Geothermal waste: An alternative replacement material of Portland cement: Fid-Bau Portal

Geothermal waste: An alternative replacement material of Portland cement

Gomez-Zamorano, L.Y. / Escalante-Garcia, J.I. / Mendoza-Suarez, G.

One of the most significant disadvantages of energy generation from geothermal resources is the generation of relatively large amounts of solid wastes that must be land filled. Such geothermal waste (GW) is obtained as a mixture of geothermal brine and steam (naturally pressurized) that undertakes a series of stages towards heat extraction. As its temperature is reduced (from about 200 deg C) a solid precipitates from the mixture, the composition of which is mostly amorphous SiO2 and also NaCl and KCl. This paper presents results from research aimed to investigate in more detail the pozzolanic behavior of GW behavior (reaction with Ca(OH)2 generated by the cement hydration) as well as its effect on the Portland cement hydration. The pozzolanic activity of the GW was evaluated by the method proposed by Luxan. A plastic flask containing 400 ml of Ca(OH)2 saturated aqueous solution was placed in a water bath at 40 deg C, then 10 g of GW was added under continuous stirring. The Ca(OH)2 saturated solution at 40 deg C had a pH of 11.92 and electrical conductivity of 8.2 mS/cm. After adding the GW, these values decreased with time; this indicated that Ca(2+) and OH(-) concentration in the solution dropped, which can be attributed to an interaction with the GW. The drop in the electrical conductivity after the addition of the GW indicates a good pozzolanic activity. The addition of 10% and 15% of GW resulted in progressively increased compressive strength, but for 20% GW the strengths were lower than those obtained for 15%. However, it seems that in all cases the addition of GW was beneficial. The hydration reactions of OPC generate Ca(OH)2 as a byproduct, and its concentration can be followed by thermogravimetry since it decomposes, loosing water, showing a weight loss step at about 450 deg C. For the P(0) mix the Ca(OH)2 values increased as the time advanced, as expected for progressive hydration reactions. For the mixes P(10), P(15), and P(20) the %Ca(OH)2 were lower compared to P(0); the values increased for the first 7 days to 14 days and then the CH concentration was depleted. This is evidence for the pozzolanic behavior of the GW since the first days of hydration. The Ca(OH)2 consumption was greater as the %GW increased. The trend towards lower Ca(OH)2 concentrations indicates that the pozzolanic reaction is still in progress and for the paste P(20) the Ca(OH)2 was totally consumed.