Alkaline Accelerators Research Articles

Produce low-CO2 sustainable cement–slag binder incorporating aluminum sulfate

Using slag to produce low-carbon building materials is a sustainable strategy to recycle industrial byproducts. However, with a high clinker substitution rate, the delayed early strength development of slag–cement binders severely limits their applicability in practical engineering. This study investigated the mechanism of action by which aluminum sulfate promotes early strength. Its influence on hydration, microstructure, hydration products, mechanical properties, and CO2 emissions was investigated. The analysis indicated that aluminum sulfate accelerated the dissolution of C3S during the induction period, but the precipitation of a large amount of ettringite hindered further hydration. However, this hindering effect decreases as hydration continues. At a 1% dosage, the CO2 emission per unit strength is the lowest. At a 2% dosage, the early strength can be effectively increased by 38.01%. By comparison, alkali-free aluminum sulfate improves the sustainability and mechanical properties better than an alkaline accelerator (sodium carbonate).

Catalyst-Free gem-Difluorination/Spirocyclization of Indole-2-carboxamides: Synthesis of C2-Spiroindoline Derivatives.

A catalyst-free gem-difluorination/spirocyclization reaction has been successfully developed for the synthesis of gem-difluorinated C2-spiroindoline derivatives from indole-2-carboxamides. The resulting gem-difluorinated C2-spiroindolines can be easily converted into 2-spiropseudoindoxyls through hydrolysis. This method offers the benefits of simple operation, convenient access to raw materials, and mild conditions. Dual function of Selectfluor in this reaction is noteworthy as it can serve as both a fluorinating agent and an alkaline accelerator precursor.

Influence of main components of accelerators on mechanical property and hydration of Portland cement in a dry-hot geothermal environment

The behavior of accelerators in the shotcrete of tunnel structure is significantly affected by the high geothermal tunnel environment. The objective of this study is to investigate the influence of main components of alkali-free accelerators (aluminum sulfate, AS) and alkaline accelerators (sodium aluminate, NA) on mechanical property and hydration of Portland cement in a dry-hot geothermal environment of 80 ℃ (DH). Setting time test, compressive strength test, XRD, TGA, MIP and SEM were performed on cement systems with AS and NA. Results show that relative to standard curing environment, the cement hydration was promoted and the pore structure at early stage was optimized in DH, thereby significantly accelerating the setting time of cement pastes and improving the early compressive strength development of mortars. In contrast, the later cement hydration was retarded and pore structure was detrimental in DH, resulting in abnormally slow development of later strength and even reduction. Additionally, the stability of hydration products such as ettringite (AFt) and monosulfoaluminate hydrate (AFm) would decline in DH. DH could accelerate the dehydration and decomposition of AFt, and promote the transformation process from AFt to AFm at early stage. After 28 days, AFm would convert into hemicarboaluminate due to carbonation. The addition of AS elevated the hydration degree and optimized the pore structure, especially at later stage, thus exhibiting more stability in long-term strength. Nevertheless, the incorporation of NA would bring about serious deterioration phenomenon in DH, and the compressive strength almost stagnated after 1 day. This work provides a guidance for the real application of accelerators in the shotcrete of tunnel structure in a dry-hot geothermal environment.

Preparation and hydration mechanism of aluminum formate/aluminum sulfate based alkali-free composite accelerator for sprayed concrete

The high alkali content of alkaline accelerators has a negative influence on the late strength and durability of concrete. Further, their corrosive nature is harmful to the skin of construction personnel, posing safety concerns. Therefore, the development of alkali-free accelerators has gained interest, especially in shotcrete-based constructions. The production process of aluminum-sulfate-based alkali-free accelerators is simple and less exothermic, with no negative impact on the later strength of concrete, aspects that are widely recognized and very important in construction. However, their high sulfate content may have a significant impact on the durability of concrete; therefore, reducing the sulfate content plays an important role in enhancing the durability of concrete. In this study, an alkali-free liquid accelerator was synthesized by replacing aluminum sulfate with aluminum formate, which effectively reduced the sulfate content. Experiments were performed on concrete samples in which aluminum sulfate was replaced with aluminum formate at different levels. The results showed that the formate and 3CaO·Al2O3 (C3A) of the alkali-free liquid accelerator prepared from aluminum formate form calcium aluminate, similar to the ettringite phase. The formate promoted the dissolution of Ca3SiO5 (C3S), thus accelerating the hydration of concrete. Meanwhile, aluminum ions hydrolyzed by aluminum formate also participated in the reaction, resulting in the dual participation of cations and anions in hydration, leading to a synergistic effect with aluminum sulfate.

Influence of alkaline and alkali-free accelerators on strength, hydration and microstructure characteristics of ultra-high performance concrete

To broaden the application of ultra-high performance concrete (UHPC) in engineering fields with high-early-strength requirements, accelerators were used to accelerate the setting and hardening of UHPC. This study investigated how alkaline accelerator (NA) and alkali-free accelerator (AS) influence the strength, hydration and microstructure characteristics of UHPC. To achieve the proposed objective, the terms of setting time, strength, released ionic concentration, isothermal calorimetry, X-ray diffraction, scanning electron microscope, energy dispersive X-ray and mercury intrusion porosimetry were performed. The results show that NA and AS significantly increase the setting and hardening rate of UHPC, and their optimal dosages are 2% and 4%, respectively. 2% NA increases the 1 d compressive, splitting tensile and flexural strengths of UHPC by 55.8%, 62.2% and 56.5%, respectively, while 4% AS increases by 10.8%, 21.6% and 21.0%, respectively. However, their later strengths decrease slightly. The first exothermic peak values of NA-2% and AS-4% are about 3 times higher than the reference paste, and their main hydration peaks appear earlier by 38.2 and 23.3 h, respectively. NA and AS contribute to the rapid formation of hydration products and refine the pore structure of pastes.

“Dormancy” and “Awakening” Method Used for Fresh Mortar Waste Recycling and Reuse

A safe and zero hazardous method is proposed to recycle and reuse fresh mortar waste. In this method, a dormancy agent and an awakening agent were used together in this study to reuse waste cement mortars. Citric acid (CA) was used as the dormancy agent to retard the hydration of cement, and an alkaline accelerator (AA) was used as the awakening agent to accelerate the hydration of cement retarded by CA at an early age. Autogenous shrinkage, dry shrinkage and quality loss, compressive strength, capillary water absorption, and rapid chloride penetration testing were performed for the mortars. A setting time test, X-ray diffraction, and thermogravimetric analysis (TGA) were performed for the pastes (without sand). The results showed that the addition of 0.3% of CA solution increased the setting time significantly. This was because the citrate ions were absorbed onto the surface of the cement particles and a protective film was formed around the cement particles to retard dissolution. However, the retarding effect was eliminated by the AA, which was mainly composed of sodium silicate due to the contribution to initial calcium consumption. With the increasing dosage of the AA, the autogenous shrinkage, dry shrinkage, water absorption, and total charges passed were increased continuously compared to the reference. However, the influence was insignificant at a low dosage of the AA (1% and 1.5%). Obviously, the compressive strengths of the mortars with the AA were increased compared with the reference at the curing times of 3 d and 7 d. On the basis of not reducing the compressive strength of mortar, the reutilization of fresh mortar waste is realized. From this work, it was found that the reuse of waste cement mortars was feasible with the combined utilization of CA and the AA, which could provide some theoretical basis and experimental data for engineering applications.

Effect of Mitigating Strength Retrogradation of Alkali Accelerator by the Synergism of Sodium Sulfate and Waste Glass Powder

This work aims to utilize waste glass powder (WGP) as a plementary material to mitigate the strength shrinkage caused by the alkaline accelerator. Waste glass power was used to replace cement by 0%, 10%, and 20% to evaluate waste glass powder on the alkaline accelerator’s strength retrogradation. The results show that the strength improvement effect of unitary glass powder is inconspicuous. Innovative methods have been proposed to use sodium sulfate and waste glass powder synergism, using the activity of amorphous silica in glass powder. Compared with the reference group, the compressive strength of 28d mortar increases by 67% when the sodium sulfate content is 2.5%, and the replacement amount of waste glass powder is 10%. Besides, XRD and SEM analysis of hydration products also confirmed that the synergistic effect of sodium sulfate and waste glass powder could reduce strength inversion. The findings presented in this paper are pivotal for using waste glass to solve the problem of strength inversion caused by the alkaline accelerator.

Influence of shotcrete accelerators on the hydration of cement pastes and their impact on sulfate resistance

Alkali-free, aluminium-based accelerators are often used for shotcrete linings in tunnels and mines, where external sulfate attack can occur. Therefore, it is essential to know, if the influence of the accelerators on the cement hydrate assemblage negatively impacts sulfate resistance. This study focuses on the effect of aluminium-based accelerators on cement hydration and the consequences for external sulfate attack. Aluminum sulfate-based accelerators cause rapid setting due to the very early formation of ettringite and accelerate alite hydration. At late ages, significant more AFm phases are formed compared to the reference without accelerator, which during storage in sodium sulfate solution react to ettringite. A significantly higher volume increase due to the formation of additional ettringite during sulfate exposure was calculated by thermodynamic modelling for the accelerated paste compared to the reference. Alkaline accelerators based on sodium aluminate form mainly amorphous calcium (sulfo-) aluminate hydrates at very early age, while ettringite seems to be destabilized. The ettringite quantities formed during storage in sodium sulfate solution are significantly higher than for the reference and the paste with the alkali-free accelerator. This finding compares well to experiments on concrete specimens, where in the mixture with the alkaline accelerator deleterious expansion was observed.

External sulphate attack of sprayed mortars with sulphate-resisting cement: Influence of accelerator and age of exposition

This work evaluates the influence of the accelerator type, the cement type and age of exposure on the degradation mechanism and the durability of sprayed mortars subjected to external sulphate attack (ESA). Cores and prisms were extracted from panels sprayed with 8 mortar compositions (with 2 sulphate-resisting cement types and 4 setting accelerators) and then exposed to a sulphate solution at the ages of 7 or 28 days for 400 days. The evolution of the ESA was assessed through XRD, SEM, compressive strength, dimensional variation and ultrasonic pulse velocity. Results show that alkaline accelerators increase drastically the vulnerability of the matrix to the ESA. The degradation is enhanced by the higher solubility of aluminate phases and the increased formation of expansive phases. Results reveal that the use of sulphate-resisting cement might not suffice to mitigate severe material degradation.

Test and simulation of cement hydration degree for shotcrete with alkaline and alkali-free accelerators

In this paper, adiabatic temperature rise test was used to evaluate the effect of the type and content of accelerator on cement hydration in shotcrete. Two kinds of accelerators, alkaline and alkali-free accelerators were used in the experiments. For each type of accelerator, two different concrete mixture were used, and the dosage of the accelerator was 3% and 7% respectively. The experimental and theoretical results show that the use of alkaline or alkali-free accelerator can significantly improve the hydration rate in the first and second stage for shotcrete, but the alkaline accelerator will reduce the final hydration degree obviously, however the effect of alkali-free accelerator is small or even slightly increase. With the increase of the content of accelerator, the final hydration degree of concrete decreases, whether for alkali-free or alkaline accelerators. The mechanism may present in two aspects. For alkali-free accelerator, the hydration products are mainly rod-shaped AFt which will make concrete lose fluidity but do not completely cover the surface of the cement particles, and the acid substances in alkali-free accelerator will accelerate the dissolution of C3S, thus promoting the hydration reaction. However, for the alkaline accelerator, the hydration products are mainly composed of C-A-H, AFm and a small amount of C–S–H gel which will wrap on the surface of cement particles, thus affect the further hydration of cement particles. The proposed cement hydration rate model can well predict the development process of the hydration degree for shotcrete, especially for the early age concrete.

The effect of sodium aluminate on the properties of the composite cements

The effect of sodium aluminate activator on properties of the Portland-composite cement (clinker factor 65 %) was studied and the results are being discussed. It is shown that the introduction of sodium aluminate (Na[Al(OH)4]) leads to an increase in the water demand of cement and a decrease in its strength. The results showed that the sodium aluminate and polycarboxylate ether (PCE) admixtures ensure the production of higher strength cements. It has been found that the addition of Na[Al(OH)4] – PCE significantly increases the compressive strength of cement mortar, particularly at 10–48 h of hydration. XRD and SEM measurements confirm that the hydration of composite cement with the addition of Na[Al(OH)4] – PCE is greatly accelerated by the way of formation of intense lines ettringite. It has been shown that the introduction of a sodium aluminate into the cement mortars allows to accelerate hardening and increase early strength and using of composite cement will provide improved performance and suitability. The results have shown that alkaline accelerator and polycarboxylate were effective for decrease shrinkage of composite cements.

Parameters controlling early age hydration of cement pastes containing accelerators for sprayed concrete

The objective of this work is to parametrize the early age hydration behavior of accelerated cement pastes based on the chemical properties of cement and accelerators. Eight cements, three alkali-free and one alkaline accelerators were evaluated. Isothermal calorimetry, in situ XRD and SEM imaging were performed to characterize kinetics and mechanisms of hydration and the microstructure development. The reactivity of all accelerators is directly proportional to their aluminum and sulfate concentrations and to the amount and solubility of the setting regulator contained in cement. Alite hydration is enhanced if a proper C3A/SO3 ratio (between 0.67 and 0.90) remains after accelerator addition and if limestone filler is employed, because undersulfated C3A reactions are avoided. Combinations of compatible materials are recommended to enhance the performance of the matrix and to prevent an undesirable hydration behavior and its consequences in mechanical strength development.

High-temperature behavior of structural and non-structural shotcretes

Sprayed concrete (shotcrete) is well known as a reliable and effective material for rock stabilization, fire proofing of metallic structures and jacketing of R/C members. Shotcrete structural applications, however, have been so far very limited mainly because of some concerns about material’s durability and high-temperature behavior. The latter issue is the starting point of this research project aimed to investigate the thermo-mechanical properties of three shotcretes containing different accelerating agents (based on sodium silicates in one mix – C1, and on sulfo-aluminates in two mixes – C2/C2F, no steel fibers/with steel fibers). The objective is to check (a) whether the heat-triggered mechanical decay of shotcrete is similar to that of ordinary concrete, and (b) how shotcrete low thermal diffusivity and relatively large porosity evolve at high temperature.The mechanical properties in compression are investigated both at high temperature (hot tests, C2 and C2F) and past cooling (residual tests, all mixes).In terms of normalized mechanical decay, the two shotcretes containing an alkali-free accelerating agent behave similarly to ordinary concrete, while the shotcrete with an alkaline accelerating agent is more heat and age sensitive. Up to 850 °C, all mixes exhibit a markedly lower thermal diffusivity compared with ordinary concrete, and a higher porosity. Furthermore, the rather low mechanical properties of the shotcrete with an alkaline accelerator with respect to the base material make it hardly fit for structural purposes, while the two shotcretes with an alkali-free accelerator are as good as any ordinary concrete even at high temperature, as demonstrated by the basic structural application presented at the end of the paper, concerning the lining of a circular deep R/C tunnel exposed to the standard fire.

Influence of spraying on the early hydration of accelerated cement pastes

In practice, most of the studies about the interaction between cement and accelerators is performed with hand-mixed pastes. However, in many applications mixing occurs through spraying, which may affect accelerators reactivity and the microstructure of the hardened paste. The objective of this study is to analyze how the mixing process influences the early hydration of accelerated cement pastes. Isothermal calorimetry, X-ray diffraction, thermogravimetry and SEM imaging were performed on cement pastes produced by hand-mixing and by spraying, using equivalent doses of an alkali-free and an alkaline accelerator and two types of cement. Results showed a great influence of the spraying process on the reactivity of accelerators and on the morphology of the precipitated hydrates. Variations in hydration kinetics caused by the mixing method are explained and the results obtained might have a significant repercussion on how future research on the behavior of accelerated mixes will be performed.

Early age hydration of cement pastes with alkaline and alkali-free accelerators for sprayed concrete

The objective of this study is to evaluate the early age hydration mechanisms of accelerated CEM I pastes. Liquid phase analysis, conductimetry, isothermal calorimetry, in situ XRD and SEM were performed on cement pastes produced with equivalent doses of an alkaline accelerator (sodium aluminate solution) and of an alkali-free accelerator (aluminum sulfate solution). Results showed that the addition of these chemicals first changes the ionic equilibria of the medium by the consumption of Ca2+ and SO42− ions from the liquid phase. The alkali-free accelerator contributes to the rapid formation of ettringite, as well as to a faster rate of alite dissolution and hydration. The aluminate accelerator leads to AFt and AFm formation, rapidly depleting gypsum and filling up the space, inhibiting further alite hydration. According to the results, variations in cement hydration mechanisms caused by accelerators with different chemical compositions are explained.

Ultrasound monitoring of the influence of different accelerating admixtures and cement types for shotcrete on setting and hardening behaviour

The possible use of ultrasound measurements for monitoring setting and hardening of mortar containing different accelerating admixtures for shotcrete was investigated. The sensitivity to accelerator type (alkaline aluminate or alkali-free) and dosage, and accelerator–cement compatibility were evaluated. Furthermore, a new automatic onset picking algorithm for ultrasound signals was tested. A stepwise increase of the accelerator dosage resulted in increasing values for the ultrasound pulse velocity at early ages. In the accelerated mortar no dormant period could be noticed before the pulse velocity started to increase sharply, indicating a quick change in solid phase connectivity. The alkaline accelerator had a larger effect than the alkali-free accelerator, especially at ages below 90 min. The effect of the alkali-free accelerator was at very early age more pronounced on mortar containing CEM I in comparison with CEM II, while the alkaline accelerator had a larger influence on mortar containing CEM II. The increase of ultrasound energy could be related to the setting phenomenon and the maximum energy was reached when the end of workability was approached. Only the alkaline accelerator caused a significant reduction in compressive strength and this for all the dosages tested.

Alkali-activated binders by use of industrial by-products

Cement kiln dust (CKD) materials are used as alkaline accelerators for latent hydraulic substances and as alkali activators for different alumosilicate materials, including ground-granulated blast furnace slag, low-calcium fly ash and metakaolin. The dusts differ in their phase composition, especially in the amount of reactive phases and the kind and amount of alkali salts. The quantitative phase composition, pore solution composition and strength behavior of the activated blends is reported.

The influence of alkali-free and alkaline shotcrete accelerators within cement systems: Influence of the temperature on the sulfate attack mechanisms and damage

The resistance to sulfate attack of mixtures accelerated with alkali-free and alkaline accelerators was found to be mainly influenced by the Al3+ and SO42− added via the admixtures. Microstructural observations showed decalcification and disintegration of the CSH gel, which acted as an additional Ca2+ supplier as compared to the CH for ettringite formation. The CSH decalcification was mainly observed with a homogeneous distribution of the alkali-free admixture. The disintegration of the CSH gel increased the porosity and allowed more sulfate solution to penetrate into the specimens. This process promoted the swelling of the specimens and directly contributed to the expansion, explaining the lack of a direct relationship between the ettringite formation and the expansion. Moreover, the CSH gel disintegration, typical for MgSO4 attack, also occurred with Na2SO4 solutions and depending on the aluminate–sulfate distribution and the extent of the CSH gel disintegration, different damage types were detected. At higher temperatures (65 °C) the damage was mainly controlled by the growth, the rearrangement and the thermal stability of ettringite.

The influence of alkali-free and alkaline shotcrete accelerators within cement systems: I. Characterization of the setting behavior

The influence of alkali-free and alkaline accelerators on the setting behavior of cementitious mixtures is investigated. The aluminium sulfate present in the alkali-free accelerators promotes the crystallization of ettringite prisms (2–7 μm) between the clinkers, thus connecting them in a compact mass and accelerating the setting. The very rapid setting promoted by the addition of the alkali KAl(OH) 4-rich admixture is mainly the result of the precipitation of CH plates and amorphous KCASSH hydrates. In this case, the crystallization of elongated ettringite rods contributes to a smaller extent to the setting.

Scientific Serials

American Journal of Science, March.—Trinidad pitch, by S. F. Peckham and Laura A. Linton. This paper gives an account of the physical and chemical properties of pitch from the Pitch Lake of Trinidad, together with a map of the lake itself. A dry sample of the true lake pitch contained 34·2 per cent, petrolene, 18·8 per cent, asphaltene, 11·4 per cent, of other organic matter, and 35·6 per cent. of inorganic matter. The pitch as it occurs is a unique substance found nowhere else in nature. It consists of a mixture of bitumen, water, sand, decayed vegetation, and gas in such definite proportions that within certain limits the composition of the entire mass is uniform.—Proofs of the rising of the land round Hudson Bay, by Robert Bell. The old shorelines in the provinces of Ontario and Quebec slope upward in a north-easterly direction at rates varying in different regions from a few inches to a foot and even two feet per mile. Many former landing-places about the bay are now high and dry. The rising is apparently still in progress.—Experiments upon the kathode rays and their effects, by A. W. Wright. In developing “shadowgraphs,” it is better not to use any alkaline accelerator at all until just at the end of the process. Röntgen rays passing through glass walls do not show magnetic deflection or mutual repulsion; but when they are made to pass through gold-leaf instead, they show traces of these phenomena, probably owing to the fact that they carry with them small portions of volatilised and electrified metal.—Triangulation by means of kathode photography, by John Trowbridge. The principle of triangulation may be applied to kathode photography when determining the situation of metallic particles in the body. By using two vacuum tubes in different positions, two pictures of, say, a bullet embedded in a hand may be obtained, and their distance apart gives the depth at which the bullet may be sought.—Notes of observations on the Röntgen rays, by H. A. Rowland, N. R. Carmichael, and L. J. Briggs. Some photographs of a coin obtained by Röntgen's method, showed no penumbra when the coin was 2 cm. from the plate. In a very high vacuum tube the source of the active rays was distinctly traced to the anode.