## 4 Test Results and Analysis
### Mechanism Analysis  
From the results above, it can be seen that the unconfined compressive strength at 7d, 28d, and 90d have particular variation features as the proportion of RBA increases. The maximum value is obtained at 50% of the specimen at 7d. While at 28d it is kept in a stable range, after 50%, the proportion increases. At 90d, the strength increases as the proportion increases, and at 50%, it has the almost same value as that at 80%. It means that the potential activity of RBA activated and offset part of the weakness caused by the weak performances of it, which ultimately affected and enhanced the strength of the cement stabilized macadam layer. Combining with the plastic index and crushed stone value showed in Table 1, 50% is the most appropriate proportion in this study. Comparing with the reference value at 7d stipulated for the first-class highway by specification (JTG/T F20--2015), 3-5 MPa is stipulated, and the strength of the 50% specimens exceeds the upper limit. The \"modified EDTA titration experiment \"shows the titration results after 28 days' maintenance; the specimens with 50% of RBA have the minimum content of Ca\({}^{2+}\). Of this proportion, the potential activity of RBA was enormously stimulated and satisfied most of the specifications. When the replacement ratio increases to 80%, the CaO in clay brick increases because RBA in 80% consumes a similar Ca\({}^{2+}\) amount of 50%, and the strength results showed that the strength generated by potential activation can offset the strength weakened by the weak performances of the part of RBA and improve the strength level.  
It can be proved that the activation of RBA activity is based on the hydration reaction of cement. The potential activation of RBA from the strength makes use of the product of the cement hydration reaction. The consumption of this product makes the activated part of RBA form strength at a rate less than that of the cement hydration reaction. xCaO-SiO\({}_{2}\)-yH\({}_{2}\)O (calcium silicate hydrate, short for C-S-H) and Ca(OH)\({}_{2}\) are major products of the hydration reaction. After a period of time, a certain amount of Ca(OH)\({}_{2}\) is produced. Then, Ca(OH)\({}_{2}\) and SiO\({}_{2}\) in RBA react as shown in Equation (1).  
\[\text{Ca(OH)}_{2}+\text{SiO}_{2}+\text{H}_{2}\text{O}\text{$\rightarrow$} \text{xCaO-SiO}_{3}\text{yH}_{2}\text{O} \tag{1}\]  
In Equation (1), Ca(OH)\({}_{2}\) (form cement hydration), SiO\({}_{2}\) (the main substances in clay brick), and H\({}_{2}\)O react and produce xCaO-SiO\({}_{2}\)-yH\({}_{2}\)O. This reaction to some extent increases the hydration product C-S-H of cement and increases the amount of the cement that participates in a hydration reaction. When the RBA content reaches a reasonable value, i.e., 50% in this paper, the SiO\({}_{2}\) in clay brick consumes a large amount of the Ca(OH)\({}_{2}\) produced by the cement hydration reaction, and produces xCaO-SiO\({}_{3}\)-yH\({}_{2}\)O with a cementitious effect and increases a part of the strength.  
For 50%, the increased strength not only offsets the weakened strength due to the weak performances of RBA, but also achieves a higher strength level than the specimen without RBA replacing CBA, and satisfied most of the specifications.