Properties and environmental assessment of eco-friendly brick powder geopolymer binders with varied alkali dosage

Highlights

• Brick powder geopolymer with good strength and water resistance is prepared.

• The water absorption and water resistance of brick powder geopolymer are evaluated.

• Brick powder geopolymer has great advantages in environmental protection.

• The optimal mix proportion of brick powder geopolymer is acquired.

Abstract

This paper presents the effect of alkali dosage on the mechanical properties and water resistance of alkali-activated brick powder geopolymers (BPG) by measurements of compressive and flexural strengths, bulk density, water absorption and softening coefficient. To reveal the micro-mechanism of BPG under varied alkali dosage, the mineralogical phases, reaction degree, micromorphology, and pore structure of BPG are analyzed by techniques of XRD, TGA, SEM and MIP, respectively. Furthermore, the environmental impacts of BPG are evaluated by considering CO₂-e emission and energy consumption. Results show that the BPG prepared with low alkali dosage (2% or 4% Na₂O) exhibits the loose microstructure with few amorphous gels and relatively high porosity (including high proportion of capillary pores) resulting in the relatively poor mechanical properties and water resistance. However, a higher alkali dosage (6% or 8% Na₂O) promotes the geopolymerization reaction resulting in the denser microstructure with more amorphous gels and relatively lower porosity, thereby greatly enhancing the mechanical properties and water resistance with maximal flexural strength, compressive strength and softening coefficient of 2.2 MPa, 31.1 MPa and 0.77 respectively. Water immersion may cause Na⁺ and OH⁻ to dissolve from BPG matrix resulting in a charge imbalance of three-dimensional network structures, which will further lead to strength degradation. In comparison to ordinary Portland cement paste, BPG can reduce CO₂-e emission by about 40%–70% and energy consumption by about 20%–50%. The optimal mixing parameters have an alkali dosage of 6%, a silicate modulus of 1.6 and a water-to-binder of 0.3 by considering the compressive strength and environmental impacts.

Introduction

Construction solid waste (CSW) is a kind of common solid waste manufactured in the process of demolition, new-built, alteration and expansion of infrastructures and buildings. Statistical data show that the annual generation of CSW has recently approached 2.6 billion tons in China, which accounts for 30%–40% of the total municipal solid waste [1,2]. Waste concrete (WC) and waste brick (WB) account for 80% of the total CSW [3], the most of which are directly landfilled or piled up due to their complex composition and cumbersome separation process. Inappropriate treatments will result in serious environmental pollution and occupation of land resources [1,4]. The past few years have seen the rapid development of green chemistry and technologies of environmental sustainability, which causes the traditional concept shift towards the new perspective that is considering the solid wastes as recycle resources [4]. At present, numerous literatures have documented on resource reutilization of WC and WB in the aspects of preparing green cementitious materials to reduce the mounting demand for natural resources in the construction industry [[5], [6], [7], [8], [9], [10]]. After separating, crushing or ball milling, the WC and WB are usually used as recycled aggregate, subgrade filling material and auxiliary cementitious material [5,7,11,12]. However, existing researches have showed that the mechanical properties of recycle products with WB or WC as part of raw materials are relatively poor [3,13,14], especially for recycle products including WB due to its poor properties such as higher-water absorption, lower strength and reactivity [13,14]. Besides the above application, in the construction industry, alkali-activation technology has emerged as an effective technology to reuse different mineral wastes (Slag, FA and MK) and industrial by-products (steel-slag and red mud) to produce green building materials [15]. In comparison to traditional Portland cement that has large energy consumption and CO₂ emission in production, alkali-activated geopolymer possess more prominent properties (such as high compressive strength and excellent durability) and lower CO₂ emission [[16], [17], [18]]. The compositions of WB are mainly including SiO₂ and Al₂O₃, which is considered to have great potential as an alkali-activated geopolymer precursor [4]. Therefore, compared with the other treatment technologies, the alkali-activated technology is a preferable way for realizing the high efficiency and high value-added reutilization of WB.

Previous researches have assessed the feasibility of brick powder (BP) as the raw materials of geopolymer composites, Table 1 summarizes the recent studies about the optimal mixing parameters and properties of BPG [[19], [20], [21], [22], [23], [24]]. For instance, Reig et al. [19] investigated that the effect of activator types (NaOH or [NaOH + Na₂SiO₃]), alkali dosage (7–12%), silicate modulus (0.73–1.6), water-to-binder (0.30–0.45) and curing age (3 or 7 days) on the compressive strength of alkali-activated brick powder geopolymers (BPG). The results indicated that the optimal mix parameters had an alkali dosage of 6.5%, a silicate modulus (Ms) of 2.0, a water-to-binder of 0.30, and the maximal compressive strength reached 50 MPa upon curing at 65 °C for 7 days. According to Table 1, consistent conclusions can be drawn that [NaOH + Na₂SiO₃] activator has an optimal activation effect on BP and the optimal Ms is about 1.6. However, the difference between the optimal alkali dosage obtained by different scholars is relatively more obvious, which indicates that alkali dosage has a prominent influence on the properties of BPG activated by [NaOH + Na₂SiO₃] activator solutions including sufficient reactive silica. Meanwhile, the analyses on microstructure evolution of BPG with varied alkali dosage in the existing research reports are not deep enough. Besides that, existing reports mainly focus on the effect of alkali dosage on the macroscopic mechanical properties [2,19,20,[23], [24], [25], [26]] and limited literatures have documented that the effect of alkali dosage on the water absorption and water resistance of BPG. In fact, currently, the researches on the preparation, properties and chemical structure compositions of BPG are still insufficient.

In order to make up for the deficiency of existing researches and better guide the practical applications of BPG, particularly in the aspects of microstructure evolution, mechanical properties, and water resistance relationship. This paper mainly studies the influence of alkali dosage on the compressive and flexural strengths, water absorption and water resistance of BPG and deeply analyzes the evolution of reaction degree and microstructure with varied alkali dosage by techniques of XRD, TGA, SEM and MIP. Meanwhile, to better understand the advantages of BPG in energy conservation and emission reduction compared with ordinary Portland cement (OPC), the evaluations of environmental impacts of BPG and OPC pastes are carried out by calculating the CO₂-e emission and energy consumption.