Comparison of the weathering behavior of a very high strength concrete with that of a standard concrete

We studied the weathering process of a very high strength concrete (VHSC) and compared it with that of a usual concrete. VHSC has compressive strengths much above 100 MPa after seven days of curing. The compressive strength is increased by lowering the value of the water/cement ratio and by improving the particle size distribution of the numerous residual anhydrous grains of clinker and of the quartz aggregates. A proportion of 15% of the cement is replaced by non-condensed silica fume, which consists of spherical particles of amorphous silica, 0.1 µm in diameter. This has the advantage to fill the space between clinker particles. Another advantage is to densify the interfacial zone between cement paste and aggregates. Afters 28 days of curing, the VHSC samples consist of quartz aggregates and residual anhydrous clinker particles linked to each other with a paste mainly composed of calcium silicate hydrate (C-S-H). Samples of VHSC were immersed in continuously renewed distilled water under inert atmosphere. After two months of exposure, chemical, mineralogical and textural changes have occurred in a superficial zone. The depth of the degraded zone is 300 µm. This value is much lower than the depth of the degraded zone formed in an usual mortar (800 µm) or in a common paste (1500 µm) leached in the same condition. At the surface of the weathered samples of VHSC, the anhydrous clinker particles have dissolved and the resulting holes of 10 µm diameter remained empty. At the frontier between the safe core and the weathered superficial zone, the holes resulting from the dissolution of clinker particles were filled with secondary C-S-H. As a conclusion, the low porosity of VHSC is a benefit for the compressive strength but also for the durability. The presence of numerous anhydrous clinker particles is not a problem.