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Strength Development and Pore Structure Characterisation of Binary Alkali-activated Binder Based on Tungsten Mining Waste

The mineralogical properties of tungsten mining waste mud (TMWM) make its valorisation and re-usage as an alumino-silicate source material to produce an alkaliactivated binder without calcination is a challenge. Moreover, the dissolution of silicate and alumina species from TMWM is very slow. Despite the crystallinity of TMWM, this study demonstrates that its combination with other sources of alumino-silicate source waste materials – such as red clay brick waste (RCBW), ground granulated blast furnace slag (GGBFS) and electric arc furnace slag (EAFS) – improved the compressive strength and the pore structure of the alkali-activated matrix. The combined precursors (90 vt.%TMWM+10 vt.%RCBW, 90 vt.%TMWM+10 vt.%GGBFS, and 90 vt.%TMWM+10 vt.%EAFS) were activated using a combination of alkaline activator solutions (sodium silicate and sodium hydroxide) with the ratio of 1:3 (66.6 wt.% sodium silicate combined with 33.33 wt.% of NaOH 10M). The results show that the compressive strength increased from 11.23 MPa at 28 days to reach 24.98 MPa when the TMWM was partially replaced by 10 vt.% RCBW. In addition, this study shows that the interconnected porosity decreased where the critical pore size was reduced from 21.28 μm to 0.55 μm for the tungsten mining waste-based alkali-activated binder and the binary alkali-activated binder based on TMWM and RCBW.


Introduction
Alkali-activated materials are attracting a substantial volume of research interest as an alternative binder to replace Portland cement to reduce CO 2 emissions and global warming. The valorisation and re-usage of some waste minerals to produce new cementitious materials using the alkaline activation process technology by blending the precursors (minerals waste) with the alkaline activator solutions are one of the solutions that allow the reduction of CO 2 emissions by re-using such waste [1]. Panasqueira is an underground mine located in the centre of Portugal which uses mechanised roomand-pillar mining methods. According to the recent technical report on the mineral just about 0,3 % of excavated rock. The re-usage of tungsten mining waste mud has been investigated since the initial study carried out by Pacheco-Torgal et al [3], in which they prepared alkali-activated materials using tungsten mining waste mud (TMWM) as a precursor, mixed with minor quantities of calcium hydroxide Ca(OH) 2, as powder, and sodium hydroxide and waterglass as alkaline activator solutions. The alkaline activation reactions is the dissolution of an alumino-silicate minerals by a highly alkaline solution prior to precipitation reactions that form a gel binder [4]. In the study, the tungsten mining waste mud was subject to a thermal treatment at 950°C for 2h. The alkali-activated mixtures that were prepared contained TMWM and 10% of Ca(OH) 2, as powder, and NaOH with a concentration of 20M, as alkaline activator solution, and H 2 O/Na 2 O molar ratio=10,8. The binder was mixed with limestone aggregates (aggregate/binder ratio =1.5). The alkali-activated concrete resulted from their study had a compressive strength of Fc (56) = 65.3 MPa after 56 curing days. Recently, several studies have been carried out to decrease the CO 2 emission and the consumption of energy during the calcination process of the TMWM by combining TMWM with other alumino-silicate rich materials due to the low reactivity of untreated (non-calcined) tungsten mining waste mud (TMWM) with the alkaline activator solutions. TMWM was blended with other mineral waste (aluminosilicate rich materials) such as glass powder [5], brick powder [6], electric arc furnace slag [7] and granulated ground blast furnace slag [8] to produce alkali-activated binders.
This research aimed to optimise the alkaline activation of TMWM by replacing 10 vt.% of TMWM with other minerals waste and understand the influence of the precursors combination on the mechanical strength and pore structure of the binders formed.

Materials
The raw materials used in this investigation consisted of four powders as precursors: TMWM, EAFS, GGBFS and RCBW. The activator solution was a mix of sodium hydroxide (NaOH) (SH) and sodium silicate (Na 2 SiO 3 ) (SS). DOI

Precursors
The main precursor used was TMWM that derived in its powder form from the mining waste disposal dam at Panasqueira mine. After the collection of the TMWM from the disposal dam, TMWM was dried at 100°C for 24 hours. Then, the dried mud was mechanically disaggregated using a crushing machine and, finally, the mud was sieved to obtain particles of sizes under 500 µm. Two types of slags were used in the current study: ground granulated blast furnace slag (GGBFS) and electric arc furnace slag (EAFS).
The GGBFS was obtained from the cement plant Ain El Kebira located in the northeast of Algeria. Before mixing, the GGBFS was sieved to obtain size particles under 63 µm. EAFS was received from the national steel industry in Maia and Aldeia de Paio Pires, Seixal, Portugal. The EAFS was placed, as received, in the oven at 60 °C for 24 hours to dry.
Afterwards, it was grinded into fine particles in a crusher mill consuming 12 kW h per ton of steel slag crushed. Then, it was further pulverized in a ball mill consuming respectively 12.5 kW h and 37.5 kW h per ton of coarser and finer particles of steel slag powder [9].
The EAFS was sieved to obtain size particles under 250 µm. The particle size distribution curves of milled TMWM, GGBFS and MK are shown in Fig. 1

Activator solutions
The alkaline solution used was a mixture of an aqueous solution of sodium hydroxide (

Samples preparation and analysis
In this investigation, four mixtures were prepared.  Table 2.

Total porosity and Pore size distribution
In this study, four alkali-activated binder samples named (100TMW, 90TMW+10EAFS, 90TMW+10GGBFS and 90TMW+10RCBW) were tested by MIP. For each sample, 290 data were collected which used to obtain the Skeletal density, the total porosity and the average pore diameter, and the pore size distribution integral curve and the pore size distribution differential curves were also plotted in Fig.3.
The skeletal density of the alkali-activated binder changed when 10 vt.% of TMWM was partially replaced by other mineral waste, for the sample 100TMW the skeletal density is 3.44 g/cm 3 . This value was increased in the sample that had 10 vt.% EAFS to reach 3.84 g/cm 3 and decreased for the samples 90TMW+10GGBFS and 90TMW+10RCBW with a skeletal density of 2.88 g/cm 3 and 2.71 g/cm 3 , respectively. It was observed that the variation of the skeletal density depends on the density of the primary precursors were the EAFS is denser than the other precursors. Additionally, it was observed that the total porosity decreased for those samples in which 10 vt.% of TMWM was partially   [10]. Pores in the four alkali-activated binders can be classified into four categories: micropores (d < 2 nm), mesopores (2nm<d<50nm), macropores (50nm<d<10mm) and voids and microcracks (d > 10 mm). The micropores (ranges d > 2 nm) cannot be measured using the MIP technique because the data given are limited and there is a minimum pore diameter of 0.005 mm (5 nm) that can be evaluated. However, it is possible to measure the mesopores, macropores and voids and microcracks [11].
The dominant range for the sample 100TMW was coarse pores with the size (d >

Characterisation of pore size
Another important measurement was obtained using MIP which is the critical pore diameter (or most probable diameter). The critical pore diameter is the inflection point on the volume intrusion versus pore diameter curve (or the maximum peak of the dV/dp curve) as shown in Fig.3b and Table 4. Katz and Thompson [13] suggested that this point corresponds to the smallest pore size diameter of the subset of the largest pores which creates a connected path through the alkali-activated samples.

Conclusions
The influence of the partial replacement of 10 vt.% of TMWM by 10 vt.% of EAFS, GGBFS and RCBW on strength and pore structure of tungsten mining waste-based alkali-activated binders are investigated in the current study. Based on the investigation, it is concluded that: • Either way, the compressive strength increased when TMWM was partial replaced with 10 vt.% of other minerals waste. Moreover, higher compressive strength was obtained when TMWM was partial replaced with RCBW.
• The pore structure of the alkali-activated binders was improved when 10 vt.% of EAFS, GGBFS and RCBW partial replaced the TMWM where the total porosity was significantly reduced for the binary alkali-activated binders.Furthermore, the main difference between 100TMW alkali-activated binder and binary-alkaliactivated binders is that 100TMW alkali-activated binder pores are predominantly composed of coarse pores (voids/microcracks with diameter (> 10 µm)), whereas binary alkali-activated binders pores contain higher proportion of macropores and mesopores.
• Blending of tungsten mining waste with other mineral waste (alumino-silicate rich powders) generate more reaction products. The reaction products improved the pore structure and formed dense binder structure which increase the compressive strength results • The compressive strength of the alkali-activated binder depends to the pore structure, while the sample 100TMW has the higher proportion of coarse pore and