Blended cement with reduced CO2 emission- utilizing the fly ash-limestone synergy

Abstract

During cement production large amounts of CO2 are emitted, about 1 tonne CO2 per tonne clinker, if no measures are taken. About 40% originates from fuel combustion, grinding and other operations, and 60% from the de-carbonation of limestone to form the clinker phases. One way to reduce these emissions on the short term is by replacing part of the clinker with other materials such as slag, limestone powder, fly ash, silica fume and natural pozzolans. The type of replacement materials used depends on their availability (e.g. amount available, price and transportation) and is therefore dependent on the geographical location of the cement plant. The aim of this study is to contribute to the development of a novel all-round Portland composite cement for the Norwegian market. When this study was started, the cements produced at the Norwegian cement plants were: CEM I Portland cements containing up to 5% limestone powder and CEM II/A-V Portland fly ash cements containing up to 18% fly ash but no limestone powder. In this study, the effect of increasing the replacement levels of the ordinary Portland cement (OPC) (up to 35% replacement), and combining siliceous fly ash (FA) and limestone powder (L) to replace OPC are investigated.

Using a combination of fly ash and limestone to replace OPC seems to be better than using only one of them. Limestone powder accelerates the early hydration more than fly ash, but fly ash contributes to strength development at later ages due to its pozzolanic reaction. Additionally a chemical interaction between fly ash and limestone has been observed, first in simplified cementitious system and later also in Portland composite cement. Limestone powder interacts with the AFm and AFt phases formed during the hydration of OPC. At first, ettringite forms during the hydration of OPC. When all gypsum is consumed, ettringite will react with the remaining aluminates and form monosulphate. In the presence of limestone, hemi- and monocarboaluminate are formed instead of monosulphate. The ettringite does, therefore, not decompose. This leads to higher volume of the hydrates, which on its turn might reduce the porosity and enhance the compressive strength. The effect of limestone powder on OPC is limited due to its low aluminate content. However, when part of the OPC is replaced by fly ash, the fly ash will introduce additional aluminates to the system as it reacts. This will lower the SO3/Al2O3 and increase the AFm/AFt ratio and thereby amplify the impact of limestone powder. These changes in the AFm and AFt phases have been experimentally observed by TGA, XRD and EDX, and predicted using thermodynamic modelling.

Only a few percent of limestone powder are required to prevent ettringite from decomposing to monosulphate. The changes in hydration products resulting from these small limestone powder contents coincides with an increase in compressive strength. Replacement of 5% fly ash with 5% limestone powder in a 65%OPC+35%FA cement resulted in a compressive strength increase ranging between 8 and 13% after 28 days of curing. At higher limestone contents the compressive strength decreases again as the additional limestone mainly serves as an inert filler. Replacing 5% of OPC with limestone powder resulted, on the other hand, in a strength reduction or a slight increase up to 4% after 28 days of curing. The beneficial effect of limestone is maximal at 28 days, and reduces slightly upon further curing. It is furthermore valid at 5, 20 and 40°C. However, at 40°C the fly ash reaction is accelerated and over time the fly ash content is more important than the synergetic effect.

The observed increase in compressive strength has to be partly due to the chemical interaction described above as an inert filler (crystalline quartz) with a similar psd does not have the same beneficial impact on strength as limestone. Additionally, the presence of limestone powder does not seem to affect the reactivity of OPC and fly ash significantly.

The observed effect between fly ash and limestone enables higher replacement levels than when only one of them is used. The applicability of the study is demonstrated by the fact that cement with the optimal composition found in this study (65%OPC+30%FA+5%L) has recently been used in the construction of the Meteorological Centre in Oslo and the Science Centre in the county of Ostfold