In-depth investigation of the long-term strength and leaching behaviour of inorganic polymer mortars containing green liquor dregs

Highlights

• Eco-friendly geopolymer mortars containing green liquor dregs were synthesised.

• For the first time the durability and leaching behaviour of dregs-containing mortars was studied.

• Dregs effectively enhanced mortars' compressive strength (up to 44%).

• Samples' compressive strength increased with ageing demonstrating their durability.

• Mortars' heavy metals leaching levels were below the contamination limits in soil.

Abstract

Green liquor dregs are the most challenging waste stream coming from the pulp and paper industry. Despite tremendous efforts, there are not currently any viable recycling alternatives for this massively produced waste (2 Mt/year), which inevitably ends up in landfills. Urgent actions must be undertaken to tackle this. In this work, a substantial amount of dregs was incorporated into eco-friendly, waste-based inorganic polymer (geopolymer) mortars as fine filler. Then, and for the first time, the long-term strength performance (up to 270 days) and heavy metals leaching behaviour of the dregs-containing mortars was evaluated. The effect of the mixture composition and dregs incorporation content on the fresh- and hardened-state properties of the mortars was also studied. Dregs were found to increase the initial and final setting time of the slurries, thus extending the open time before their in-situ application. The use of dregs as fine filler effectively enhances the compressive strength of the mortars, and decreases their water absorption levels. These eco-friendly building materials showed excellent long-term performance, as their strength continuously increases up to the 270^th day (after mixture), and no signs of efflorescence formation were detected. Moreover, the heavy metals leaching levels of the mortars were well below the contamination limits in soil, which demonstrates the feasibility of this recycling methodology.

Introduction

The pulp and paper sector is one of the most resource intensive, energy demanding and pollutant emitting manufacturing industries (Corcelli et al., 2018; Sun et al., 2018), and production is still expected to increase in coming years (Corcelli et al., 2018). In fact, recent data show that worldwide paper and board production has increased around 1.5% in 2017, reaching 420 Mt (CEPI, 2017), of which ∼22% was produced in Europe. In 2016, the contribution of this industry to the EU economy reached €81 billion, while the number of paper and pulp employees was estimated to be 175,000 people (CEPI, 2016). These values show the huge economic impact of this industry in the EU context. On the other hand, this activity also generates a vast amount of wastes - 11 Mt/year in Europe (Monte et al., 2009). Among the residues produced (e.g. biomass ash (Modolo et al., 2018; Modolo et al., 2015), grits (Saeli et al., 2018), primary and biological sludge, lime mud (Modolo et al., 2014)), green liquor dregs (Novais et al., 2018c) are a major inorganic stream, for which there is not currently any viable and sustainable recycling methodology. Green liquor dregs are undissolved smelts from chemical recovery boilers mixed with weak white liquor created in kraft pulp production. This waste, whose global production was projected to reach 2 Mt in 2016 (Novais et al., 2018c), is usually disposed of in landfills (Kinnarinen et al., 2016), which raises serious environmental concerns due to the presence of hazardous metals such as Co, Cr, Cu, Cd, Ni, Pb and Zn in their chemical composition (Golmaei et al., 2018a). However, it should be highlighted that some of these metals (e.g. Cu and Zn) are also known as nutrients in the soil (Pöykiö et al., 2006).

Dregs are generated in the chemical recovery cycle employed to recover sodium hydroxide and sodium sulphide, this waste being formed in the dissolving tank where the weak white liquor and smelt from the chemical recovery boiler are mixed together. Recently, the possibility of extracting heavy metals from the dregs has been exploited using cyclones (Golmaei et al., 2018a) or chelating agents (Golmaei et al., 2018b), in order to allow their use as fertilizer or as a soil amendment. Despite the interesting results of these investigations, the disposal of the highly-heavy metals concentrated dregs fraction when using cyclones (Golmaei et al., 2018a) further intensifies their recycling challenge, while the use of chelating agents is complex (Golmaei et al., 2018b), requiring the purification of the generated supernatant phase containing metal-chelating complexes, which could not only contaminate water resources, but would add cost to the papermaking process. Our strategy is less complex, allowing the direct use of this waste, without the need to perform expensive chemical treatments, in the production of eco-friendly and low cost building materials.

The incorporation of wastes into building materials is an excellent recycling methodology, decreasing the consumption of virgin raw materials, while at the same time preventing the landfill disposal of these wastes. However, the incorporation of some residues is particularly challenging, this being the case of the green liquor dregs. Dregs present very high pH (>10) and alkaline and alkaline-earth oxide contents, which hampers their incorporation in cement or concrete. These features explain the reason why this massively produced waste remains yet unexplored. Despite this, some authors have attempted to use dregs as cement replacement in concretes and mortars (Martínez-Lage et al., 2016), or as aggregate in road pavement construction (Modolo et al., 2010; Pasandín et al., 2016). Nevertheless, the properties of such dregs-containing materials were not suitable for the envisioned applications (hot-mix asphalt (Pasandín et al., 2016) and concretes (Martínez-Lage et al., 2016)). One exciting possibility could be the incorporation of dregs in the production of inorganic polymers (geopolymers), which are binder systems widely acknowledged as being an environmentally friendlier alternative to conventional Portland cement (Turner and Collins, 2013). The proof-of-concept has been recently demonstrated by the authors (Novais et al., 2018c). This study showed the feasibility of using as-received dregs as fine fillers in the production of a sustainable binder, without compromising the performance of the mortars. Indeed, an increase in both the flexural (up to 71%) and compressive strength (up to 34%) was achieved for the dregs-containing mortars in comparison with the reference. Nevertheless, the leaching behaviour and the long-term strength performance of the mortars was not considered.

In this work, and for the first time, the long-term strength performance and leaching behaviour of geopolymer mortars containing significant amounts of dregs was evaluated. This investigation is of the utmost importance to demonstrate that the hazardous elements present in the dregs composition are effectively retained inside the geopolymeric mortars and, therefore, that their incorporation in these mortars is a safer and environmentally better management strategy in comparison with their landfill disposal. This investigation represents a significant, and essential, step forward in comparison with the only previous study reported to date (Novais et al., 2018c), reducing the existing knowledge gap regarding the long-term strength performance and leaching behaviour of these innovative materials. Moreover, the maximum dregs incorporation content was increased by a factor of 4, and the specimens were cured at ambient temperature (23 °C) instead of using 40 °C (Novais et al., 2018c), this being a more sustainable and cost effective strategy. The influence of the mixture composition and dregs content on the fresh-state (workability and setting time of the mortars) and hardened-state properties (compressive strength, water absorption and heavy metals leaching) was also considered.