Utilization of Concrete Waste Aggregates Using ...

June 2010, Volume 4, No.6 (Serial No. 31) Journal of Civil Engineering and Architecture, ISSN 1934-7359, USA

Utilization of Concrete Waste Aggregates Using Geopolymer Cement Sotya Astutiningsih1, Henki Wibowo Ashadi2, Hendra Widhatra2, Kresnadya Desha Rousstia2 and Maria Elizabeth Suryatriyastuti2 1. Department of Metallurgy and Materials Engineering, University of Indonesia, Depok 16424, Indonesia 2. Department of Civil Engineering, University of Indonesia, Depok 16424, Indonesia Abstract: Reuse of concrete waste, especially in large quantity, can save not only material but also cost for its disposal. This paper presents experiment results on the use of fine and coarse aggregates from concrete waste in geopolymer mortars and concretes. Geopolymeric cement is an inorganic compounds of aluminosilicates synthesized from precursors with high content of silica and alumina activated by alkali silicate solutions. Geopolymer in this experiment was synthesized from fly ash as the precursor and sodium silicate solution as the activator. Hardening of geopolymers was performed by heating the casted paste in an oven at ~60℃for 3 to 36 hours. Compressive strength of geopolymer pastes and mortars using either fresh or waste fine aggregates were in the range of 19-26 MPa. Hardening time of 3 hours at 60℃ followed by leaving the test pieces at room temperature for 7 day before testing results in similar strength to that of mortars cured for 36 hours at 60℃ followed by leaving the samples at room temperature for 3 days. It suggests that optimum strength can be achieved by combination of heating time and rest period before testing, i.e the specimens age. Applying mix design with a target strength of 40 MPa, conventional Portland cement concretes using fresh aggregates reached 70% of its target strength at day-7. Compressive strength of geopolymer concretes with waste aggregates was ~25 MPa at day-3 while geopolymer concretes with fresh aggregates achieved ~39 MPa at day-3. It can be concluded that geopolymer concretes can achieve the target strength in only 3 days. However, the expected reinforcing effect of coarse aggregates in concrete was ineffective if waste coarse aggregates were used as the strength of the concretes did not increase significantly from that of the mortars. On the other hand, waste fine aggregates can be reused for making geopolymer mortars having the same strength as the geopolymer mortars using fresh aggregates. Key words: Geopolymer, concrete waste aggregates, mortar, concrete, compressive strength.

1. Introduction Concrete wastes are composed of hydrated Portland cement, fine and coarse aggregates. Except the cement, fine and coarse aggregates have not changed either physically nor chemically and therefore can be reused. The reuse of waste materials not only saves cost but also contributes to a cleaner environment. In this paper, the compressive strength of geopolymer mortars and concretes using fresh and waste aggregates will be compared. The waste aggregates used were sand and coarse aggregates, from Portland cement concrete. Geopolymer concrete made for this experiment used Corresponding author: Sotya Astutiningsih, PhD, research fields: cements and geopolymers. E-mail: sotya.astutiningsih@ ui.ac.id.

the same mix design as Portland cement concrete without Portland cement at all. Geopolymer cement as the binder will be synthesized from fly ash and sodium silicate solution. Curing will be performed at ~60℃ for 3 to 36 hours and testing at 3 and 7 day. The compressive strength of geopolymer mortars and concretes using waste aggregates will be compared to those using fresh aggregates. The popular terminology of “geopolymer” for alkaliactivated aluminosilicate-based inorganic polymers was introduced by Joseph Davidovitts in 1976, as mentioned by himself in his visit to Curtin University in 2002 [1], in recognition of the geological nature of their major constituents: silica and alumina. Geopolymers can be made from precursors containing

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Utilization of Concrete Waste Aggregates Using Geopolymer Cement

high amount of silica and alumina in their reactive glassy phases, such as dehydroxylated kaolin minerals or industrial wastes like blast furnace slag and fly ash. Reactive precursors will dissolve, set and cured or hardened when activated. The available and inexpensive activators for the precursors are the alkali silicate solutions. The chemical composition of cured geopolymer resembles that of zeolites, but having amorphous structure [2]. These materials can be defined as inorganically solid materials with large 3D molecular structures analogous to polymeric materials. The repeat unit forming the network are [SiO4] and [AlO4] tetrahedra. Some geopolymers contains calcium, such as fly ash- and blast furnace slag-based geopolymers, however the role of calcium in geopolymer is still under investigations [3-5]. Because the strength contributor in the geopolymeric system is not predomainated by calcium compounds, as in Portland cement, it is claimed that geopolymers are more resistant in acids than Portland cements are. The principal differences between geopolymer and Portland cements lies not only in the chemical make ups of the material but also in hardening mechanisms and thus the molecular structure formed. Different from hydration of clinker phases in Portland cement hardening, geopolymers cure or harden in two steps: dissolution followed by polycondensation. First the alumina and silica precursors dissolve into oligomers through an exothermic reaction, and then polycondense, which is an endothermic reaction, into framework aluminosilicate and water molecules as by-product [6]. For that reason, the optimum mechanical strength of geopolymer concrete can be achieved at much shorter time than that of Portland cement concrete depending on curing temperature. Density of geopolymer concrete is similar to that of Portland cement concrete. Mechanical strength of geopolymer concrete, like its Portland cement counterpart, depends upon the mix design and curing methods [7]. Special geopolymer concretes can achieve optimum compressive strength up to 100 MPa, and have

high early strength of 20 MPa in 4 hours [8]. Geopolymers have also been used to substitute organic polymers as adhesives for fire-proof and UV resistant structural reinforcement [9]. Palomo, Grutzeck and Blanco [10] studied the effect of curing temperature and time, and alkaline solution to fly ash ratios on the mechanical strength of geopolymer. It was found that curing temperature and time affect the mechanical strength of geopolymer. The material achieved compressive strength of 60 MPa upon curing at 85℃ for 5 hours. Similar to that, geopolymer that was synthesized by Swanepoel and Strydom from a mixture of fly ash, kaolin and sodium silicate solutions showed an optimum strength upon curing at 60℃ for 48 hours [11]. Xu and Van Deventer [12] studied the geopolymerisation of 15 aluminosilicate minerals and found that minerals with high degree of dissolution would produce geopolymers having higher compressive strength. Physical characteristics of geopolymer cements and binders with a compressive strength at 28 days of 90 MPa and flexural strength of 10-15 MPa are compiled in Table 1. With the major constituents of ceramics, i.e., SiO2 and Al2O3, geopolymers obtain the good properties of ceramics such as high hardness, strength, and heat resistance and at the same time taking the advantage of low temperature processing and formability of the organic polymers. Due to the novel and improved engineering properties, these materials promise a wide range of applications, including resin for composites, adhesives, sealants, waste encapsulation, refractory, Table 1 Physical characteristics of geopolymer cements and binders [13]. Properties Young modulus Shrinkage during setting Linear expansion Heat conductivity Specific heat Bulk density Open porosity Geopolimerisation shrinkage

Values >2 GPa