# Crossover Effect in Cement-Based Materials: A Review
## 3 COE and Mechanism  
CBMs exposed to elevated curing temperatures at the early ages attain higher early-age compressive strength, but a loss of ultimate strength may occur at later ages because of the unsuitable arrangement of hydrates during fast hydration. This phenomenon is called COE [44, 45, 46]. The scanning electron microscope (SEM) images of cement paste cured at room temperature and elevated temperature of 60 \({}^{\circ}\)C at the ages of three and 28 days were studied by [PERSON] et al. [47] as shown in Figures 3 and 4, respectively. From the images, they observed more uniformly distributed hydration products in the cement pastes cured at room temperature than in the cement pastes cured at 60 \({}^{\circ}\)C. The observed that cement paste cured at room temperature had porous microstructure with uniformly distributed capillary pores at early age of three days. This porous microstructure densified due to curing at elevated temperature and an increase in compressive strength of cement paste was observed. A dense rim of hydration product, generally composed of calcium silicate hydrate (C-S-H), was formed around cement particles cured at elevated temperature of 60 \({}^{\circ}\)C at the ages of three and 28 days. The concluded that increase in heterogeneous phases in microstructure of cement paste cured at elevated temperature might be the reason of compressive strength reduction at the age of 28 days. According to the research reports [18; 48], the detection of COE depends on the range of temperature, duration of testing, type of cement used, proportions of the mineral additions and mineralogical composition of the cement used. Reports from the researchers [49; 50] showed that an elevated temperature speeds up the binder hydration process and increases the quantity of formed hydration products (for example calcium-silicate-hydrate). Moreover, accelerated curing temperature increases strength gain at initial ages by faster precipitation of hydration products and refining pore structure of CBMs. According to several reports [49; 51; 52; 53; 54] COE is caused by harder and thicker formation of hydrated shells, low porosity hydration products round cement grains, slow diffusion of hydration products in secondary cement hydration reaction and non-uniform scattering of reaction products in pores of the hardening cement paste at elevated temperature. [PERSON] et al. [55] reported that COE can appear even after seven days. [PERSON] et al. [56] reported that the non-uniform distribution of hydration products results in porosity of bulk cement paste. Additionally, speed  
Figure 4: SEM images of cement (OPC) paste at the age of 28-days, (**a**) cured at room temperature, and (**b**) cured at 60 \({}^{\circ}\)C. Adapted with permission from [47], MDPI, 2016. Figure 3: SEM images of cement (ordinary Portland cement (OPC)) paste at the age of three days, (**a**) cured at room temperature, and (**b**) cured at 60 \({}^{\circ}\)C. Adapted with permission from [47], MDPI, 2016.  
during early ages can increase inside stresses that surpass tensile strength of immature concrete, leading to increased porosity, cracking and decreased potential strength. According to the reports [11; 49; 52], the rate of hydration and concrete strength increase rapidly by increasing early curing temperature, but porosity increases and micro cracks appear because of non-homogeneous distribution of hydration products and variance in coefficients of thermal expansion of the concrete's ingredients, which finally leads to reduced compressive strength at the later ages. [PERSON] et al. [33] stated that the hydration products do not have sufficient time for their uniform distribution within the pores of hardening CBMs at elevated temperature. Consequently, hydrated shells with low porosity are formed near the cement particles by ending further hydration. This non-uniform scattering of the hydration products causes large capillary pores by decreasing later-age compressive strength.