Hydration Pathways and C-A-S-H Morphology Control Creep in Blended Cement Paste

Concrete keeps changing even after it hardens because the cement continues to react with water. At the same time, concrete slowly changes shape when it carries a constant load. This slow deformation is called creep. Because hydration and creep happen together, it is hard to separate their effects, especially when the concrete is still young and reacting quickly.

The problem becomes more complicated when some of the cement is replaced with materials such as fly ash or slag. These materials also keep reacting after the concrete hardens and can change C-A-S-H, the primary phase that gives cement paste its strength. Fly ash and slag react differently, so the amount used alone does not tell us how much creep the concrete will develop over time.

Ultimately, creep comes down to what’s happening at a microscopic level inside the paste. The actual makeup and shape of the C-A-S-H gel dictate how much the concrete will give way under pressure. Then there’s the issue of dissolution. Some particles naturally dissolve as the concrete cures, but researchers suspect that the heavy, constant weight itself might actually force parts of the material to dissolve. Trying to separate these overlapping processes experimentally is difficult because they can occur at the same time.

Hydration makes the problem even more complicated because several properties change together as the cement ages. The pore structure becomes different, strength and stiffness increase, and the hydrate phases continue to change. When creep changes with age, it can be difficult to tell whether the cause is ongoing hydration, changes in C-A-S-H, a denser pore structure, or simply the increasing maturity of the cement paste.

In a recent research paper published in Cement and Concrete Research, Dr. Jiawei Wang, Dr. Jun Wang, Dr. Yuanpeng Liu, Professor Zhangli Hu and Professor Jiaping Liu from Southeast University, working with Dr. Yang Chen from China Construction Eighth Engineering Division Co., Ltd., and Professor Xiaohong Zhu, Professor Roya Maboudian and Professor Paulo J.M. Monteiro from the University of California, Berkeley, investigated the aging creep behavior of blended cement paste containing supplementary cementitious materials. Wang and colleagues addressed this problem by asking a more controlled question: what happens to creep when two materials have essentially the same hydration state and comparable physical properties, but continuing hydration is allowed in one and largely removed from the other? The study introduced, for the first time, an equivalent-system approach to blended cement paste, enabling the influence of continuing hydration on aging creep to be isolated under comparable hydration states and physical properties. They examined cement pastes containing fly ash or slag, allowing the contributions of SCM reaction, clinker hydration and C-A-S-H development to be considered separately.

The main idea was to compare cement pastes that were still reacting with similar pastes in which further hydration had been largely removed. In the real samples, clinker and supplementary cementitious materials continued to react normally. In the comparison samples, the clinker and SCMs that would still have been unreacted at a given age were replaced with inert quartz. These samples were then cured for more than 180 days. This allowed the researchers to study creep at a similar hydration level without the added effect of ongoing reactions. The authors examined four blended pastes containing either fly ash or slag at replacement levels of 40 or 60%. They quantified how much clinker, fly ash and slag had reacted and measured pore structure, compressive strength and elastic modulus. These comparisons were important because the real and equivalent samples needed to have similar mechanical properties, allowing differences in creep to be linked mainly to continuing chemical reactions instead of changes in stiffness or pore structure.

The authors found that the two systems had similar compressive strength, elastic modulus and pore structure, but their creep behavior was clearly different. The real hydrating samples generally showed more creep than the equivalent samples, even though their pore structure was denser. This points to ongoing hydration reactions and changes in the hydrate phases as having a stronger effect on creep than pore structure alone. They also found clear differences between fly ash and slag. Fly ash continued reacting gradually over a longer period, and this ongoing pozzolanic reaction was associated with continued creep development. The effect also depended on fly ash content, with higher replacement levels generally producing more creep at the same loading age. In the equivalent system, where continued hydration was removed, creep became much less sensitive to loading age and SCM type. The team noted that slag to have a different pattern and its reaction was much stronger at early ages and slowed substantially after about 28 days. Slag-containing pastes experienced pronounced early creep, but then creep development slowed. The change coincided with the inferred transformation of C-A-S-H from a more fibrillar structure toward a denser foil-like morphology. The study associated this foil-like structure with fewer connected gel pores and greater resistance to creep deformation.

The research work distinguished the roles of SCM and clinker hydration. In these highly substituted blended systems, clinker hydration became limited relatively early, whereas the SCM reactions continued. For slag blends, calcium released into the pore solution can suppress further clinker dissolution, reducing the contribution of clinker hydration to later creep. The researchers consequently found SCM hydration to have a stronger influence on creep development than continuing clinker hydration.

The work by Wang and colleagues represents an important advance in understanding the aging creep of low-carbon blended cementitious materials. In the Wang et al. study, the comparison shows that creep in blended cement paste depends on more than porosity, strength and elastic modulus. Samples with similar physical and mechanical properties still behaved differently when hydration was continuing, indicating that ongoing chemical reactions in the paste, including changes in C-A-S-H, play an important role in how the material slowly deforms under sustained load. Fly ash and slag also affected creep in different ways. Fly ash continued reacting over a longer period and was associated with continued creep development. Slag reacted more strongly at early ages, leading to greater early deformation, but its later formation of foil-like C-A-S-H was associated with reduced creep. The timing of SCM reactions and the structure of the hydrates they produce are important when interpreting the long-term behavior of blended cement.

The long-term behavior of blended cement depends strongly on what continues to happen inside the material after it has hardened. Creep is closely connected to the reactions of supplementary cementitious materials and to the changes these reactions produce in C-A-S-H over time. This helps explain why concretes with similar strength, stiffness and pore structure can still deform differently under sustained load.

Dissolution may also play a role in aging creep and with clinker and SCM particles dissolution during hydration, they contribute to the deformation of the paste. Creep continued even after these reactions had largely slowed which increase the possibility that C-A-S-H itself may also dissolve under sustained load. The authors emphasize that direct experimental evidence is still needed to confirm this mechanism.

For engineers, predicting creep in concrete containing fly ash or slag may require looking beyond age, strength and pore structure. The reaction rate of the SCM and the C-A-S-H structure that develops over time may also influence long-term deformation and this could help in choosing the type and amount of SCM and also when estimating how concrete will behave if it is loaded at different ages. These findings provide a valuable scientific basis for improving creep prediction and for designing low-carbon cementitious materials with more reliable long-term performance.