Compatibility of melamine formaldehyde- and polycarboxylate-based superplasticizers on slag/sintering ash-based geopolymer paste

Abstract

The high viscosity of a geopolymer based on fresh slag/ash represents a notable drawback in terms of workability. This research delves into the potential advantages of incorporating a superplasticizer to address the low workability of geopolymers. Innovatively, geopolymer pastes were formulated using energy production system ash (EPA) and ground granulated blast furnace slag (GGBFS), alongside melamine formaldehyde and polycarboxylate-based superplasticizers (SPs). Furthermore, paste formulations incorporated maximized emission reduction of sintering (MEROS) ash (SA), a by-product of the steel industry. Flow table and marsh funnel time tests were conducted to assess the impact of 1 %, 2.5 %, and 5 % SPs and SA on fresh-state behavior of the geopolymer pastes. Comprehensive examinations of physical, mechanical, and morphological characteristics of the geopolymer pastes were undertaken under two conditions: without and with NaOH (at various NaOH concentrations). The findings indicate that, the presence of SPs has minimal impact on the workability of the pastes. However, when the NaOH is used instead of the Na2SiO3 at 10 %, inclusion of 1 % polycarboxylate-based (PCE), melamine formaldehyde-based (MF) SPs and SA results in a dramatic decrease in the marsh funnel time by 12, 8.5 and 5.3 times, respectively. While the inclusion of 1 % PCE and MF increases the 28-day compressive strength of the geopolymer paste by about 48 % and 4 %, respectively, the addition of 1 % SA causes a significant improvement in the 28-day compressive strength by about 87 % under NaOH-free condition. When 10 % NaOH is used, an increase in the SPs content from 0 % to 5 % leads to a reduction in compressive strength, while the use of 5 % SA leads to an increase in 7-, 28-, and 90-day compressive strength by approximately 33 %, 35 %, and 16 %, respectively. Microstructure analysis reveals that geopolymer gels such as calcium aluminum silicate hydrate and sodium aluminum silicate hydrate can contain sulfur derived from the admixtures. The use of SA not only improves mechanical properties but also promotes sustainable utilization of industrial by-products in construction materials.