Hydration kinetics and shrinkage behavior of hybrid alkaline cements

Hydration kinetics and shrinkage behavior of hybrid alkaline cements

Hybrid alkaline cement (HAC) which is composed of more than 70% supplementary cementitious materials (SCMs), less than 30% Portland cement and a small dosage of alkali activator is considered as a promising alternative material to Portland cement due to its low carbon emissions and excellent mechanical properties. An overview on the hydration mechanisms of HACs and durability is provided in this paper. It is evident that HAC is more durable than Portland cement in a number of environments; however, the lack of long- term track records in field is the barrier for application. The knowledge gap to facilitate the future research and development of HAC materials is also discussed.

In this study, HACs were prepared using 75% granulated blast furnace slag (GBFS) and/or fly ash (FA) and 25% Portland cement with external addition of 3%-5% alkali activator. The influence of precursors and Na2O content on hydration heat, reaction extent and microstructural development were investigated to understand the early hydration kinetics. Increasing the Na2O content improved the compressive strength and reaction extent gradually. The fly-ash based HAC shows slower hydration rate and lower reaction extent than that of slag-based HAC due to the much lower reactivity of fly ash, resulting in reduced compressive strength and slightly delayed setting.

One industrial by-product gypsum is used to retard the rapid setting of HAC. The influence of gypsum on setting time, hydration products and microstructure of HACs were investigated. The setting time of HACs were lengthened with the increase in the gypsum and reached their maximum when the dosage was 20%. The XRD and TG analysis results show that the monosulfate and thaumasite were generated in HACs with gypsum rather than ettringite The incorporation of gypsum led to the reduction of hydration products and a loose microstructure. The retarding effect of gypsum was considered to be the decline of iron dissolution rate caused by chemical reaction instead of coverage theory.

Shrinkage is a vital factor for hybrid alkali activated cement (HAC) related to durability. In this work, the drying shrinkage behavior of HAC mortars with various precursors, curing regimes and activator states was investigated. Portland cement mortar and alkali activated cement (AAC) mortar were used as control groups. Results showed that the drying shrinkage magnitude of HAC mortar is lower than that of AAC mortar but higher than that of Portland cement mortar. Slag-based HAC mortar shows the largest shrinkage value and the incorporation of fly ash could temper shrinkage slightly. The steam curing and solid activation could mitigate the magnitude of drying shrinkage for HAC mortar. MIP, SEM, and XRD analyses results show that the pore size distribution which is related to the capillary pressure, amount of crystalline phase and Ca/Si ratio in product gel are the primary factors affecting the shrinkage behavior of HAC.

The impact of Global Warming Potential (GWP) is employed to evaluate and quantify the carbon dioxide emissions of one cubic HAC concretes. The CO2 footprint of HAC concrete is approximately 43-55% less than comparable concrete containing 100% OPC binder, whereas the carbon footprint of HACs is slightly higher than that of geopolymer concrete and AAC concrete. The carbon emission of HAC decreases when the fly ash is substituted for slag. Increasing the alkali contents of HAC results in an increase in carbon emissions. In terms of activator, HAC activated by waterglass shows the highest carbon emission, while HAC with sodium hydroxide shows the lowest. In conclusion, HAC can be considered as a low-carbon cementitious material due to its lower impact on global warming.

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