The effect of thiocyanates on the hydration of portland cement at low temperatures

Abstract

Sodium thiocyanate, potassium thiocyanate, ammonium thiocyanate, calcium thiocyanate, and lithium thiocyanate were added to Normal Type 10 portland cement in amounts of 1.5 and 3% (with respect to cement on weight basis) and cured at temperatures of 20, 0 and −5°C. The rate of development of heat of hydration, calcium hydroxide content and strength development were followed from a few hours up to 28 days. All thiocyanates increase the early rate of reaction of cement at 20°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°C, there was acceleration in the presence of thiocyanates and the heat of hydration was higher in all the samples containing thiocyanates. All thiocyanates accelerate hydration at −5°C with respect to the reference paste. The reference sample showed practically no hydration even up to 4 days as the water in the pores remained frozen. Some freezing occurred in the presence of NH 4SCN, 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 thiocyanate for increasing strengths at low temperatures. After 28 days of curing at −5°C, the cement paste with 3% calcium thiocyanate attains a strength that is 74% of the strength of cement paste cured at 20°C. Sodium thiocyanate is also an effective accelerator and increases strength at low temperatures. The least effective thiocyanates with respect to the development of strengths at −5°C are LiSCN, KSCN and NaSCN at a dosage of 1.5%. A linear relationship exists between the amount of lime formed and strength, within the range of curing periods studied. However, when strength is compared at the same 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 degree of hydration. At the same degree of hydration at −5°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.