Optimering av sulfatinnehållet hos blandcement

Sammanfattning: Abstract The production of concrete has a large environmental impact because of the carbon dioxide emissions from cement production. To reduce emissions of carbon dioxide, part of ordinary portland cement can be replaced with different supplementary cementitious materials to create blended cements. Supplementary cementitious materials are often waste products from other industries, such as fly ash, silica fume and granulated blast furnace slag, or materials like calcined clay that can be produced without emitting large amounts of greenhouse gases. The objective of this project was to investigate whether an optimization of the sulfate content of blended cements is necessary and how the addition of sulfate to an already sulfate optimized portland cement combined with a supplementary cementitious material affects the product. Three different supplementary cementitious materials were used: a calcined clay, a fly ash and a granulated blast furnace slag, and the cement that was used was a rapid-hardening portland cement. These materials were used to make mortar or cement paste with different amounts of added sulfate to find an optimum sulfate level and to examine the effect of an increased amount of added sulfate to the blended cement. A literature study was conducted to examine the role of sulfate in the cement hydration process, the philosophy of sulfate oprimization, and the methods you can use to do a sulfate optimization. Three different methods were used to evaluate the role of sulfate in blended cements. The compressive strength tests on mortar prisms after 1, 3, 7 or 14 days have been done. Measurement of total heat of hydration on mortar have been done and the evaluation of the shape of heat flow curves using isothermal calorimetry after 1, 3, 7 or 14 days. The setting time was measured with an automatic Vicat. The result of heat of hydration and the compressive strength have been evaluated to investigate whether there is a correlation between the two methods. The results show that the granulated blast furnace slag and fly ash did not require added sulfate to improve the compressive strength or total heat of hydration. The results indicated that an increased amount of added sulfate reduces the compressive strength and the total heat of hydration. However, according to the heat flow curves, a small amount of sulfate may be added to increase the distance of the main hydration peak and the sulfate depletion peak. An increased distance makes the blended cements less affected by external factors that affects the sulfate solubility, such as temperature changes during the hydration of cement. The compressive strength of the material with calcined clay and its total heat of hydration could be slightly improved with added sulfate. The apparance of the heat flow curves clearly show that there is a need for an additional amount of sulfate to make the blended cement less affected by external factors that affects the sulfate solubility, such as temperature changes. The setting time of the granulated blast furnace slag and fly ash were almost unchanged with increasing addition of sulfate but the calcined clay showed reduced setting time with an increasing amount of sulfate. A correlation between total heat of hydration and compressive strength was found in the present measurements. This indicates that heat flow can be used for sulfate optimization instead of compressive strength, but this needs more validation.