Hydration, Hardening Mechanism, and Performance of Tuff Normal Concrete

Using admixtures to replace part of cement can effectively reduce environmental pollution. Many concrete gravity dams have been planned for Tibet, where the resource of fly ash (FA) is scarce but the resource of tuff is abundant. Compared with the more mature and widely used FA concrete in gravity dams, the application of tuff concrete is rare, and the hardening mechanism of the cement–tuff binary cementation system and the performance of tuff concrete still need to be further studied. In view of this, the hydration and hardening mechanism of the cement–tuff binary cementation system using varying percentages of tuff contents (0%, 25%, and 50%) are discussed, and the mechanical properties (e.g., strength, modulus) and durability performance (e.g., freeze-thaw, carbonization) of tuff normal concrete are investigated in this paper. The microstructure of the cement–tuff system shows that additional tuff powder promotes the hydration of clinker in cement, which increases the calcium hydroxide content [$\mathrm{Ca}(\mathrm{OH}{)}_2$] and leads to a higher porosity at early curing age. Results show that the amount of tuff content is almost positively correlated with porosity. The additional tuff in concrete increases the earlier splitting tensile strength, ultimate tensile value, and drying shrinkage, which also slightly reduces the frost resistance and little affects the compressive strength, tensile strength, elastic modulus, impermeability, and carbonation depth. Tuff normal concrete shows better performance than FA concrete in some aspects. The results show that tuff materials can be used to replace FA, and tuff normal concrete can be applied to the construction of concrete gravity dams in areas where fly ash is scarce and tuff is abundant.