This article reports the effects of different dosages of silane coupling agent (KH-550) on the properties of geopolymer that have been cured for 360 days. The mineral phase formation and microstructure changes of geopolymer were analyzed combined with mechanical properties, XRD, FT-IR, SEM and 29Si NMR. The results show that adding an appropriate amount of silane coupling agent can improve the toughness of geopolymer, and the best performance is obtained when the silane coupling agent content is 0.1 wt%. The highest compressive strength and flexural strength of geopolymer samples containing 0.1 wt% silane coupling agent cured for 90 days reached 51.4 MPa and 12.93 MPa, respectively. The gel phase in the geopolymer is significantly enriched after adding silane coupling agent. Combined with 29Si NMR analysis, it can be seen that the doping of silane coupling agent promotes the polymerization reaction, and gradually develops towards low-aluminum bonded silica and Q4(0Al) structure. The decrease in Q4(0Al) structure of the geopolymer samples cured for 360 days indicates the weakening of the geopolymerization reaction and also explains the decrease in strength after long-term curing.

Introduction

Concrete is one of the most widely used building materials, traditionally produced by using ordinary portland cement (OPC) as major binder. However, the environmental problems caused by the emission of large amounts of carbon dioxide (CO2) during the production of cement have attracted more and more attention [[1], [2], [3], [4], [5]]. The production of 1 ton of cement is accompanied by the production of 0.6–۱ ton of CO2, and it is accompanied by the production of other harmful gases [6,7]. This promotes the use of geopolymer, a new type of green cementitious material, and readily available raw materials such as metakaolin (MK), fly ash (FA) and ground blast furnace slag powder (GGBS) can be used for preparation. In addition, geopolymer can also reduce CO2 emissions by 26–۴۵% without losing economic benefits [8,9]. The excellent properties of geopolymer gradually make it an alternative to OPC [10]. Recently, more researches have shown that the use of alkaline activators can activate cementitious materials, and can directly prepare geopolymer concrete without using OPC [[11], [12], [13]]. The zeolite-like gel (N-A-S-H) with a high degree of polymerization can be produced through the polymerization reaction of the silico-alumina material under alkaline conditions. The basic structure of geopolymer is a three-dimensional network structure with random distribution of [SiO4]4- and [AlO4]5- tetrahedron, and alkali metals distribute between network pores to balance the electric charge [14].