THE EFFECT OF THIOCYANATES ON THE HYDRATION OF PORTLAND-CEMENT AT LOW-TEMPERATURES

Sodium thiocyanate, potassium thiocyanate, ammonium thiocyanate, calci um thiocyanate, and lithium thiocyanate were added to Normal Type 10 p ortland cement in amounts of 1.5 and 3% (with respect to cement on wei ght basis) and cured at temperatures of 20, 0 and - 5 degrees C. The r ate of development of heat of hydration, calcium hydroxide content and strength development were followed from a few hours up to 28 days. Al l thiocyanates increase the early rare of reaction of cement at 20 deg rees C. The most efficient early acceleration occurs with 3% Ca(SCN)(2 ). The total heat of hydration in samples containing KSCN is about 30% more than that registered for the reference at 3 days. At 0 degrees C , there was acceleration in the presence of thiocyanates and the heat of hydration was higher in all the samples containing thiocyanates. Al l thiocyanates accelerate hydration at - 5 degrees C with respect to t he reference paste. The reference sample showed practically no hydrati on even up to 4 days as the water in the pores remained frozen. Some f reezing occurred in the presence of NH4SCN, LiSN and 1.5% Ca(SCN)(2). Calcium thiocyanate accelerates the hydration and strength development in the paste at all curing temperatures. It is the most effective thi ocyanate for increasing strengths at low temperatures. After 28 days o f curing at - 5 degrees C, the cement paste with 3% calcium thiocyanat e attains a strength that is 74% of the strength of cement paste cured at 20 degrees C. Sodium thiocyanate is also an effective accelerator and increases strength at low temperatures. The least effective thiocy anates with respect to the development of strengths at -5 degrees C ar e LISCN, KSCN and NaSCN at a dosage of 1.5%. A linear relationship exi sts between the amount of lime formed and strength, within the range o f curing periods studied. However, when strength is compared at the sa me degree of hydration (in terms of lime formed), some pastes exhibit better strengths than others. The relative strengths, therefore, seem to be dependent more on the microstructure of the pastes than on the d egree of hydration. At the same degree of hydration at - 5 degrees C, 1.5-3% Ca thiocyanate, or 3% Na, K and Li thiocyanates exhibit better strengths than those containing 1.5% Na, Li and K thiocyanates.