Application of Air-cooled Blast Furnace Slag Aggregates as Replacement of Natural Aggregates in Cement-based Materials：A Study on Water Absorption Property

The influence of air-cooled blast furnace slag aggregates as replacement of natural aggregates on the water absorption of concrete and mortar was studied, and the mechanism was analyzed. The interface between aggregate and matrix in concrete was analyzed by using a micro-hardness tester, a laser confocal microscope and a scanning electron microscope with backscattered electron image mode. The pore structure of mortar matrixes under different curing conditions was investigated by mercury intrusion porosimetry. The results showed that when natural aggregates were replaced with air-cooled blast furnace slag aggregates in mortar or concrete, the content of the capillary pore in the mortar matrix was reduced and the interfacial structure between aggregate and matrix was improved, resulting in the lower water absorption of mortar or concrete. Compared to the concrete made with crushed limestone and natural river sand, the initial absorption coefficient, the secondary absorption coefficient and the water absorption capacity through the surface for 7 d of the concrete made from crushed air-cooled blast furnace slag and air-cooled blast furnace slag sand were reduced by 48.9%, 52.8%, and 46.5%, respectively.

Investigation on the Use of Local Sand in Burkina Faso as Standard Sand for Testing Compressive Strength of Cement

The aim of this work is to identify a substitute for imported standardized sand for determining the strength class of cements using local sands. To this end, three (03) local sands (Manga, Kaya and Dori) were characterized and then used to formulate sand mixtures fitting into the granular spindle of standardized sand noted SM1, SM2 and SM3. SM1 is composed of 80% Manga sand plus 20% Dori sand. SM2 is a mixture of 80% of the refusal and 20% of the passing through the 0.25 mm sieve of the Manga sand. SM3 is composed of 77% Manga sand 0.25 mm sieve refusals plus 23% Kaya sand. These sands were used to characterize a CEM II/B-M (P-L) 42.5 R cement by comparing it with the results of a standardized sand noted SS. Characterization of the local sand showed them to be less dense and more absorbent than the standardized sand. Mortars made with SM1, SM2 and SM3 thus have lower workability and bulk densities in the fresh and hardened states than the SS-based mortar, with higher porosities accessible to water after 28 days of curing. Mechanical characterization of the cement gave lower compressive strength values with local sands at 2 and 28 days cure. At 2 days cure, local sands gave 17.42 MPa, 14.68 MPa and 17.13 MPa respectively for SM1, SM2 and SM3, compared with 21.12 MPa for standard sand. At 28 days, mortars based on SM1 and SM3 sands give higher strength values than the 42.5 MPa required by standard NF EN 197-1, with 44.21 MPa and 43.17 MPa respectively, compared with 49.84 MPa for standardized sand. On this basis, correction factors have been proposed for the use of local sands in cement characterization. They nevertheless require repeatability studies before validation.