Microstructure and strength of blended FBC-PCC fly ash geopolymer containing gypsum as an additive

Abstract

This study focused on the microstructure and strength of blended fluidized bed coal combustion fly ash (FBC-FA) and pulverized coal combustion fly ash (PCC-FA) geopolymers containing gypsum as an additive. The source materials consisted of 100% FBC-FA and a blend of 75% FBC-FA and 25% PCC-FA. Gypsum was used as an additive at the dosage levels of 0, 5, and 10%wt of the source materials. NaOH and Na 2 SiO 3 were used to activate aluminosilicate sources and temperature curing to accelerate the geopolymer reaction. The microstructures of the geopolymer pastes were examined using XRD, FTIR, MIP and SEM tests. The compressive strengths of the geopolymer mortars were also tested. Test results showed that the blending of FBC-FA and PCC-FA improved the geopolymerization and resulted in a dense matrix with reduced porosity and increased compressive strength as compared to those of the FBC-FA geopolymer. The improvement is due primarily to the high glassy phase content of PCC-FA. In addition, the use of 5% gypsum as an additive further improved the geopolymerization. The sulphate ions enhanced the leaching of alumina from the source materials forming additional aluminosilicate and increased calcium in the system which resulted in the formation of additional CSH.

Use of Gypsum as a Preventive Measure for Strength Deterioration during Curing in Class F Fly Ash Geopolymer System

“…The 2% replacement resulted in the best strength result, observed at 28 days (see Figure 1); however, the higher replacement ratios, more than 4%, markedly lowered the overall strength. Similar trends were observed in earlier studies that investigated the influence of gypsum with a range of replacement ratios for fly ash activated with NaOH and Na-silicate [4,8,13,14]. Those studies reported that the strengths of fly ash geopolymers increased up to a certain point of gypsum addition (~1% to 10%) and decreased above that addition; however, no detailed discussion concerning the causes of the strength decrease was made.…”

“…Note that sample F-NW was made within a typical range of successful synthesizing conditions of activator and curing temperature for fly ash geopolymer, as reported in the literature [3,4,6,8,14,18]. Nonetheless, although the F-NW sample showed a high increasing rate of strength gain over 30 MPa/day at one day and reached its highest value (44.74 MPa) after seven days, the sample deteriorated to half of the seven-day strength value by 28 days.…”

“…[6] found that samples activated with NaOH showed a porous matrix, while those activated with NaOH and Na-silicate solutions showed a denser matrix in the microstructural study. The weight ratio of NaOH and Na-silicate in solution also affects the degree of strength gained, and the optimal ratio varies with the type of fly ash; excessive addition of Na-silicate tends to lower the strength of the hardened geopolymer [3,8]. …”

“…Gypsum was used in a few earlier studies to improve the strength of fly ash based geopolymers when using NaOH and Na-silicate to activate the fly ash [4,8,13,14]. The studies observed that the addition of a small quantity of gypsum increased the material strength, but the addition of more than an optimal quantity reduced the material strength.…”

“…The studies observed that the addition of a small quantity of gypsum increased the material strength, but the addition of more than an optimal quantity reduced the material strength. Although the studies claimed that the strength improvement was due to the formation of more C-S-H [8] or a higher degree of Al or Si dissolution from the fly ash [13,14], the claims were not confirmed. No sufficient explanation for the reduced strengths when the gypsum concentration exceeded certain limits was provided.…”