Investigation on high-volume fly ash pastes modified with micro-size metakaolin subjected to high temperatures

Portland cement(PC) containing high-volume fly ash(HVFA) is usually used to obtain economical and more sustainable merits, but these merits suffer from dramatically low compressive strength especially at early ages. In this work, the possibility of using micro-size metakaolin(MSK) particles to improve the compressive strength of HVFA paste before and after subjecting to high temperatures was studied. To produce HVFA paste, cement was partially substituted with 70% fly ash(FA), by weight. After that, FA was partially substituted with MSK at ratios fluctuating from 5% to 20% with an interval of 5%, by weight. The effect of MSK on the workability of HVFA mixture was measured. After curing, specimens were subjected to different high temperatures fluctuating from 400 to 1000 ℃ with an interval of 200 ℃ for 2 h. The results were analyzed by different techniques named X-ray diffraction(XRD), thermogravimetry(TGA) and scanning electron microscopy(SEM). The results showed that the incorporation of MSK particles into HVFA mixture exhibited a negative effect on the workability and a positive effect on the compressive strength before and after firing.

Accelerated Carbonation Assessment of High-Volume Fly Ash Concrete

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.

High-Volume Fly Ash Concrete-A Relevant Step to Sustainable Development

HVFA （high-volume fly ash） concrete could be a sustainable way for by-product utilization to conserve natural resources and protect environment. HVFA concrete can play the role of a high-performance material that may be comparable to the conventional Portland cement concrete. The results of the research programme concerning the relationships between the composition of concrete （w/b ratio, fly ash content and type of cement） and their physical and mechanical properties are presented and discussed in the paper. It is found that the introduction of high-volume fly ash into concrete has caused a decrease in compressive strength at the early age of storage. The significant increase in strength was observed between 28 days and 90 days of curing. The high-volume fly ash concretes were characterized with lower water absorbability and sorptivity than control concrete.