Effects of Adding Neutralized Red Mud on the Hydration Properties of Cement Paste

Abstract

Red mud is a highly alkaline waste by-product of the aluminum industry. Although recycling of red mud is being actively researched, a feasible technological solution has not been found yet. In this study, we propose that neutralization of red mud alkalinity could assist in its use as a construction material. Neutralized red mud (LRM + S; pH 6–8) was prepared by adding sulfuric acid to liquefied red mud (LRM; pH 10–12). After adding LRM and LRM + S to cement paste, the heat of hydration, compressive strength, and hydration products were examined. The experiments revealed that the calorific value of the cement paste with LRM was low, and its peak was delayed, when compared with that of plain cement paste (referred to as Plain), whereas the calorific value of the cement paste with LRM + S was similar to that of Plain. At the age of 28 d, the compressive strength of the cement paste with 10% LRM + S was 99% whereas that with 20% LRM was only 55% of the strength of Plain. Thus, our results help to resolve the issue of strength degradation of cementitious materials observed upon the addition of red mud and enable its reuse as a construction material.

Hydration Properties of Cement with Liquefied Red Mud Neutralized by Nitric Acid

“…In particular, the neutralized samples had higher compressive strengths than Plain after 1 day of curing. In addition, LRM 20 had a particularly low compressive strength ratio of 2%, which was consistent with the results of a previous study showing that the addition of LRM greatly reduced the early compressive strength of cement [ 16 ]. In a previous study, a cement paste with LRM neutralized by sulfuric acid had a compressive strength ratio of less than 90% after 1 day of aging [ 16 ], while our LRMN samples neutralized by nitric acid had a higher strength than the Plain sample.…”

“…In addition, LRM 20 had a particularly low compressive strength ratio of 2%, which was consistent with the results of a previous study showing that the addition of LRM greatly reduced the early compressive strength of cement [ 16 ]. In a previous study, a cement paste with LRM neutralized by sulfuric acid had a compressive strength ratio of less than 90% after 1 day of aging [ 16 ], while our LRMN samples neutralized by nitric acid had a higher strength than the Plain sample. This is attributed to the accelerated hydration and highly developed strength in the presence of NaNO 3 , which was generated by the nitric acid neutralization of LRM, as confirmed by the XRD results in Figure 3 [ 19 ].…”

“…In this study, RM sludge (KC Co., Mokpo, Korea) was used as LRM and nitric acid-neutralized LRM (hereafter, LRM + N). Similar to previous studies, LRM was prepared by mixing RM sludge, water, and polycarbonate thickener at a ratio of 1:0.2:0.0036 w.r.t mass of RM sludge with a moisture content of approximately 36 wt.% [ 16 ]. First, RM sludge was mixed with water for approximately 3 min using a homomixer (K&S Co. Ltd., Seongnam, Korea), as shown in Figure 1 .…”

“…These samples were dried in an oven at 40 °C for 3 days. The dried samples were crushed and passed through a 200-mesh sieve for the XRD analysis (Rigaku, SmartLab, Tokyo, Japan) [ 16 ]. XRD analysis was performed using a CuKa wavelength at 45 kV and 200 mA and at 4°/min in the range of 2θ = 5–75°.…”

“…By adding 10% LRM neutralized by sulfuric acid to cement, its 28-day compressive strength can represent about 99% of that plain of the plain mixture. However, the use of nitric acid-neutralized red mud significantly lowers the initial strength of the cement paste, particularly the daily strength by up to 83% and the 3-day strength by up to 68% [ 16 ].…”