Effect of Sodium Aluminate Dosage as a Solid Alkaline Activator on the Properties of Alkali‐Activated Slag Paste

Abstract

Extensive research into alkali-activated slag as a green gel material to substitute for cement has been done because of the advantages of low-carbon dioxide emissions and recycling of industrial solid waste. Alkali-activated slag usually has good mechanical properties, but the too fast setting time restricted its application and promotion. Changing the composition of alkaline activator could optimize setting time, usually making it by adding sodium carbonate or sodium sulfate but this would cause insufficient hydration reaction power and hinder compressive strength growth. In this paper, the effect of sodium aluminate dosage as an alkaline activator on the setting time, fluidity, compressive strength, hydration products, and microstructures was studied through experiments. It is fair to say that an appropriate amount of sodium aluminate could obtain a suitable setting time and better compressive strength. Sodium aluminate provided enough hydroxyl ions for the paste to promote the hydration reaction process that ensured obtaining high compressive strength and soluble aluminium formed precipitate wrapped on the surface of slag to inhibit the hydration reaction process in the early phase that prolonged setting time. The hydration mechanism research found that sodium aluminate played a key role in the formation of higher cross-linked gel hydration products in the late phase of the process. Preparing an alkali-activated slag with excellent mechanical properties and suitable setting time will significantly contribute to its application and promotion.

One part alkali activated materials: A state-of-the-art review

“…Replacing a proportion of Na 2 SiO 3 with either Na 2 CO 3 , NaAlO 2 or Ca(OH) 2 in FA-and GGBS-based one-part mixtures caused a reduction in flow [83,[167][168][169]. Ouyang et al [170] combined silicate-based activators (either quick-dissolving Na 2 SiO 3 or Na 2 SiO 3 ⋅5H 2 O) with either Na 2 CO 3 or Ca(OH) 2 to activate FA-and GGBS-based one-part mixtures and showed a higher reduction in workability with Ca(OH) 2 replacement as opposed to Na 2 CO 3 .…”

“…A higher replacement of Na 2 SiO 3 reduces the pH of the mix, causing a more moderate reaction rate [167,174]. Also, the delay in setting due to NaAlO 2 addition occurs as aluminate species coagulate the silicate species in Na 2 SiO 3 and absorb to the precursor particles, which could generate negative charge sites that repel the OH − responsible for catalysing the dissolution of aluminosilicate species [169]. A high solid activator dosage typically leads to shorter setting times.…”

Study on the Influence of Alkali Activator Solutions on Strength Improvement of Pozzolan Calcium Hydroxide Binders

“…Increase in volume due to alkali-silica formation at early age of concrete or mortar is limited due to confinement in formworks or moulds. Apart from alkali-silica reaction in pozzolanic binders but also alkali-aluminum reaction can take place in the same system provided there are reactive alkali and aluminum elements in the solution (Chen et al, 2021). There have been no reports of concrete and mortar being destroyed because alkali-aluminum gels formed during the early and late days of curing.…”

Experimental Study on the Durability of Alkali‐Activated Slag Concrete after Freeze‐Thaw Cycle

“…Owing to the effects of frost heaving and hydration, the microstructural surface began to form reticulated or flocculent gel materials, and small voids appeared. Meanwhile, a small amount of needle-shaped ettringite (Aft) was found to affect the mechanical properties [20]; after 300 freeze-thaw cycles, different degrees of avulsion and breakage were generated in the cement mortar, which split, and the compressive strength decreased sharply. With the number of freeze-thaw cycles increased, the microstructure of the OCC was destroyed rapidly.…”

“…Based on the EDS analysis, it was found that the Ca/Si ratio inside the C-S-H gel generated from the AASC hydration product was approximately 1.0, which was considered to possess low Ca/Si, and was denoted as the C-S-H(I) gel [20]. is was different from the C-S-H (Ca/Si approximately 1.5) gel and the Ca(OH) 2 generated in the OCC hydration product.…”