MICROSTRUCTURE AND FLEXURE STRENGTHS OF DOLOMITE PARTICULATE- REINFORCED GEOPOLYMER COMPOSITES
Patrick F. Keane1 and Waltraud M. Kriven2
1 Department of Nuclear, Plasma, and Radiological Engineering, University of Illinois at Urbana-Champaign, IL, USA
2 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 USA
ABSTRACT
Potassium-based geopolymer was fabricated at room temperature by high shear mixing of potassium metasilicate and metakaolin, forming a composition of K2O•Al2O3•4SiO2•11H2O. Dolomite dust waste product was used as a particulate reinforcement to form composites containing 0, 20, 30 and 40 wt % dolomite (CaMg(CO3)2). The composites were heat treated at 400, 800 and 1,000°C for 1 hour at a slow ramp rate of 1 °C/min. While single phase geopolymer exhibited 3-pt flexural strengths of ∼7MPa, geopolymer composites containing 20 wt % dolomite and cured at room temperature achieved ∼16 ± 3.5 MPa. In general, strengths drastically decreased due to dehydration, but began to recover after 1000 °C heat treatments. Dolomite particulates helped to dissipate crack energy and enhanced flexural strengths. The geopolymer composites experienced increased porosity due to water loss at 400°C and liberation of CO2 gas from carbonates at ≥ 800 °C.
INTRODUCTION
Geopolymers a type of chemically bonded ceramics of chemical formula M2O•Al2O3•4SiO2•11H2O where M = Na, K, Cs or Li. They are refractory inorganic polymers formed from both aluminum and silicon sources containing AlO4- and SiO4 tetrahedral units, under highly alkaline conditions (NaOH, KOH, CsOH) at ambient temperatures. They are a rigid, hydrated, alumino-silicate solid containing group I, charge-balancing cations which result in an amorphous, cross-linked, impervious, acid-resistant, 3-D structure.1-3
Dolomite is a common rock forming mineral of chemical formula CaMg(CO3)2. It is produced as a fine powder during cutting of dolomite grave stones. Fine dolomite is produced in abundance during crushing and grinding of aggregate material for roads and pavements, and has too small of a particle size to be used solely when pouring large scale pavements. Fine dolomite dust by-product was obtained from an Illinois asphalt company, Gallagher Asphalt Corporation.4 Upon heating, dolomite begins to decompose at ∼850 °C according to the equation:
The final decomposition endotherm peaking at ∼920 °C corresponds to calcite freed from the dolomite decomposition to form lime (CaO) according to the equation:
The aim of this research was to develop geopolymer composites containing dolomite waste dust and to investigate the associated 3-point flexure strengths as a function of heat treatment temperatures. The fine dolomite particulates imparted possible benefits of crack energy dissipation upon heating geopolymer-dolomite composites, by causing a crack deflection mechanism to operate in the composite. It was also anticipated that dispersing dolomite particles into the KGP matrix enhanced stability against moisture exposure which normally produces cracking due to the 6.8 nm average diameter, nano porosity of the pure potassium-based geopolymer matrix.
EXPERIMENTAL PROCEDURES
Sample Preparation
Potassium geopolymer was produced by high shear mixing amorphous fumed silica (Cab-o-Sil, Cabot Corp., Boston, MA), DI water, 97% pure KOH, (Sigma Aldrich Inc.) and metakaolin (BASF GmbH, Germany). This was a two-step process in which fumed silica, DI water, and KOH were mixed to produce potassium water glass (K-WG) of composition K2O + 2SiO2+ 11H2O. The K-WG was then mixed with metakaolin of average particle size 1.2 m, using both a high shear IKA mixer (Model RW20DZM, IKA, Wilmington, NC) as well as a Thinky ARE-250 planetary conditioning mixer (Intertronics, Kidlington, Oxfordshire, UK). After mixing and de-airing the geopolymer paste, the dolomite particles were added, followed by high shearing (IKA), gyroscopical mixing (Thinky), and de-airing (Thinky).
The average particle size of the dolomite particles was measured by sieving and weighing. The sample mixtures were poured into a vi...
