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In cement-based pastes, the relationship between the complex phase assemblage and mechanical properties is usually described by the “gel/space ratio” descriptor. The gel/space ratio is defined as the volume ratio of the gel to the available space in the composite system, and it has been widely studied in the cement unary system. This work determines the gel/space ratio in the cement-silica fume-fly ash ternary system (C-SF-FA system) by measuring the reaction degrees of the cement, SF, and FA. The effects that the supplementary cementitious material (SCM) replacements exert on the evolution of the gel/space ratio are discussed both theoretically and practically. The relationship between the gel/space ratio and compressive strength is then explored, and the relationship disparities for different mix proportions are analyzed in detail. The results demonstrate that the SCM replacements promote the gel/space ratio evolution only when the SCM reaction degree is higher than a certain value, which is calculated and defined as the critical reaction degree (CRD). The effects of the SCM replacements can be predicted based on the CRD, and the theological predictions agree with the test results quite well. At low gel/space ratios, disparities in the relationship between the gel/space ratio and the compressive strength are caused by porosity, which has also been studied in cement unary systems. The ratio of cement-produced gel to SCM-produced gel (GC to GSCM ratio) is introduced for use in analyzing high gel/space ratios, in which it plays a major role in creating relationship disparities.

One of the main concerns in the modern cement industry is on reducing the CO2 emissions. As an alternative binder of Portland cement, belite-ye’elimite-ferrite (BYF) cement can reduce the CO2 emission by over 20% due to its benefits concerning the chemical composition and industrial processing. This paper proposed a ternary cementitious system consisting of BYF cement, limestone filler, and silica fume. The ternary system containing 10% limestone and 5% silica fume showed higher compressive strength than the BYF cement, while the CO2 emission factor further reduced by ∼13%. The effect of limestone and silica fume on the hydration was studied. It was found that the hydration reaction of limestone induced the formation of hemicarbonate and stabilized ettringite. This reaction formed dissolution rims around the limestone particles. At the early hydration ages, silica fume was bonded by aluminum hydroxide, forming aluminate silicate hydrate. During the hydration, the silicate dissolved and reacted with the hydration products in the dissolution rims of limestone. This reaction increased the Si content in the rims, and may potentially contribute to the compressive strength development of the ternary cementitious system.

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