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Stephen Wibiatma Wijaya

Bio: Stephen Wibiatma Wijaya is an academic researcher from Petra Christian University. The author has contributed to research in topics: Geopolymer & Fly ash. The author has an hindex of 3, co-authored 4 publications receiving 67 citations.
Topics: Geopolymer, Fly ash, Borax, Compressive strength

Papers
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Journal ArticleDOI
TL;DR: In this paper, the setting time of fly ash-based geopolymer concrete is investigated. And the experimental results showed that fly ash particle size, CaO and MgO content, in addition to ratio of sodium silicate and sodium hydroxide in the alkali solution, molarity of NaOH, initial temperature of the mixture, curing temperature, and mix volume could potentially influence the set time of the geopolymers mixture.
Abstract: Fly ash is a waste from coal burning, that are generated with fluctuation both in its physical and chemical characteristics. This characteristics of fly ash when used in the making of geopolymer concrete will greatly affect the final products obtained. The pH value measured in fly ash, according previous research, can influence the setting time of geopolymer and fly ash with high pH values can cause flash-setting in the concrete. Understanding more clearly about the factors that affect the setting time of fly ash based geopolymer is important for further progress and development of the material. It was found that factors that influence the setting time of geopolymer was not only from the physical and chemical properties of the fly ash itself. Other factors such as composition and mix design, manufacturing process and environmental conditions can also affect its setting time. The experimental results showed that fly ash particle size, CaO and MgO content, in addition to ratio of sodium silicate and sodium hydroxide in the alkali solution, molarity of NaOH, initial temperature of the mixture, curing temperature, and mix volume could potentially influence the setting time of the geopolymer mixture.

52 citations

Journal ArticleDOI
TL;DR: In this article, the effect of borax addition to the high calcium fly ash-based geopolymer mixture to reduce the occurrence of flash setting was investigated, and it was found that the setting time can be extended significantly, with the addition of 5% borsax, by mass, of fly ash.
Abstract: Geopolymer that was made with high CaO content fly ash was found to have higher compressive strength than the low CaO fly ash, using the same mixture composition. This effect could be due to the physico-chemical properties of the fly ash, in respect to its particle size or the chemical composition. Although it was not widely published, the occurrence of flash setting of geopolymer was known to occur when using high CaO content fly ash as the precursor. Geopolymer paste may solidify within minutes after the addition of alkali activators, making it very difficult to cast in big volume. This paper investigate the effect of borax addition to the high calcium fly ash-based geopolymer mixture to reduce the occurrence of flash setting. It was found that the setting time can be extended significantly, with the addition of 5% borax, by mass, of fly ash. The addition of borax also have positive effect on increasing the compressive strength of geopolymer.

32 citations

Journal ArticleDOI
TL;DR: In this paper, the compressive strength of fly ash-based geopolyme, manufactured using fly ash obtained from different power plants, was investigated and it was found that the size of the fly ash granules, the CaO and MgO content, and the carbon content in fly ash are the internal factors that may affect the compressed strength of geopolymer.
Abstract: . This study was conducted to determine the factors that may affect the compressive strength of fly ash-based geopolyme, manufactured using fly ash obtained from different power plants. Fly ash obtained from different sources may have very different characteristics that affect the properties of geopolymer product. The source of variations in the geopolymer properties from the view point of the source materials can be classified into internal and external factors. Internal factors include the physical and chemical properties of the fly ash, while external factors associated with mixture proportion of geopolymer and the manufacturing process. From the experimental results, it was found that the size of the fly ash granules, the CaO and MgO content, and the carbon content in fly ash are the internal factors that may affect the compressive strength of geopolymer. On the other hand, the ratio of sodium silicate to sodium hydroxide in the alkaline solution and the molarity of the sodium hydroxide solution are the external factors influencing the compressive strength of geopolymer.

15 citations

01 Jan 2013
TL;DR: In this article, metode Direct Displacement-Based Design (DDBD) merupakan metode baru dalam perhitungan struktur bangunan terhadap gempa ying membutuhkan verifikasi secara global.
Abstract: Metode Direct Displacement-Based Design (DDBD) merupakan metode baru dalam perhitungan struktur bangunan terhadap gempa yang membutuhkan verifikasi secara global. Metode ini memiliki proses perhitungan yang lebih singkat dan sederhana jika dibandingkan dengan metode Force Based Design (FBD) yang telah banyak diaplikasikan saat ini. Penelitian kali ini bertujuan untuk mengevaluasi kinerja bangunan yang dihitung dengan masing-masing metode untuk melihat performanya terhadap target desain sehingga dapat diverifikasi keandalannya. Bangunan yang diteliti memiliki bentuk irregular plan setinggi 6-lantai yang berada pada wilayah gempa 2 dan 6 Peta Gempa Indonesia. Verifikasi terhadap kinerja bangunan didapatkan dari hasil analisis dynamic nonlinear time history. Hasil verifikasi dari penelitian ini menunjukkan bahwa kedua metode memberikan kinerja yang baik, namun bangunan yang didesain menggunakan metode DDBD lebih mendekati target desain yang telah ditentukan sebelumnya dibandingkan dengan yang menggunakan metode FBD. Selain itu, metode DDBD cocok untuk diterapkan pada konsep Performance Based Design yang memakai drift sebagai batasan desain.

2 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, the effect of mix proportioning on fresh, hardened and durability properties of fly ash and fly ash-slag geopolymer mortar is discussed. But, the main focus of this paper is on the quality of the mortar.

109 citations

Journal ArticleDOI
TL;DR: In this article, the effects of different w/b ratios, admixtures (such as superplasticizers and/or retarders) and the dosage of the selected admixture on the fresh and hardened properties of one-part alkali activated fly ash/slag based binders were investigated.

86 citations

Journal ArticleDOI
TL;DR: A comprehensive overview of the design and properties of concrete with alkali-activated binder (AAC) is presented in this paper, where 184 published studies reported on concrete with AAC binder were reviewed critically.

69 citations

Journal ArticleDOI
TL;DR: In this article, a review of known factors that influence the setting time of fresh geopolymers and discusses the underlying chemical and physical-mechanisms of each parameter is discussed.
Abstract: In recent years, environmentally sustainable geopolymers have been widely investigated to obtain comparable mechanical properties with Portland cement, and they are now close to being in a position where they can substitute parts of the cement production utilized in construction. However, in order for geopolymers to reach their full potential, it is crucial to understand the influence of various parameters (e.g., particle size, treatment of precursor, chemical composition) on the hardening evolution at earlier ages of the pastes and the resulting final mechanical strength of the set system. Hence, this paper reviews known factors that influence the setting time of fresh geopolymers and discusses the underlying chemical- and physical-mechanisms of each parameter. Next to this, the interconnection between the setting time and mechanical strength is discussed. This will provide a platform for researchers and engineers to understand the hardening evolution of geopolymers in a broader range.

64 citations

Journal ArticleDOI
TL;DR: A systematic review of the reported studies to improve the buildability of concrete can be found in this article, where two methods to enhance the concrete's buildability are discussed: (1) Additives during initial mixing of concrete and (2) Buildability enhancement at the print-head by an intervention (set-on-demand).
Abstract: Buildability is the ability to 3D print by adding concrete layer by layer continuously to the required level without significant deformation or collapse of the freshly printed component. This paper presents a systematic review of the reported studies to improve the buildability of concrete. The paper also presents original concepts and some results for improving the buildability of geopolymer concrete. Two methods to enhance the buildability are discussed: (1) Additives during initial mixing of concrete (2) Buildability enhancement at the print-head by an intervention (set-on-demand). The former method is the most widely studied, however, it affects the pumpability. The latter includes mixing accelerators, heating, ultrasonication, or magnetorheological control, performed at the print-head to rapidly increase the yield strength of the material immediately prior to extrusion. This method does not affect the pumpability of the mix until it reaches the print-head. Finally, the prospects of a new set-on-demand method are presented.

54 citations