Issue 38

U. Haider et alii, Frattura ed Integrità Strutturale, 38 (2016) 305-318; DOI: 10.3221/IGF-ESIS.38.41 315 It can be seen from Fig. 7(b) that as the age of all cementitious samples increase the compressive strength increases. Till 14 days age of specimens, much differences in compressive strengths are not seen for all the specimens. However as the age of specimen’s increases, the compressive strength curve for second layer becomes steep and shows that hydration of second layer particles continue to produce additional C-S-H gel which gives 7% more compressive strength to it at the age of 90 days, as compared to raw fly ash particles. Whereas it can also be seen that hydration of particles of third layer slows down and its samples show less compressive strength at the age of 28 and 90 days as compared to raw fly ash and second layer samples. The reason for strength gain of samples as age of specimens increase is because in the mixtures as the cement particles start reacting with water and they produce calcium silicate hydrates and calcium hydroxides while silica and alumina in raw fly ash, second, and third layer particles react with calcium hydroxide to form additional calcium silicate hydrates and calcium aluminate hydrates which starts to give strengths mainly at and after 28 days [20-21]. However, the reason for which samples containing second layer particles show more compressive strength is because second layer particles have small particle size and large surface area which makes them reacts abruptly in cementitious mixtures to produce hydration products as compared to samples containing raw fly ash and third layer particles. Fig. 7(c) shows load - deflection relationship when the specimens are tested in flexure at 90 days. Here it can be seen that samples of third layer particles have more flexural load taking ability and deflect more as compared to raw fly ash and second layer particles. The reason of this is because angular particles of third layer create better interlocking between particles which improves the strength and therefore large deformations occur [47]. Fig. 7(d) shows the curves between vicat needle penetration and setting time of paste for pure cement, 60% raw fly ash replaced with cement, 60% second layer replaced with cement, and 60% third layer replaced with cement. It can be seen here that vicat needle penetration which is an indication of the hardening or setting of cementitious pastes [48] is seen to be lowest with time for cement paste, which indicates that initial and final setting times of cement is lower than that of raw fly ash, second layer, and third layer containing cementitious pastes. Whereas third layer paste show lower setting time as compared to raw fly ash and second layer but more setting time as compared to cement paste. Second layer particles show more setting time as compared to cement, third layer, and raw fly ash pastes. The reason for increase of setting times of raw fly ash paste mixture as compared to cement paste mixture is that when the raw fly ash is replaced with cement, the concentration of cement particles decrease and the distance between the particles of cement increase, along with it lesser hydration products are formed, so both factors contribute to an increase in the setting time of raw fly ash paste mixture [49]. Whereas second layer comparatively finer particles fill in the voids between the cement particles and decrease further the concentration of cement in the second layer paste mixture and thus increase the setting time more as compared to raw fly ash paste [50]. The third layer particles have relatively large size of particles which lack finer particles in them and replacing cement with third layer particles in third layer cementitious paste firstly decreased the concentration of cement particles as compared to pure cement paste sample but cement also filled in the pores within third layer particles and cement concentration increased as compared to raw fly ash particles therefore as compared to raw fly ash paste third layer paste show decrease of setting time. Further it can be seen in Fig. 7 that raw fly ash specimens showed properties that were closer to the second layer samples as compared to the third layer samples because raw fly ash particles, as seen in Fig. 1, were found to contain up to about 55-60% second layer particles and up to 35-40% third layer particles. Behaviour under harsh environment Fig 8 shows the graph between charge passed through a 50 mm cementitious sample vs time measured, when the specimens were exposed to 3% NaCl and 0.3 M NaOH solution on the negative and positive ends of a 20 V DC supply. It can be seen here that as the time increased from 0 to 48 hours, more charge passed through all the samples. However in 48 hours the charge passed through the third layer samples was the highest and lowest charge passed was recorded through second layer samples as compared to samples containing raw fly ash particles. The high permeability of samples containing third layer particles seen in Fig. 8(a) is due to the fact that the third layer particles contains very large angular particles, in which small size rounded particles are missing and when angular particles are packed together they create porosity in them. However, the second layer particles are more rounded particles which pack more tightly together and thus lesser charge passes through them as compared to samples having third layer angular particles [43]. Chloride concentration profiles per gram of the specimens are plotted versus their average depths in Fig. 8(b). Here it can be seen that samples containing third layer particles show high concentration of chlorides at depths between 2.5 and 17.5 mm as compared to samples containing raw fly ash and second layer particles. It is further seen that chloride penetrates the third layer particles till the depth of 17.5mm, whereas in samples having raw fly ash and second layer particles chlorides penetrate till depth of 12.5mm.