Effect of Quartz/Mullite Blend Ceramic Additive on Improving Resistance to Acid of Sodium Silicate-Activated Slag Cement

摘要

We evaluated the usefulness of manufactured quartz/mullite blend (MQMB) ceramic powder in increasing the resistance to acid of sodium silicate-activated slag (SSAS) cementitious material for geothermal wells. A 15-day exposure to 90 deg CO2-laden H2SO4 revealed that the MQMB had high potential as an acid-resistant additive' for SSAS cement. Two factors, the appropriate ratio of slag/MQMB and the autoclave temperature, contributed to better performance of MQMB-modified SSAS cement in abating its acid erosion. The most effective slag/MQMB ratio in minimizing the loss in weight by acid erosion was 70/30 by weight. For autoclave temperature, the loss in weight of 1OO deg C autoclaved cement was a less than 2 %, but at 300DG C it was even lower. Before exposure to acid, the cement autoclaved at 100DG C was essentially amorphous; increasing thq temperature to 200DG C led to the formation of crystalline analcime in the zeolitic mineral family during reactions between the mullite in MQMB and the Na from sodium silicate. In addition, at 300DGC, crystal of calcium silicate hydrate (1) (CSH) was generated in reactions between the quartz in MQMB and the activated slag. These two crystalline phases (CSH and analcime) were responsible for densifing the autoclaved cement, conveying improved compressive strength and minimizing water permeability. The CSH was susceptible to reactions with H2SO4, forming two corrosion products, bassanite and ionized monosilicic acid. However, the uptake of ionized monosilicic acid by Mg dissociated from the activated slag resulted in the formation of lizardite as magnesium silicate hydrate. On the other hand, the analcime was barely susceptible to acid if at all. Thus, the excellent acid resistance of MQMB-modified SSAS cement was due to the combined phases of lizardite and analcime.