# Microstructure of Mortar with Ballast Waste as a Cement Replacement
## 3 Results and Discussion
### Preparation of Mortars with Ballast Waste
#### 3.2.1 Workability  
In cement-based materials, the fresh state corresponds to the time between cement hydration and setting, during which the material exhibits plastic behavior. Workability tests were conducted on cements with 10%, 15%, and 20% ballast waste replacement (sample B-1 and sample B-2). The selected water/cement ratio for the test was 0.5, and the cement replacement/sand ratio was 1:3.  
Before mixing, the reference cement (OPC) and the ballast replacement dosages were blended in a tubular device for five minutes to prevent segregation during mixing, which was carried out using Controls-brand equipment [47].  
The mortar consistency results are shown in Figure 5. For each type of cement, average values from three consistency determinations are provided. Additionally, the loss of fluidity in the cement with ballast replacement, relative to OPC, is represented and calculated using Equation (1).  
\[Loss\;of\;fluidity=\frac{(medium\;consistency\;reference\;-\;medium\; consistency\; cement)}{medium\;consistency\;reference}\times 100 \tag{1}\]  
In general, a decrease in the spread diameter (mean consistency) is observed as the replacement percentage increases from 10% to 20%. The loss of fluidity in the cement with the replacement follows a linear trend as the ballast waste replacement percentage increases (from 10% to 20%) (Figure 5).  
The mean consistency of both samples studied is similar, and decreases as the replacement percentage increases. The results are also similar for fluidity loss, which increases with the ballast waste replacement percentage in OPC.  
Studies on cement with replacement using siliceous materials [52; 53; 54; 55] show that the resulting mixtures require a higher water demand to maintain workability. The replacement  
Figure 5: Representation of the values of fluidity loss and average consistency in all mortars.  
of small amounts of cement (5% by mass) with kaolin has little effect on paste fluidity. However, at higher replacement percentages (above 10%), the addition of water-reducing agents or superplasticizers is usually necessary to ensure workability.  
Regarding workability, the pozzolanic activity of cements with the studied sample substitutions has been determined using the Frattini test, with testing times of 8 and 15 days, according to the standard. The values obtained have been presented in Figure 6.  
All samples with substitutions at 8 days are located above the CaO solubility curve, with a tendency to be positioned to the left of the CaO/OH\({}^{-}\) solubility curve relative to OPC. This corresponds to an increase in the concentration of Ca\({}^{2+}\) released during the hydration process of the new cements. The sample B-2 mortars at 8 days deviate from the rest and from OPC, although they maintain the tendency to be positioned above the solubility curve.  
Meanwhile, the sample B-2 mortars with the three substitutions at 15 days are below the CaO solubility limit curve, due to a lower calcium ion content released during the hydration reaction. This condition allows these cements to be classified as Class IV pozzolanic-type cements [56].  
The situation appears to coincide with the pozzolanic activity data, that is, with the low activity at an early age up to 28 days, attributed to a filler effect during this period [57], causing a nucleation process in which more space is available for the precipitation of hydration products. In contrast, at ages beyond 28 days, the filler effect is replaced by the pozzolanic effect, leading to a decrease in portlandite content and the release of calcium ions.