Properties and environmental assessment of eco-friendly brick powder geopolymer binders with varied alkali dosage
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.
Section snippets
Raw materials
Brick powder (BP) was obtained by grinding broken waste brick (<7 mm) in a ball mill for 30 min, whose chemical composition, mineralogical composition, particle size distribution and micro morphology are obtained by XRF, XRD, Laser particle size and SEM tests respectively, as shown in Table 2 and Fig. 1, Fig. 2, Fig. 3. The XRF and XRD results show that the chemical composition of BP is mainly composed of SiO2 and Al2O3 (Si/Al molar ratio is 3.08) and the mineralogical composition of BP mainly
Setting times of BPG pastes
Fig. 5 presents the setting times of BPG pastes prepared with different levels of alkali dosage. The setting time of BPG is first shortened and then prolonged with the alkali dosage increases from 2% to 8%. The fastest setting process has an initial setting time of 46 min and final setting time of 60 min at the alkali dosage of 4%. The BPG paste prepared with alkali dosage of 2% or 8% exhibits a relatively long setting and hardening process with the initial and final setting time of 110/101 min
Conclusions
The effect of alkali dosage on mechanical properties and water resistance of brick powder geopolymers has been studied by macro tests of compressive and flexural strengths, bulk density, water absorption and softening coefficient, and the mineralogical phases, reaction degree, micromorphology, and pore structure of brick powder geopolymers have been analyzed by techniques of XRD, TGA, SEM and MIP, respectively. Furthermore, the environmental impacts of brick powder geopolymers have been
Author statement
Yue Li: Conceptualization, Methodology, Writing-Original draft preparation, Review & Editing, Jiale Shen: Investigation, Formal analysis, Writing-Original draft preparation, Visualization, Data Curation, Hui Lin: Conceptualization, Methodology, Visualization, Review & Editing, Jianfeng Lv: Supervision, Review & Editing, Shan Feng: Project administration, Methodology, Junchang Ci: Visualization, Review & Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (52078015) and Ministry of Housing and Urban-Rural Development of the People's Republic of China (2020-K-079).
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