# Long-Term Behavior of Concrete Containing Wood Biomass Fly Ash
## 3 Results and Discussion
### Drying Shrinkage
A smaller peak in the range between 400 and 450 \({}^{\circ}\)C indicates the decomposition of portlandite, while the large peak between 600 and 800 \({}^{\circ}\)C belongs to the decomposition of dolomite [74; 75]. Since brucite decomposes in the temperature range of 350-450 \({}^{\circ}\)C [76], this part of the DTG curves is shown enlarged in Figure 7b. The peak occurring around 350 \({}^{\circ}\)C could indicate the possible decomposition of brucite. This peak is most pronounced in mixtures M3 and M4, which contain WFA2. This supports the assumption that brucite could partly be the cause of the initial volume increase. Figure 6: Results of the length change measurements: (**a**) Mixes M0, M1, and M2; (**b**) Mixes M0, M3, and M4. However, free MgO from WFA is not the only possible source of brucite in the mixes analyzed in this work. In concrete made with dolomite aggregate, brucite (Mg(OH)\({}_{2}\)) and calcite (CaCO\({}_{3}\)) form as a result of the dedolomitization reactions of the reaction between dolomite and portlandite [77]. The rate of this reaction is influenced by the temperature and the content of alkalis [77; 78]. The rate and extent of the reaction also depend on the particle size of the dolomite. Larger grains are almost always less reactive because they can form a protective shell of reaction products [78; 79]. The number of particles passing the 0.063 mm sieve of the crushed dolomite aggregate used in this work was about 2.5% of the concrete mass. It can therefore be assumed that brucite was present in all the mixes analyzed in this work. It has been discussed that the dedolomitization reaction does not cause expansion [80; 81]. For the aggregate used in this work, there have been no previous reports that alkali aggregate reactions cause any volume instabilities, which was also further confirmed by the drying shrinkage results. Therefore, the cause of the volume increase is probably due to the reactions caused by the wood ash. Figure 7: Results of the TG analysis. (**a**) DTG curves in the entire temperature range from 30 to 1000 \({}^{\circ}\)C; (**b**) enlarged DTG curves in the temperature range up to 600 \({}^{\circ}\)C.  
Figure 8 shows an example of detrimental cracking in concrete caused by the expansive effect of wood ash in concrete (results of a previous study [82]). The sample in Figure 8 was made with a 30% replacement of Portland cement with WFA containing 57.6% CaO and 0.7% MgO. In the same study, no cracks were found in the concrete mix made with 15% of cement replacement [82]. The probable mechanism that leads to the cracking of the sample in Figure 8 is related to the crystallization pressures caused by expansion during the hydration of free CaO [83]. The formation of Ca(OH)\({}_{2}\) from CaO is accompanied by the molar volume increase by a factor of \(\approx\)2 [84]. The growth of Ca(OH)\({}_{2}\) crystals in the pore space fills the voids without causing much expansion of the cement paste, but as crystals become enclosed, their expansion becomes restrained and stresses build up. The energy forcing the Ca(OH)\({}_{2}\) crystals to grow can lead to pressures exceeding 150 MPa in fully restrained samples [85]. In the cement paste matrix, crystallization pressures are controlled by the amount and reactivity of free CaO, the possibility of the diffusion of Ca\({}^{2+}\) and OH\({}^{-}\) ions through the pore space, and the degree of restraint [86]. During storage, the amount of free CaO and free MgO in WFA decreases due to reactions with CO\({}_{2}\) or water present in the air [27]. In the case of concrete from Figure 8, WFA was used in the concrete two weeks after sampling at the power plant, and the first cracks were observed about 6 months after placement. The WFA2 used in this work had a CaO content of 57.9% and was stored in the laboratory for two months before being used in the concrete. Although no detrimental effects were found, the initial swelling of the concrete indicates its reactivity.