The excellent adaptability to large temperature differences of alkali-activated slag using Na2SO4+ Ca(OH)2 as the activator

Abstract

The thickening time of NaOH-activated slag system is difficult to control by retarders, which limits its application in oil and gas well cementing. To facilitate the application of alkali-activated slag, the high-temperature thickening and strength characteristics of Na2SO4+Ca(OH)2 activated slag, Na2CO3+Ca(OH)2 activated slag and NaOH activated slag were studied in this paper. It is found that the strength properties of the three are similar, with a strength of about 18 MPa for 72 h. The Na2SO4+Ca(OH)2 activated slag system has superior high-temperature thickening performance (up to 180℃) and extensive temperature difference adaptability (25–180℃). With similar thickening time at 120℃, the Na2SO4+Ca(OH)2 activated slag system has a strength of up to 15 MPa at 48 h at a low temperature of 25℃, and the Portland cement don’t set at 48 h. The Na2SO4+Ca(OH)2 activated slag system shows good thickening properities at 180℃ while still maintaining rapid strength development at 25℃. The large temperature difference adaptability of Na2SO4+Ca(OH)2 activated slag is apparently better than that of Portland cement. The mechanism analysis shows that the effective control of the retarder on the reaction of Na2SO4 and Ca(OH)2 to generate NaOH, and the low-temperature sensitivity of the reaction of Na2SO4 and Ca(OH)2 under the action of the retarder are the main reasons why the ease of adjusting the high-temperature thickening time of the Na2SO4 and Ca(OH)2 activated slag, and the good performance of large temperature difference adaptability. This provides a new way to solve the problem of over-retarding at the top of cement slurry column for long sealing section in deep wells.