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CONCRETE TECHNOLOGY46 CPI %u2013 Concrete Plant International %u2013 4 | 2025 www.cpi-worldwide.comthe compressive strength increases from 8.1 MPa (R) to 18.0 MPa (X) due to the increased fineness of the slag. This corresponds to an increase in compressive strength after 24 hours of 121%. At clinker contents of 40 wt.-% and 30 wt.-%, comparable percentage-increases in early strength of 128% and 162%, respectively, are observed after one day. Furthermore, it can be seen that the coarse slag at this early stage is not yet involved in the hydration reaction, as the corresponding data points nearly coincide with the substitution line for limestone powder (LL). For these mortar compositions, only the remaining clinker content contributes to the strength-forming reaction within the first 24 hours.In summary, granulometric optimization of the slag in the considered ternary binders leads to an increase in compressive strength already in the very early phase. By increasing the fineness of the slag, the clinker content can be reduced, for example, from 50 wt.-% to 30 wt.-%, without impairing the compressive strength after 24 hours, which directly corresponds to a significant reduction in the resulting CO2 emissions (see Table 1).Regarding the compressive strength at 28 days of age (Fig. 3b), it can be observed that it increases almost linearly with increasing clinker content between 20 wt.-% and 50 wt.-%, regardless of the fineness of the slag. The mortar composition with 50 wt.-% clinker and ultrafine slag (X) exhibits a comparable compressive strength of 60.6 MPa after 28 days, similar to the reference concrete with CEM I 52.5 R, which has a strength of 64.2 MPa. When using the fine slag (F), a slightly reduced compressive strength of 60.1 MPa can be achieved after 28 days at a clinker content of 50 wt.-%. The results show that granulometric optimization of the binder depending on the fineness of the slag allows for a significant reduction in clinker content. Thus, with increasing fineness of the slag (R %u00e0F %u00e0 X) at a constant clinker content, a denser pore structure and a more efficient microstructure can be assumed.Influence of curing temperature in combination with accelerating admixturesTo further optimize early strength, the compressive strength of the previously described mortar compositions was tested with the addition of accelerating admixtures (Master X-Seed 100) combined with curing at elevated temperatures of 40%u00b0C and 60%u00b0C for 8 hours. No pre-tempering of the constituent materials was performed, so the fresh mortar temperature was approximately 20%u00b0C immediately after mixing. From about 30 minutes after mixing, the specimens were exposed to the elevated temperatures until the compressive strength test after 8 hours.Figure 4 shows the 8-hour compressive strength results for the investigated mortar compositions with varying clinker contents and ultrafine slag (Mikrodur RX). The significant influence of the curing temperature during the first 8 hours is clearly visible. At a curing temperature of 20%u00b0C, regardless of the clinker content, only very low compressive strength values in the range of 3.0 to 6.7 MPa are observed. Increasing the curing temperature to 40%u00b0C has a particularly significant effect on early strength development at a clinker content of 40 wt.-%. Here, the compressive strength rises to 24.0 MPa, which corresponds to an increase of 274% compared to curing at 20%u00b0C. A further increase in curing temperature to 60%u00b0C leads to a disproportionately high increase in compressive strength after 8 hours to 43.7 MPa, even at a clinker content of only 30 wt.-%.Fig. 3: Compressive strength after 24 hours (a) and 28 days (b) as a function of clinker content, considering the fineness of the slag.a) b) Compressive strength [MPa] Compressive strength [MPa]Clinker content [m.-%] Clinker content [m.-%] Slag = coarse (R) Slag = coarse (R) Slag = fine (F) Slag = fine (F) Slag = ultra fine (X) Slag = ultra fine (X)dilution effectdilution effect Limestone powder Limestone powder Increasing slag finenessIncreasing slag fineness