be used for process design is limited. Thus, a study of CO₂ absorption process into monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP) and their respective blend were developed with the aim of evaluating the absorption performance and of determining a design parameter for a single-amine absorption system. The experimental tests were carried out in a wetted wall column employing a film promoter, a thin stainless steel woven. The experimental device was operated in countercurrent and the different tests were developed at 298 K, atmospheric pressure, a constant gas load (10% CO₂) and three different liquid loads. The results show that the interfacial area is influenced by increasing the liquid load. The K_Ga values of MEA and the blend based on it are higher in relation to AMP. Moreover, the results of the thermodynamical-kinetic parameter show that this absorption process is independent of liquid load, characteristic of rapid chemical reactions. The K_Ga value for the different MEA:AMP mixtures shows that it increases when the MEA proportion is increased in the blend.     

Optimization,formation, and evolution of the photoinduced curing gradients and in-situ lamellar gaps in additive manufacturing of ZrO2 ceramics: From curing to sintering behaviors

The photoinduced curing gradients and in-situ lamellar gaps were related to the three-dimensional curing mechanism of stereolithography and researched in this work. Based on the photocuring mathematic theories presented by our team, photoinduced curing gradients were influenced by printing parameters such as laser power and frequency, scanning angle, and printing thickness. Among these curing gradients, the inadequate curing area was the main factor affecting the surface quality and physical properties of ceramics. The in-situ lamellar gaps were derived from the inadequate curing area. The lamellar gap size of green bodies was enlarged in brown bodies (sintering at 1170 ℃) and shrunk in sintered bodies (sintering at 1500 ℃). The correlations between the surface quality and printing parameters were clarified by establishing mathematical formulations of the lamellar gap size. Furthermore, anisotropic volume shrinkage, anisotropic surface topography, and density and hardness evolutions were associated with the wrapped particle density differences induced by the photoinduced curing gradients. Finally, the optimal printing parameters (75% of laser power, laser frequency of 150 kHz, scanning angle of 90º, printing thickness of 25 µm, and scanning speed of 2.5 m/s) were obtained by the orthogonal experimental analysis. This study provides a distinct understanding of the high-quality forming and extreme-performance manufacturing of stereolithography.     

Nearly stoichiometric BN fiber by curing and thermolysis of a novel poly[(alkylamino)borazine]

Boron nitride (BN) fiber with a composition of BN_1.09 was fabricated by curing and thermolysis of a novel poly[(alkylamino)borazine]. The processes have been studied by a combination of gel-content test, TGA, elemental analysis, IR, XPS, XRD, SEM and TEM. The results show that curing made polymer fiber infusible and resulted in a significant improvement of ceramic yield from 53.2 wt% to 73.8 wt% at 1000 °C. Moreover, pyrolysis in NH₃ at 1200 °C generated a nearly stoichiometric BN without carbonaceous impurities while in Ar led to a BNC material with carbon content of 6.13 wt%. The obtained amorphous BN fiber with a diameter of 13 μm displayed a tensile strength of approximately 600 MPa. Furthermore, the BN fiber illustrated good oxidation resistance in air.     

巴顿铅粉生极板固化工艺

本文主要讨论了巴顿铅粉生极板固化工艺对产品性能的影响,认为巴顿铅粉在生产应用中,采用高温高湿分阶段固化会提高蓄电池性能。     

Effect of curing time on the physicomechanical characteristics of the hardened cement pastes containing limestone

The effect of curing time on the physico-mechanical properties of the hardened Portland cement pastes containing limestone was studied. Five cement-limestone blends were prepared using 0%, 5%, 10%, 15%, and 20% of limestone as a partial substituent of Portland cement. The cement pastes were prepared using the standard water of consistency of 0.255, 0.255, 0.258, 0.261, and 0.263, respectively. The fresh pastes, thus produced, were moulded into 2×2×2-cm cubes. The pastes were first cured within the moulds at 100% relative humidity for 24 h, then the specimens were demoulded and cured under tap water for 3, 7, 14, and 28 days. At each hydration age, the hardened pastes were tested for bulk density, compressive strength, differential scanning calorimetery (DSC), and X-ray diffraction analysis (XRD). The results obtained were related as much as possible to the mechanical properties of the hardened cement pastes. The inclusion of limestone results in a notable improvement of the mechanical properties of the cement pastes containing limestone.     

Influence of high-temperature and low-humidity curing on chloride penetration in blended cement concrete

The influence of high-temperature and low-humidity curing on chloride penetration in concrete containing cement replacement materials was investigated. Three different mixes were studied: a control mix in which no cement replacement materials were added and two mixes where cement was partially replaced by 20% fly ash and 9% silica fume (by weight), respectively, at a constant water-to-binder ratio of 0.45. High-temperature curing was employed to simulate concrete temperature in hot climate. The results show that at early periods of exposure, initial curing has a substantial influence on chloride penetration in concrete. The effect of initial curing is much reduced after a long period of exposure. The chloride penetration at early ages of exposure is directly related to the porosity of the binder phase and the absorption of concrete. Higher chloride penetration resistance was observed when cement is partially replaced with either fly ash or silica fume.     

以淀粉作造孔剂制备多孔陶瓷

本文选用了普通陶瓷原料粉为主要原料,适当添加造孔剂和高温活性剂,经半干压成形,常规烧成,制备出了表观密度为1．15～1．52g／cm^3;显气孔率为30．9～47．7％;抗压强度为14．2～21．6MPa的多孔陶瓷。并对工艺参数的影响及微观结构进行了分析。