4.11
Pozzolanen
Pozzolen zijn silicium- of aluminiumhoudende materialen die gemengd worden met Portlandcement. Ze reageren met het calciumhydroxide dat ontstaat tijde…
Portland cement is sometimes interground or blended with pozzolans, which are siliceous and aluminous materials.
The finely divided pozzolans, in the presence of moisture, react with the calcium hydroxide released during cement hydration to produce cementitious compounds.
Pozzolans are naturally available, such as volcanic ash and pumicite, or they are synthesized, such as fly ash, which is obtained from the flue gases of coal-fired power plants.
Pozzolans are added with cement used in roller-compacted concrete, mass concreting, and chemically resistant concrete.
When cement is replaced with pozzolan on a weight basis, the volume of pozzolan equivalent to the cement it replaces will be higher, as the specific gravity of pozzolan is less compared to cement.
The hydration of pozzolans is slower compared to that of Portland cement, resulting in a low rate of heat development, longer setting time and longer curing periods.
Lastly, the pozzolanic activity with cement can be assessed using the strength activity index, which is the ratio of the compressive strength of cement partially replaced with pozzolans to that without any replacement.
Q1: What are pozzolans and where do they come from?
Pozzolans are siliceous and aluminous materials blended with Portland cement to improve concrete performance. They occur naturally, such as volcanic ash and pumicite, or are synthesized from industrial by-products like fly ash collected from coal-fired power plant flue gases. Pozzolans react with calcium hydroxide during cement hydration to form cementitious compounds that enhance strength and durability.
Q2: How does pozzolanic activity affect concrete strength development?
Pozzolanic hydration is slower than Portland cement hydration, resulting in longer setting times and extended curing periods. However, Portland-pozzolan cements achieve high long-term strength, depending on the pozzolan proportion used. The strength activity index, defined in ASTM C 618-93, measures a pozzolan's effectiveness by comparing compressive strength of mixtures with and without pozzolan replacement.
Q3: Why does pozzolan replacement require more cement by volume than by weight?
Pozzolans have lower specific gravity compared to Portland cement. When cement is replaced on a weight basis, the volume of pozzolan needed to match that weight is higher. This volumetric difference is important for concrete mix design calculations and material proportioning in construction applications.
Q4: What are the benefits of using pozzolans in different concrete applications?
Pozzolans are added to cement used in roller-compacted concrete, mass concreting, and chemically resistant concrete. Their slower hydration produces low heat development, making them ideal for large-volume pours. Fly ash and other pozzolans also improve sulfate resistance and long-term durability while reducing strength and heat of hydration compared to ordinary Portland cement.
Q5: What types of pozzolanic materials are used in modern concrete?
Common pozzolans include fly ash from coal combustion, rice husks burned between 500 and 700°C to yield amorphous material, metakaolin from calcined kaolinitic clay, and silica fume from silicon and ferrosilicon production. Silica fume consists of highly reactive glassy particles that fill voids in concrete, significantly enhancing its strength and durability.
Q6: How is pozzolanic activity measured and assessed?
Pozzolanic activity is assessed using the strength activity index, which calculates the ratio of compressive strength in cement mixtures partially replaced with pozzolans to that without any replacement. This metric, standardized in ASTM C 618-93, quantifies a pozzolan's effectiveness in contributing to concrete strength development and is essential for quality control in cement blending.
Q7: What chemical reaction occurs between pozzolans and Portland cement?
In the presence of moisture, finely divided pozzolans react with calcium hydroxide released during hydration of cement to produce additional cementitious compounds. This pozzolanic reaction improves concrete's mechanical properties and durability. The reaction proceeds more slowly than primary cement hydration, contributing to extended curing times but ultimately higher long-term strength.