By Lianyang Zhang
A full version of this article, which includes a complete description of the laboratory process used to gain these findings, will be in a future issue of Construction and Building Materials Journal.
Decades of underfunding and inattention has left the U.S. infrastructure at great risk. According to the American Society of Civil Engineers (ASCE) 2009 Report Card for America’s Infrastructure, our nation’s infrastructure receives an overall grade of “D”.1 Repairing and upgrading the deteriorating infrastructure will use a large quantity of new concrete and in the meantime generate a significant amount of waste concrete. Ordinary Portland cement (OPC) is commonly used as the binder in the production of concrete. It is well known that the manufacture of OPC not only consumes a significant amount of natural resources and energy but also releases a substantial quantity of greenhouse gases.2-4 Production of concrete also uses sand and aggregate from quarrying operations that are energy intensive and may release high level of waste materials. Furthermore, the shortage of natural construction materials in many regions has led to long-distance haulage and significantly increased costs.5-7 Growing environmental awareness, the need to ensure sustainability of construction materials, and public concern to safeguard the countryside limit the use of quarrying sites and encourage the construction industry to look for alternative materials.7-11
On the other hand, it is a great challenge to handle the significant amount of waste concrete to be generated from repairing and upgrading the deteriorating infrastructure systems.12-15 In the United States, concrete waste occupies one third of the volume of waste materials in landfills.15 Finding areas suitable for new landfilling is getting harder and disposing is getting more expensive. The recycling of waste concrete is encouraged by different agencies and sought by various institutions. Although extensive research has been conducted,16-23 current recycling of waste concrete is still predominately limited to the use of concrete aggregates in low-specification applications such as base course and non-structural fill with the remainder still being landfilled.24,25 The old cement paste/mortar attached to the stone particles is the main reason for the lower quality of the concrete aggregate than the natural aggregate. Compared to natural aggregates, the concrete aggregate has increased water absorption, decreased bulk density, decreased specific gravity, increased abrasion loss, increased crushability, and increased quantity of dust particles. The low quality of the concrete aggregate generally leads to new OPC concrete with inferior strength, durability and shrinkage properties. The utilization of concrete aggregates in structural concrete is very limited. In cases that the concrete aggregate is used together with natural aggregate for production of structural concrete, a limit of 30% of concrete aggregate is usually recommended.24,26 Aggregate refining methods such as “heating and rubbing”27 and “mechanical grinding”28 have been developed for refining the quality of concrete aggregates by removing the attached paste/mortar, but these methods are energy intensive and produce additional fines which need to be disposed of. It is also noted that utilization of concrete aggregates as base course and non-structural fill may cause environmental problems such as contaminant leaching and pH changes in the surrounding soil and water.29-31
Obviously, an ideal solution to address the significant amount of waste concrete is complete recycling and utilization of it in production of new structural concrete. Few researchers have investigated complete recycling of waste concrete.32,33 These complete recycling methods, however, need to re-clinker the hydrated cement using the standard cement kiln procedures and thus consume significant amount of energy and release a large quantity of CO2. To completely recycle and use waste concrete in a sustainable and environmentally friendly way, a method that does not need re-clinkering at high temperature should be used.
The researchers at the University of Arizona have embarked on a multi-year research program on complete recycling and utilization of waste concrete through geopolymerization. Geopolymerization is a relatively new technology that transforms aluminosilicate materials into geopolymer. Geopolymerization involves a chemical reaction between solid aluminosilicate oxides and an alkaline activation solution at ambient or slightly elevated temperatures, yielding an amorphous to semi-crystalline polymeric structure with Si–O–Al and Si–O–Si bonds.2,34-37 Geopolymer not only provides performance comparable to OPC in many applications, but has additional advantages, including abundant raw material resources, simple production method, rapid development of mechanical strength, no/low alkali-silica reaction (ASR) related expansion, excellent durability, high fire resistance, superior resistance to chemical attack, and the ability to immobilize toxic and hazardous wastes. These characteristics have made geopolymer of great research interest as “an ideal material for sustainable development.” 2,34-37,38-42 The following briefly describes the research approach and the accomplishments to date.
To recycle and use waste concrete, it is first crushed and separated it into aggregates (coarse and fine) and fines. For complete recycling, both the aggregates and the fines need to be used. Our research is guided by the following two hypotheses:
- The fines of crushed waste concrete can be used together with Class F fly ash as the source material to produce high performance geopolymer binder. Depending on the chemical composition, mainly calcium, silica and alumina, and their chemical forms, of the fines, the relative amount of the fines and the fly ash can be adjusted to obtain the desired properties.
- The recycled aggregates with original “porous” cement paste/mortar adhering to them can be (partially) geopolymerized to generate a good bond between the aggregate and the geopolymer binder and thus a good performance geopolymer concrete can be produced from completely recycled waste concrete.
The research takes a multi-scale and multi-disciplinary approach and consists of systematically designed experiments to investigate the effect of different factors on the properties of geopolymer binder and concrete produced from waste concrete. The experiments include uniaxial compression and split tensile tests to evaluate the unconfined compressive strength (UCS), deformability and tensile strength of geopolymer binder and concrete, four-point bending tests to investigate the bonding between the recycled concrete aggregate and the geopolymer binder, scanning electron microscopy (SEM) imaging to investigate the development of micro/nano-structure, energy-dispersive X-ray spectroscopy (EDX) analysis to study the change of surface elemental composition, X-ray diffraction (XRD) analysis to quantify the phases (crystalline and amorphous) taking part in geopolymerization, and Fourier Transform Infra Red (FTIR) analysis to study the change of chemical bonds. By linking the macro-scale behavior and the macro/nano-scale properties, the underlying mechanism for geopolymerization of waste concrete can be better understood and geopolymer concrete with desired properties can be produced from waste concrete.
So far the research has focused on production of geopolymer binder using the waste concrete fines (WCF) together with Class F fly ash (FA). Sodium hydroxide (NaOH) and sodium silicate (SS) solution was used as the alkaline activator. The geopolymer binder samples were produced by first mixing WCF with FA at different proportions and then mixing the alkaline solution with the WCF/FA mixture. The resulted paste was placed in cylindrical Plexiglas molds with the mold being shaken by a vibrator during the casting to release the trapped air bubbles. The mold was capped and left in room temperature for curing. The specimens were de-molded after 24 hours and then placed in a plastic bag for six more days’ curing before tested. The geopolymer binder samples were prepared and tested at different NaOH concentrations, SS to NaOH ratios (SS/N), and WCF contents. Key observations from the study include:
- Utilization of WCF helps improve the UCS of geopolymer binder up to a certain WCF content and further increase of WCF content leads to decrease of UCS. In the current study, 50% was found as the optimum WCF content at 5 and 10 M NaOH and with SS/N =1 and 2.
- The presence of calcium compounds in WCF improves the mechanical properties of the WCF/FA geopolymer binder due to the coexistence of the geopolymer gel and the calcium silicate hydrate (CSH) gel. The optimum initial Ca/Si ratio (the Ca/Si ratio at the highest UCS) is low (0.15 to 0.25) for the WCF/FA geopolymer binder, which suggests formation of low-Ca CSH gel in the geopolymer system.
- The SEM/EDX, XRD and FTIR analyses confirm the Ca in WCF enhances the strength mainly due to the formation of low Ca semi-crystalline CSH gel which coexists with the geopolymer gel and the incorporation of Ca+ into the geopolymer network as charge balancing cation.
- Increased NaOH concentration results in higher UCS, especially at WCF content less than 50%. Addition of SS also improves UCS due to provision of additional SiO2 and delayed setting. The optimum initial Si/Al (the Si/Al ratio at the highest UCS) for the WCF/FA geopolymer binder is around 3.38.
- The geopolymer in the WCF/FA geopolymer binder is close to poly(sialate-disiloxo) and stronger than the geopolymer in the pure FA geopolymer binder which is close to poly(sialate-siloxo).
With more infrastructure systems to be repaired and upgraded, larger amounts of waste concrete (WC) will be produced. For sustainable development, it is urgent to completely recycle and reuse the WC in a cost-effective and environmentally-friendly way. Utilization of the geopolymerization technology provides an avenue for doing so. By using WC fines together with fly ash as the geopolymer binder source material and WC aggregates as the filler, high performance geopolymer concrete can be produced. This method does not use the energy-intensive OPC and eliminates the associated greenhouse gas emissions.
Lianyang Zhang, Ph.D., P.E., is an assistant professor in the Department of Civil Engineering and Engineering Mechanics at the University of Arizona in Tucson, Ariz. His research interests include recycling and utilization of wastes as construction material, rock mechanics and rock engineering, deep foundations, and sustainable geotechnics and geoenvironmental engineering. He can be reached at firstname.lastname@example.org.
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