# Long-Term Behavior of Concrete Containing Wood Biomass Fly Ash
## 3 Results and Discussion
### Drying Shrinkage  
The change in length of the concrete samples was monitored over a period of more than one year (395 days for mix M0 to 380 days for mix M4), with the first measurement taken at 24 h of age. Three samples of each mix were tested. Subsequently, the average value and the corresponding standard deviation of the change in length were calculated and are presented in Figure 6. Mix M1 with 15% cement replacement by WFA1 exhibited the same shrinkage deformation as mix M0 during the measurement period. For mix M2, the average shrinkage deformation decreased by about 37% after one year compared to the reference mix M0. It was found that increasing the aggregate/cement ratio of the concrete reduced the volume of the contracting cement paste and the overall shrinkage capacity [48]. Replacing cement with WFA could lead to a similar effect, since WFA is not as reactive as cement, as shown by the compressive strength test (Figure 5), but also by previous studies with wood ash from the same source [2]. Measurements on specimens from mixes M3 and M4 showed that WFA2 had a larger influence on the drying shrinkage behavior than mixes with WFA1. For mixes M3 and M4, where 15% and 30% cement were replaced by WFA2, shrinkage deformations after one year were reduced by about 32% and 65%, respectively, compared to mix M0. Mix M4 also showed an increase in the specimen length in the early aging phase. Since swelling of the specimens can be accompanied by cracking and weakening of the structure, additional investigations were carried out on the specimens used for drying shrinkage. Visual inspection of the specimens was performed periodically up to an age of the specimens of 2 years. Visual inspection did not reveal any cracks that would indicate deterioration of the specimens. Additionally, to verify that the initial swelling had not caused internal damage of the concrete structure, the compressive strength was measured as described in Section 2.3. All mixes showed very similar trends in drying shrinkage at later ages, suggesting that the main difference between mixtures was due to early age reactions. [PERSON] et al. [27] analyzed the chemical properties of WFA from the same power plants as the WFA used in this work and found that WFA2 had a significant amount of free MgO and free CaO. Both free CaO and free MgO are recognized as the main causes of unsoundness of Portland cement [48]. However, they are also often used as expansive agents to reduce shrinkage deformations in cement-based materials [63; 64; 65; 66; 67; 68; 69]. In contact with water, free CaO forms Ca(OH)\({}_{2}\) (portlandite), followed by volume expansion. The reactivity of CaO depends on the calcination temperature. A higher calcination temperature results in less reactive CaO, but regardless of the calcination temperature, CaO reacts within the first few days if sufficient water is available for the reaction [70; 71; 72]. The increase in the length of the concrete prisms from mix M4 could have been caused by the formation of portlandite. In Portland cement, the expansion of MgO is attributed to the formation of Mg(OH)\({}_{2}\) (brucite), which is formed by the reactions between magnesium and water [48]. This reaction proceeds very slowly and can lead to long-term unsoundness. MgO was found to be much more reactive at calcination temperatures up to 900 \({}^{\circ}\)C during early hydration than MgO in cement clinker calcined at \(\sim\)1450 \({}^{\circ}\)C [73]. The WFA used in this work was burned at temperatures up to 950 \({}^{\circ}\)C. It is therefore possible that the MgO contained in the WFA was much more reactive than the MgO contained in Portland cement and could have contributed to the early age expansion of the concrete prisms. Evidence for the presence of brucite in the concrete mixes studied in this work was additionally provided by thermogravimetric (TG) analysis. The DTG curves are shown in Figure 7. Two distinct peaks can be seen in Figure 7a.