Abstract
The issue of concrete carbonation has gained importance in recent years due to the increase use in supplementary cementing materials (SCMs) in concrete mixtures. While there is general agreement that concrete carbonation progresses at maximum at a relative humidity of about 60%, the rate may differ in the case of cements blended with SCMs, especially with high-volume fly ash replacements. In this study, the effect of high-volume fly ash concrete exposed to low ambient relative humidity (RH) conditions (57%) and accelerated carbonation (4% CO2) is investigated. Twenty-three concrete mixtures were produced varying in cementitious contents (310, 340, 370, and 400 kg/m3), water-to-cementitious materials ratio (0.45 and 0.50), and fly ash content (0%, 15%, 30%, and 50%) using a low and high-calcium fly ash. The specimens were allowed 1 and 7 days of moist curing and monitored for their carbonation rate and depth through phenolphthalein measurements up to 105 days of exposure. The accelerated carbonation test results indicated that increasing the addition of fly ash also led to increasing the depth of carbonation. Mixtures incorporating high-calcium fly ash were also observed to be more resistant against carbonation than low-calcium fly ash due to the higher calcium oxide (CaO) content. However, mixtures incorporating high-volume additions (50%) specimens were fully carbonated regardless of the type of fly ash used. It was evident that the increase in the duration of moist curing from 1 day to 7 days had a positive effect, reducing the carbonation depth for both plain and blended fly ash concrete mixes, however, this effect was minimal in high-volume fly ash mixtures. The results demonstrated that the water-to-cementitious ratio (W/CM) had a more dramatic impact on carbonation resistance than the curing age for mixtures incorporating 30% or less fly ash replacement, whereas those mixtures incorporating 50% showed minor differences regardless of curing age or W/CM. Based on the compressive strength results, carbonation depth appeared to decrease with increase in compressive strength, but this correlation was not significant.
Highlights
The need for innovative infrastructures and urban housing is enormous and the burden of fulfilling these necessities is heavily reliant on the concrete industry
The results demonstrated that the water-tocementitious ratio (W/CM) had a more dramatic impact on carbonation resistance than the curing age for mixtures incorporating 30% or less fly ash replacement, whereas those mixtures incorporating 50% showed minor differences regardless of curing age or W/CM
This is likely attributed to the higher calcium oxide (CaO) content present in the chemical composition of high-calcium fly ash
Summary
The need for innovative infrastructures and urban housing is enormous and the burden of fulfilling these necessities is heavily reliant on the concrete industry. In order to meet these expectations, more than 4.1 billion metric tons of ordinary portland cement (OPC) are mass-produced every year contributing approximately 3.4 billion metric tons of CO2 [1] These emissions are estimated to add up to 7% of the total emissions from all sources indicating the enormity of the impact on environment [2]. Utilization of SCMs such as fly ash and blast furnace slag presents many advantages, such as improving fresh and hardened properties, minimizing thermal cracking, and resisting chemical threats such as alkali-silica reaction, sulfate attack, and chloride penetration [5]. These properties are enhanced by using more SCMs, provided that the concrete is proportioned, placed, and cured appropriately. It is one of the few cases where high dosages of SCMs do not improve the durability of reinforced concrete structures
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