Durability Of Alkali-activated Slag Research Articles

Preparation of coal gasification coarse slag-based alkaline activator and its activation mechanism in alkali-activated slag

The good performance and durability of alkali-activated slag (AAS) make it to be a potential alternative to ordinary Portland cement. However, the preparation of activators such as water-glass is energy-intensive. In this paper, the feasibility of using a novel activator derived from coal gasification coarse slag (CGCS) for the preparation of AAS was investigated. The CGCS-based activators were the filtrates of CGCS and NaOH compound solution at 150 °C with different CGCS/NaOH ratios. By comparison with water glass-activated slag and NaOH-activated slag, the hydration reaction activity, mechanical property, hydrates composition, and microstructure of the CGCS-activated slag were studied. The results indicate that the compressive strength of CGCS-activated slag reached to 55.25 MPa at 28 d, much higher than NaOH-activated slag (43.08 MPa). CGCS-activated slag also performed the similar hydration dynamic to water glass-activated slag in terms of early heat release and hydrates assemblage. The amorphous silicon from CGCS dissolved in the filtrate played a crucial role in the activation effect of CGCS-based activator, which promoted the early hydration of CGCS-activated slag and contributed to the C-(A)-S-H with longer chain length. These findings proposed the mechanism of slag activated by CGCS-based activator, and provided an eco-friendly strategy for AAS production.

Improving sulfate and chloride resistance in eco-friendly marine concrete: Alkali-activated slag system with mineral admixtures

This study investigates the effects of various mineral admixtures, such as fly ash, metakaolin, and limestone powder, on the mechanical, chloride, and sulfate resistance properties of alkali-activated slag (AAS). The resistance of alkali-activated concrete (AAC) to chloride and sulfate is evaluated using the rapid chloride migration (RCM), wetting-drying cycles, SEM and XRD, TG/DTG. The results reveal that introducing fly ash improves workability but degrades the strength and durability of AAS. The 15% slag substitution by metakaolin is recommended to improve durability. Incorporating 15% limestone powder increases the compressive strength and reduces porosity due to filling and nucleation effects. The replacement ratio of 30% for each mineral admixture induces a considerable Ca2+ dilution, resulting in reducing the strength and durability of AAC. Under short-term wetting-drying cycles, the continued hydration of raw materials under MgSO4 erosion and the subsequent production of gypsum, hydrotalcite, and brucite from the matrix initially increase the AAC strength. However, the continued substitution of calcium in the C-(A)-S-H gel by magnesium and precipitation of gypsum under excessive cycles adversely deteriorates the compressive strength. The chloride migration degree in AAC is closely related to the porosity. Although the porous gel N-A-S-H facilitates chlorine migration in accelerated tests, it prevents further chlorine penetration by adsorbing chloride ions in large quantities, exhibiting the dominant effect on chloride immobilization. The formation of Friedel's salt, stemming from the chemical bonding of chloride, is present only in the high-calcium specimen G100 and the highly reactive aluminum specimen G85M15. Improvements in the gel chemistry and porosity are key factors in bolstering the durability of AAC. The findings of this study contribute to advancing the application of eco-friendly AAC in marine environments.

Degradation of alkali-activated slag subjected to water immersion

In this study, the impacts of tap water immersion on the pore solution, phase assemblages, gel chemistry and structure, and pore structure of alkali-activated slag (AAS) pastes were studied. AAS degrades under such condition and the potential mechanisms can be concluded as lower reaction rates, gel decomposition and carbonation. The leaching of Na+ and OH− at early stages hinders the reaction of slag, which leads to a slower formation of reaction products. Long-term leaching can result in gel decomposition after 90 d. Coarsened gel pores and capillary pores are both identified in water-immersed samples. Additionally, the leached Ca2+ can react with the dissolved CO2 in tap water to form calcium carbonate. A calcium carbonate layer is observed surrounding the paste while the inner matrix is free of carbonation. The insights provided by this paper contribute to understanding the behaviors and durability of AAS in underwater conditions.

Internal Curing Effect of Pre-Soaked Zeolite Sand on the Performance of Alkali-Activated Slag.

This study clarifies the effects of pre-soaked zeolite sand as an internal curing material on the hydration, strength, autogenous shrinkage, and durability of alkali-activated slag (AAS) mortars. The liquid-to-binder ratio (L/b) of all of the AAS mortars was 0.55. Sodium hydroxide solution was used as an alkali activator and an internal curing liquid. Calcined zeolite and natural zeolite sand replaced the standard sand at 15% and 30%, respectively. The setting time, autogenous shrinkage, compressive strength, ultrasonic pulse velocity, and surface electrical resistivity were tested. The following conclusions were drawn: (1) The addition of zeolite significantly reduces the autogenous shrinkage of AAS mortar. Compared with the control group, 30% calcined zeolite reduced the autogenous shrinkage by 96.4%. Moreover, the autogenous shrinkage of the AAS mortars was noticed in two stages (a variable temperature stage and an ambient temperature stage), and the two stages split at one day of age. (2) The compressive strength of all of the specimens increased as the zeolite sand content increased, and the highest compressive strength was obtained for AAS combined with 30% natural zeolite sand. (3) Internal curing accelerated the formation of the second peak of heat flow and reduced the accumulated heat release. (4) Calcined zeolite sand delayed the setting time of the AAS mortars. (5) The addition of zeolite significantly reduced the surface electrical resistivity of the AAS mortars. In summary, zeolite sand is extremely useful as an internal curing agent to reduce autogenous shrinkage and to increase the compressive strength of AAS mortars.

Durability of alkali activated slag in a marine environment: Influence of alkali ion

The durability of alkali-activated steel electric arc furnace slag (EAFS) in a marine environment was evaluated with respect to the chemical composition of the alkaline activator. Two different alkaline activators have been used: a mixture of NaOH and Na2SiO3 solutions (Na-activator), as well as a mixture of KOH and K2SiO3 solutions (K-activator). The obtained results gave the insight into the influence of alkaline activator chemistry on the compressive strength and durability of alkali-activated slag (AAS), which was exposed to the damaging seawater environment. The porosity of AAS was found to be the most important factor with regards to the strength and durability of these materials in marine environment. Sodium based alkali-activated slag (Na-AAS) displayed lower porosity and higher compressive strength compared to potassium based AAS (K- -AAS). Lower porosity and thus a lower rate of water uptake by AAS matrix, i.e., the lower sorptivity was exhibited by the Na-AAS when compared to K-AAS. Hence, Na-AAS exhibited better durability in a marine environment.

Resistance of alkali-activated slag concrete to acid attack

This paper presents an investigation into the durability of alkali-activated slag (AAS) concrete exposed to acid attack. To study resistance of AAS concrete in acid environments, AAS concrete was immersed in an acetic acid solution of pH=4. The main parameters studied were the evolution of compressive strength, products of degradation, and microstructural changes. It was found that AAS concrete of Grade 40 had a high resistance in acid environment, superior to the durability of OPC concrete of similar grade.

Sulfate attack on alkali-activated slag concrete

This paper presents an investigation into durability of alkali-activated slag (AAS) concrete in sulfate environment. Two tests were used to determine resistance of AAS concrete to sulfate attack. These tests involved immersion in 5% magnesium sulfate and 5% sodium sulfate solutions. The main parameters studied were evolution of compressive strength, products of degradation, and microstructural changes. After 12 months of exposure to the sodium sulfate solution, the strength decrease was up to 17% for AAS concrete and up to 25% for ordinary Portland cement (OPC) concrete. After the same period of exposure to the magnesium sulfate solution, the compressive strength decrease was more substantial, up to 37% for OPC and 23% for AAS. The main products of degradation were ettringite and gypsum in the case of Portland cement and gypsum in AAS. OPC samples had significant expansion, cracking, and loss of concrete, while AAS samples were not expanded but cracked in the test. During experiments with the sodium sulfate solution, some increase in strength of AAS concrete was recorded, likely due to continuing hydration.

Resistance of alkali-activated slag concrete to carbonation

This paper presents an investigation into durability of alkali-activated slag (AAS) concrete exposed to carbonation. Two tests were used which simulated exposure of AAS concrete to carbonated solution and to atmosphere high in carbon dioxide. These tests involved immersion of the concrete in 0.352 M sodium bicarbonate solution, and exposure to atmosphere with 10–20% of CO 2 at 70% relative humidity. It was found that the resistance of AAS concrete to carbonation was lower than that of ordinary Portland cement (OPC) concrete, and AAS concrete had higher strength loss and depth of carbonation than OPC concrete in both tests.

Resistance of alkali-activated slag concrete to alkali–aggregate reaction

This paper presents an investigation into durability of alkali-activated slag (AAS) concrete exposed to alkali–aggregate reaction (AAR). In the concrete prism test method (ASTM C 1293), reactive aggregates introduced in the AAS matrix reacted with alkali–silica gel formed around aggregates. The expansion of AAS concrete prisms was 0.04% after 50 days and 0.1% after 22 months. The expansion in OPC concrete was about 0.03% after 22 months of experiment. It was found that AAS concrete had lower resistance to alkali–aggregate attack than that of OPC concrete of similar grade.