Effect of Synthesized Fe(OH)3 in Aluminum Sulfate Alkali-Free Liquid Accelerator on the Properties of Portland Cement

Abstract

Aluminum sulfate alkali-free liquid accelerators are shotcrete admixtures used in the construction of tunnels and culverts. However, the hydration rate of ordinary portland cement (OPC) with aluminum sulfate alkali-free liquid accelerators is slow, and it takes a long time for it to set. Nanomaterials or ultrafine materials can accelerate the hydration rate, reduce the setting time, and increase the early strength of cement. At present, the effect of ultrafine ferric hydroxide in an aluminum sulfate alkali-free liquid accelerator on OPC hydration has not been reported. In this study, ultrafine iron hydroxide [Fe(OH)3] was synthesized by the co-precipitation method, and the effect of the synthesized iron hydroxide (IH) in an aluminum sulfate alkali-free liquid accelerator on the hydration, setting time, and compressive strength of OPC was studied. The results show that the synthesized IH was amorphous, with an average particle size of approximately 260 nm. The introduction of IH provided nucleation sites for monosulfate (AFm), followed by Ca(OH)2 and AFm. Moreover, the synthesized IH could release Fe ions at pH 13, participate in the formation of hydration products, replace aluminum ions to form iron-containing ettringite, replace silicate to form iron-containing calcium silicate (C-F-S-H), and reduce the crystallite size of Ca(OH)2. The nucleation effect and the release of iron ions of synthesized IH acted synergistically to promote the hydration of the slurry, reduced the setting time, and adjusted the pore structure distribution, thereby improving the compressive strength at each age.