Formation Of Low Ca/Si Ratio C-S-H Gel And Its Mechanism In Controlling ASR In High Performance Cement

This thesis is sponsored by the National Basic Research Program of China "973" —"Fundamental Research on the Manufacture and Application of High Performance Cement" (No. 2001CB610706).The sustainable development of construction industries has been one of the very important issues of national economy. It is the basic thesis for further development of cement-based materials to make the best use of industry wastes as mineral admixtures to manufacture high performance cement-based materials (HPCM) and it accords to the strategy of national sustainable development. The morphology, chemical composition, structure and stability of hydration products, as well as the durability of cement and concrete are influenced in HPCM system with the adding of mineral admixtures. The use of mineral admixtures as partial replacement for Portland cement in concrete is an effective means for controlling expansion due to alkali-silica reaction (ASR), however, the mechanism has not been clearly understood. Most researchers have put emphasize on the interaction mineral admixtures with alkali and Ca(OH)₂, i.e. adsorption and dilution of alkali by mineral admixtures and the secondary pozzolanic reaction reducing the content of Ca(OH)₂, while ignoring the effect of lower Ca/Si ratio C-S-H gel produced by secondary pozzolanic reaction on ASR.The purpose for this thesis is to investigate the formation of lower Ca/Si ratio C-S-H gel and its mechanism in controlling ASR in HPCM system. Based on the study of evolvement of Ca(OH)₂ in HPCM system and the suppressing effect of HPCM system on ASR, the morphology, chemical composition and structure of C-S-H gel in HPCM system and the alkali released from HPCM were studied. It is proposed that the increased alkali-binding capability of the low Ca/Si ratio C-S-H gel can significantly reduce the alkali available for reactive aggregate, resulting in the suppression of ASR. The relationship between the chemical composition and structure of C-S-H gel and its alkali-binding capability was further studied. A new mechanism of mineral admixtures in suppressing ASR is proposed from the essential characteristic of materials. The progresses are as follows:The effect of mineral admixture on the content, distribution and morphology of Ca(OH)₂ and the effect of HPCM system on ASR were studied. Results indicate thatthe content of Ca(OH)2 in paste decreased significantly by secondary pozzolanic reaction with the increase of fly ash. Ca(OH)2is an index of the degree of pozzolanic reaction to a certain extent. Fly ash has strongly preventive effect on ASR. There was almost no expansion in mortar bars when the content of fly ash reached 30% and 45 %.The chemical composition of C-S-H gel is determined by TEM/EDS and SEM/EDS. Results showed that with the increase of fly ash, the Ca/Si ratio of C-S-H gel produced by secondary pozzolanic reaction decreased significantly and more alkali (NaOeq.) was retained in C-S-H gel.Comparative study on the alkali release from HPCM and ordinary Portland cement (OPC) system found that about 80% of alkali in OPC system was released after curing for 4 months at 38°C 100% R.H., while a much lower alkali release from HPCM system, at about 20% of total alkali. It indicates that the hydration products from HPCM have a stronger capability in retaining alkali than OPC system. The result was also confirmed by TEM/EDS analysis.The alkali-binding capability of C-S-H gel is determined by its chemical composition and structure. The structure of C-S-H gel was analysed by FT-IR and 29Si MAS NMR. It was found that the average length of [SiC?4]4" tetrahedron chains increased. More acidic silanol (Si-OH) bonds and broken bonds caused by the loss of bridging [SiO,*]4" tetrahedron exist in the HPCM system. Na+ and K+ ions can reacted with Si-OH and provide electrical neutrality with the net negative surface charge on C-S-H gel. Furthermore, the surface force caused by the high specific surface area of C-S-H gel can attract more Na+ and K+ on the surface of C-S-H gel, resulting in high alkali-binding capability of the low Ca/Si ratio C-S-H gel.In view of above results, a new controlling mechanism of mineral admixture on ASR was proposed. Low Ca/Si ratio C-S-H gel formed by secondary pozzolanic reaction has excellent alkali-binding capability. It plays an important rule in reducing the available alkali for reactive aggregate and suppressing ASR.