Abstract
Alkali activation is studied as a potential technology to produce a group of high performance building materials from industrial residues such as metallurgical slag. Namely, slags containing aluminate and silicate form a useful solid material when activated by an alkaline solution. The alkali-activated (AA) slag-based materials are promising alternative products for civil engineering sector and industrial purposes. In the present study the locally available electric arc furnace steel slag (Slag A) and the ladle furnace basic slag (Slag R) from different metallurgical industries in Slovenia were selected for alkali activation because of promising amorphous Al/Si rich content. Different mixtures of selected precursors were prepared in the Slag A/Slag R ratios 1/0, 3/1, 1/1, 1/3 and 0/1 and further activated with potassium silicate using an activator to slag ratio of 1:2 in order to select the optimal composition with respect to their mechanical properties. Bending strength of investigated samples ranged between 4 and 18 MPa, whereas compressive strength varied between 30 and 60 MPa. The optimal mixture (Slag A/Slag R = 1/1) was further used to study strength development under the influence of different curing temperatures at room temperature (R.T.), and in a heat-chamber at 50, 70 and 90 °C, and the effects of curing time for 1, 3, 7 and 28 days was furthermore studied. The influence of curing time at room temperature on the mechanical strength at an early age was found to be nearly linear. Further, it was shown that specimens cured at 70 °C for 3 days attained almost identical (bending/compressive) strength to those cured at room temperature for 28 days. Additionally, microstructure evaluation of input materials and samples cured under different conditions was performed by means of XRD, FTIR, SEM and mercury intrusion porosimetry (MIP).
Highlights
Alkali activation technology offers the possibility to utilize large amounts of aluminosilicate-rich secondary products such as fly ash from thermoplants [1,2,3], slag from metallurgical processes [4,5] and waste glass [6] for a useful new group of building products
Signals attributed to the O–H symmetric stretching and H–O–H bending vibrations in activated materials appear at around 3300 and 1640 cm−1 respectively and decrease over time due to a lower concentration of available water as it is consumed into products of the reaction [33]
The distribution of particles within the matrix of all samples is similar for all samples, but some cracks and voids are visible in the sample cured at room temperature, especially around single grains (Figure 11a,b)
Summary
Alkali activation technology offers the possibility to utilize large amounts of aluminosilicate-rich secondary products such as fly ash from thermoplants [1,2,3], slag from metallurgical processes [4,5] and waste glass [6] for a useful new group of building products. Steel slag-based alkali activated materials (AAM), have a high mechanical strength, show good fire resistance and high thermal resistance at elevated temperatures, and, in the case of low density, they further exhibit low thermal conductivity [7]. Altan et al suggested that when alkali activated slag mortars, using alkali hydroxide and a sodium silicate activator, were cured at room temperature (23 ◦ C) for a sufficiently long time (70 days), an equal or higher strength could be attained compared to those cured at 80 ◦ C for only 4 days [23]. Trends were reported whereby the compressive strength of activated fly ash with 8 M NaOH increased in a near linear fashion with both elevated temperatures (45, 65 and 85 ◦ C) and curing time (5, 12, 24 h and 7 days). Input materials as well as samples cured under different conditions were further microstructurally evaluated by means of XRD, FTIR, SEM and mercury intrusion porosimetry (MIP)
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