Effects of Borax, Sucrose, and Citric Acid on the Setting Time and Mechanical Properties of Alkali-Activated Slag

Abstract

The setting time of alkali-activated slag (AAS) binders is extremely short, while traditional retarders of Portland cement may be invalid for AAS. To find an effective retarder with a less negative impact on strength, borax (B), sucrose (S), and citric acid (CA) were selected as potential retarders. The setting time of AAS with different admixtures dosages of 0%, 2%, 4%, 6%, and 8%, and the unconfined compressive strength and beam flexural strength of 3 d, 7 d, and 28 d AAS mortar specimens were tested. The microstructure of AAS with different additives was observed by scanning using an electron microscope (SEM), and the hydration products were analyzed by energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD), and thermogravimetric analysis (DT-TGA) to explain the retarding mechanism of AAS with different additives. The results showed that the incorporation of borax and citric acid could effectively prolong the setting time of AAS more than that of sucrose, and the retarding effect is more and more obvious with the increase in borax and citric acid dosages. However, sucrose and citric acid negatively influence AAS’s unconfined compressive strength and flexural stress. The negative effect becomes more evident with the increase in sucrose and citric acid dosages. Borax is the most suitable retarder for AAS among the three selected additives. SEM-EDS analysis showed that the incorporation of borax does three things: produces gels, covers the surface of the slag, and slows down the hydration reaction rate.

Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar

“…In comparison to the effect of electrostatic repulsion, the limited availability of free water for dispersing GGBS particles made the fluidity more sensitive to the loss of free water 51 , 52 . Borax hydrolysis produced boric acid and borate ions under alkaline conditions 40 . The hydrolysis reaction equations for borax are shown in (4) and (5).…”

“…Contrary to gelatinization, this complex reaction mainly affected the AAS mortar strength at the 3 d due to the semipermeable membrane properties of the complex on GGBS surface. As hydration progressed, the complex gradually permeated and disintegrated, normalizing the hydration reaction 40 . The complex reaction was shown in formula ( 6 ).…”

“…Bagheri et al 39 compared the environmentally friendly substitution of borax in AAMs and geopolymers and found that a 10–30% borax content improved the mechanical properties of the geopolymers more significantly than AAMs. Li et al 40 investigated the impact of borax as an admixture on the performance of AAS mortar. Their research revealed that, despite the initial formation of calcium borate complexes due to borax addition, which slowed down the hydration reaction, it had no effect on the later-stage strength development of the AAS mortar.…”

Effect of Retarders on the Reactivity and Hardening Rate of Alkali-Activated Blast Furnace Slag Grouts

“…Various patents and studies have shown that admixtures can effectively delay the setting of activated slags [10][11][12][13]. A recent study showed the use of borax and citric acid was more effective than sucrose in extending the setting time of alkali-activated slags [14]. The research revealed that both sucrose and citric acid had an adverse impact on the compressive strength.…”

“…Sodium tetraborate can extend the setting time by altering the gel structure and mortar properties through the transformation of Al-O-Si bonds to B-O-Al-O-Si [18,19]. Another study stated that the incorporation of borax produced gels, covered the surface of the slag, and slowed down the hydration reaction rate [14]. While the design of cement-based materials requires various retarders to achieve optimal results, these same retarders cannot be used in the same conditions for alkali-activated materials due to certain limitations [14].…”