EFFECTS OF BIO-BASED PLASTICIZERS, MADE FROM STARCH, ON THE PROPERTIES OF FRESH AND HARDENED METAKAOLIN-GEOPOLYMER MORTA
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EFFECTS OF BIO-BASED PLASTICIZERS, MADE FROM STARCH, ON THE PROPERTIES OF FRESH AND HARDENED METAKAOLIN-GEOPOLYMER MORTAR: BASIC INVESTIGATIONS ADRIAN TUTAL *
, STEPHAN PARTSCHEFELD, JENS SCHNEIDER, AND ANDREA OSBURG
1
Faculty of Civil Engineering, F. A. Finger-Institute for Building Materials Engineering, Chair of Building Chemistry and Polymer Materials, Bauhaus-Universität Weimar, Weimar, Germany
Abstract—Conventional superplasticizers based on polycarboxylate ether (PCE) show an intolerance to clay minerals due to intercalation of their polyethylene glycol (PEG) side chains into the interlayers of the clay mineral. An intolerance to very basic media is also known. This makes PCE an unsuitable choice as a superplasticizer for geopolymers. Bio-based superplasticizers derived from starch showed comparable effects to PCE in a cementitious system. The aim of the present study was to determine if starch superplasticizers (SSPs) could be a suitable additive for geopolymers by carrying out basic investigations with respect to slump, hardening, compressive and flexural strength, shrinkage, and porosity. Four SSPs were synthesized, differing in charge polarity and specific charge density. Two conventional PCE superplasticizers, differing in terms of molecular structure, were also included in this study. The results revealed that SSPs improved the slump of a metakaolin-based geopolymer (MK-geopolymer) mortar while the PCE investigated showed no improvement. The impact of superplasticizers on early hardening (up to 72 h) was negligible. Less linear shrinkage over the course of 56 days was seen for all samples in comparison with the reference. Compressive strengths of SSP specimens tested after 7 and 28 days of curing were comparable to the reference, while PCE led to a decline. The SSPs had a small impact on porosity with a shift to the formation of more gel pores while PCE caused an increase in porosity. Throughout this research, SSPs were identified as promising superplasticizers for MK-geopolymer mortar and concrete. Keywords—Bio-based . Geopolymer . Metakaolin . Starch . Superplasticizer
INTRODUCTION Used at a rate of ~4 Gt/a, cement is the most widely deployed binding agent in construction materials (Andrew). In spite of constant improvements in the energy efficiency of manufacturing processes over recent decades and the increasing substitution of cement clinker by materials such as ground granulated blast-furnace slag (GGBS), fly ash (FA), and metakaolin (MK), cement production is responsible for the emission of ~1.5 Gt/a of CO2. This corresponds to ~4% of the annual global anthropogenic CO2 emissions (Le Quéré 2018). Cement-substitute materials such as GGBS, FA, and MK have a glassy alumosilicate structure and also react with alkaline activators (alkali hydroxide solution or alkali silicate solution) to form an aluminosilicate network (ASN). In highly alkaline environments, the X-ray amorphous aluminosilicates dissolve and condense to an ASN and transform further to a zeolite-like structure in which the activat
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