A silane admixture to achieve slump retention in ultra-high calcined clay blended cement

To achieve climate neutrality, cement must significantly reduce its total CO₂ emissions. Substantial clinker replacement with SCMs such as calcined clay (CC) presents one strategy. However, it is well known that high CC contents (>60 wt%) in composite cements and CCs rich in meta kaolin severely affect workability retention. In this study, combinations of slump-retaining PCEs with two silanes (isobutyl triethoxysilane, IBTEOS and tetraethoxysilane, TEOS) were investigated in an ultra-high CC blend (OPC/CC 20:80 wt./wt.) to achieve extended (>4 h) slump retention. Mortar tests confirmed that IBTEOS in combination with specifically tailored PCEs holding hydrolyzing esters can substantially improve slump retention as compared to using PCEs alone, with 0.3% IBTEOS identified as optimal dosage. Moreover, it was found that the positive effect of IBTEOS derives from its non-hydrolysable isobutyl group whereas TEOS which is fully hydrolysable does not provide slump retention. Isothermal heat flow calorimetry evidenced that IBTEOS delays early cement hydration, but does not negatively impact long-term compressive strength. FT-IR and 1 H NMR spectroscopy revealed partial hydrolysis and slower condensation of IBTEOS as compared to TEOS which presents the key to its effectiveness. A mechanistic explanation for the extended workability obtained from IBTEOS is that when adsorbed, it imparts a hydrophobic effect onto the binder surface and slows down initial water uptake. According to our experiments, a combination of specifically selected ready-mix type PCEs with IBTEOS can provide sufficiently long slump retention even in ultra-high CC blended cements where conventional PCEs fail.