•The engineering properties of alkali-activated concretes (AAC) are determined.•Results are compared to existing models for portland cement concrete (PCC).•Models are proposed to predict tensile ...strength and modulus of AAC.•AAC exhibits higher tensile strength and lower Poisson’s ratio than PCC.•Alkali-activated slag concrete is much more brittle than PCC.
This paper presents an investigation into the tensile strength, modulus of elasticity, Poisson’s ratio, and stress–strain relationships of alkali-activated portland-cement-free concrete made with fly ash or ground granulated blast furnace slag (GGBFS) as the sole binder. Alkali-activated concrete is shown to be stronger in tension and have lower Poisson’s ratio than portland cement concrete. Relationships are proposed to estimate the tensile strength and modulus of elasticity based on the compressive strength of alkali-activated concrete, which are of the same form as those currently employed for portland cement concrete.
The early age reaction kinetics and microstructural development in alkali-activated slag binder are discussed. In-situ isothermal calorimetry was used to characterize the reaction progression in ...sodium hydroxide and sodium silicate-activated slag binders cured at ambient temperature. Microstructure and strength development were monitored to correlate the heat evolution with the property development. In-situ isothermal calorimetric data for sodium hydroxide-activated systems exhibited only one major heat evolution peak with no dormant period. Sodium silicate-activated pastes exhibited multiple peaks and extended dormant periods. Microstructural evolution, monitored using BSE–SEM, showed rapid product formation on the surface of slag grains in sodium hydroxide-activated systems, forming thin reaction shells—the thickness of which was related to the activator concentration—and leading to diffusion controlled hydration at a very early stage. Sodium silicate-activated systems exhibited slow and progressive product formation, predominately nucleated from the solution. These results are supported by electron mapping and electron dispersive X-ray spectroscopy.
•The effects of activators and temperature on activation kinetics are investigated.•Elevated temperature and increased activator alkalinity greatly accelerate hydration.•Increased silica retards ...hydration but improves later-age strength.•The main product is C-S-H with varying levels of hydrotalcite.
The early-age reaction kinetics of alkali-activated ground granulated blast-furnace slag (GGBFS) binders as determined by in-situ isothermal calorimetry are discussed in this paper. Particular attention is paid to the effects of activator type (sodium hydroxide and sodium silicate) and concentration, as well as curing temperature (23°C and 50°C). The mechanical strength development, microstructure, and product phase composition are also discussed to provide context for the phenomena observed in the kinetics results. It is shown for both activators that elevated temperature curing greatly accelerates hydration, resulting in more rapid product formation and strength development. High-molarity sodium hydroxide activators are shown to accelerate early hydration at ambient temperature, but tend to present a barrier to advanced hydration thereby limiting the later-age strength. Elevated temperature curing is shown to remove this barrier to advanced hydration by improving solubility and diffusivity. Hydration of sodium silicate-activated slag is comparatively slow, resulting in the delayed formation of very dense products with higher mechanical strength. Increasing sodium oxide tends to accelerate hydration, resulting in improved early- and later-age strength, while increasing the silica tends to retard the reaction, resulting in slower, more complete hydration as well as improved mechanical strength.
•We compare chloride permeability tests for alkali-activated concrete.•High pore solution conductivity causes problems with electrical methods.•Resistivity and RCPT work but classifications should be ...changed.
This paper presents the results of an experimental study of chloride permeability in alkali-activated fly ash, alkali-activated slag, and Portland cement concrete. Test methods include the rapid chloride permeability test (RCPT), AC and DC electrical resistivity, and the 90-day salt ponding test. We hypothesize that differences in pore solution chemistry between alkali-activated and Portland cement binders render electrical methods unable to accurately estimate chloride permeability in alkali-activated concrete. The present study seeks to evaluate this hypothesis by comparing results from electrical tests with diffusion coefficients from salt ponding tests. Contrary to previous claims, the RCPT provided a good estimate of chloride permeability in both alkali-activated slag and alkali-activated fly ash concrete. RCPT results showed excellent correlation with diffusion coefficients determined from salt ponding tests. Resistivity-measurements exhibited poor correlation to diffusion coefficients and overestimated the resistance to chloride ion penetration. Furthermore, AC and DC resistivity measurements showed significant disagreement for alkali-activated concrete. Finally, evidence form salt ponding tests suggests differences in chloride binding potential between alkali-activated and Portland cement concretes.
This paper presents an experimental investigation of the alkali-silica reactivity of alkali-activated concrete (AAC). Some researchers suggest that the high alkalinity of the pore solution could make ...AAC more susceptible to alkali-silica reaction (ASR) than comparable ordinary Portland cement (OPC) concrete, perhaps even with aggregates that are normally considered non-reactive. Many have further questioned the suitability of standardized ASR test methods for use with AAC. In an attempt to resolve this controversy, the authors subjected OPC, alkali-activated fly ash (AAF), and alkali-activated slag (AAS) concrete made with both reactive and non-reactive aggregates to accelerated and long-term ASR test protocols (ASTM C1567 and C1293). Paradoxically, some AAS mixtures with non-reactive aggregates showed significant expansion while AAF mixtures with reactive aggregates expanded minimally. AAS mixtures with reactive aggregates underwent extremely high expansions. Microstructural evaluation using scanning electron microscopy revealed significant cracking but did not identify ASR gel in the majority of specimens.
An experimental investigation into the micromechanical properties of alkali-activated slag cement (AASC) binders was carried out using targeted and grid nanoindentation. The results of grid ...indentation techniques were deconvolved using Gaussian mixture modeling with Bayesian model selection to determine the appropriate number of component phases for the model. The information given by the resulting mixture models and from targeted indentation experiments was disseminated in the context of existing information about the composition and development of the microstructure in AASC binders. The microstructure of sodium silicate-activated slag cement contains only two components (ground mass gel and unreacted slag cement) upon microscopic examination, but indentation data suggest that it is much more complex and varied. The microstructure of sodium hydroxide-activated slag cement contains ground mass gel, unreacted slag cement, and an inner product ring surround the unreacted slag. The inner product is denser, harder, and stiffer than the surrounding product phases. The micromechanical properties in sodium hydroxide-activated slag cement are not affected by activator molarity; the macroscale strength is similarly unaffected. Conversely, the micromechanical properties of sodium silicate-activated slag show a slight improvement with increased silica modulus, while the macroscale strength shows a significant improvement. The macroscale improvement is likely due to the increased size of unreacted slag cement grains, which are shown to be very hard and stiff.
Early hydration and setting of oil well cement Zhang, Jie; Weissinger, Emily A.; Peethamparan, Sulapha ...
Cement and concrete research,
07/2010, Letnik:
40, Številka:
7
Journal Article
Recenzirano
A broad experimental study has been performed to characterize the early hydration and setting of cement pastes prepared with Class H oil well cement at water-to-cement ratios (w/c) from 0.25 to 0.40, ...cured at temperatures from 10 to 60
°C, and mixed with chemical additives. Chemical shrinkage during hydration was measured by a newly developed system, degree of hydration was determined by thermogravimetric analysis, and setting time was tested by Vicat and ultrasonic velocity measurements. A Boundary Nucleation and Growth model provides a good fit to the chemical shrinkage data.
Temperature increase and accelerator additions expedite the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier setting; conversely, retarder and viscosity modifying agents delay cement nucleation, causing later setting times. Lower w/c paste needs less hydration product to form a percolating solid network (i.e., to reach the initial setting point). However, for the systems evaluated, at a given w/c, the degree of hydration at setting is a constant, regardless of the effects of ambient temperature or the presence of additives.
Over the years, the use of class F fly ash has been proven to be an efficient strategy to mitigate alkali–silica reaction (ASR) in concrete mixtures containing reactive aggregates. Prior research has ...identified the major mechanisms driving the mitigating action of fly ash to be, among others, alkali dilution and reduction in pore solution alkalinity due to the pozzolanic activity of fly ash. In this study, the relationship between these different mechanisms, their relative prevalence during the course of the ASR reaction, and their influence on the pore solution chemistry and nature of ASR products were explored in detail in a highly reactive model mortar system using a multi-analytical approach. Addition of fly ash to the mortar resulted in generation of fewer ASR products of similar nature than those produced in a control system. Raman spectroscopic analysis revealed the presence of aluminate monomer at the mouth of cracks in the aggregates, confirming that, in addition to the above-mentioned mechanisms, passivation of silica sites on the surface of aggregates due to the presence of aluminum in the pore solution was also a contributing factor to how fly ash mitigates ASR.
Characterization of a nontraditional binding material containing cement kiln dust (CKD) and ground granulated blast furnace slag (GGBFS) is discussed in this paper. Significant compressive strength ...was obtained for a CKD–GGBFS blend with 70% CKD and 30% GGBFS at a water-to-binder ratio of 0.40 after 2
days of curing at elevated temperature. Similar strength was also obtained for the samples subjected to normal moisture curing over a period of 28
days. The compressive strength increased with additional moist curing in both the cases. The microstructural and the mineralogical examinations show that the strength development was mainly due to the formation of calcium silicate hydrate (C-S-H). In addition to normal C-S-H, aluminum and magnesium incorporated C-S-H phases were also present in the CKD–GGBFS blends. The formation of ettringite appears to be a contributing factor in early age strength development of CKD–GGBFS binder.
This paper presents various mechanisms that can explain the cause of excessive autogenous shrinkage exhibited by alkali activated slag mortars (AASM) compared to Portland cement mortars (OPC). The ...influence of activator concentration parameters such as Na2O content and of the silica modulus (Ms) on the magnitude of autogenous shrinkage are evaluated, as are the proposed mechanisms. The results show that the shrinkage kinetics of AASM is strongly dependent upon the rate of reaction, internal relative humidity (RH) and surface tension of the pore solution. Pore size variation between AASM and OPC mortars and the corresponding tensile stresses based on the capillary tension approach were also calculated using experimentally measured internal RH and surface tension values. A higher amount of both Na2O and Ms. resulted in larger capillary stress resulting in greater autogenous shrinkage. Use of internal curing and of shrinkage reducing admixture were very effective in reducing the autogenous shrinkage of AASMs.
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