硅酸盐学报2026，Vol.54Issue(5)：1576-1592,17.DOI:10.14062/j.issn.0454-5648.20250380
偏高岭土对再生水泥碳化反应活性的影响及其机理
Effect of Metakaolin on Carbonation Reactivity of Recycled Cement
摘要
Abstract
Introduction The construction industry is a primary contributor to global CO2 emissions,making the development of novel low-carbon building materials a critical scientific and engineering challenge.A promising strategy involves the valorization of waste concrete fines,which are generated in vast quantities.The thermal activation of these fines at 600-800℃transforms the hydrated cement paste into a recycled cement rich in dicalcium silicate(i.e.,β-C2S),a clinker phase with a high intrinsic carbonation reactivity.This makes thermally activated recycled cement an ideal precursor for carbon-sequestering materials where CO2 is utilized as a curing agent to enhance mechanical properties.However,its practical application is hindered by a key technical bottleneck,i.e.,relatively low reaction kinetics and insufficient mechanical strength development.Therefore,identifying an effective modifier to stimulate its carbonation potential and optimize the final microstructure is of significant urgency.Metakaolin(MK)as a highly reactive pozzolan is a promising candidate due to its known ability to promote hydration and serve as a nucleation site.This study was to systematically investigate the role of MK as a synergistic component to overcome the performance limitations of carbonated recycled cement,providing a scientific basis for the development of high-performance,green building materials. Methods The raw materials used were a waste concrete powder(WCP)and a commercial metakaolin(MK).A thermally activated recycled cement(TARCP)was produced via first pressing a mixture of WCP and MK(0%,10%,and 20%by mass,denoted as TARCP,MK10,and MK20,respectively)and then calcining it in a muffle furnace at 800℃for 1 h.After cooling and grinding,the resulting powders were mixed with water at a water-to-binder ratio of 0.55 to prepare paste specimens.The pastes were cast and cured for 24 h before being demolded and subjected to one of two curing regimes for 7 d and 28 d,i.e.,(1)water curing(25℃,75%RH)and(2)accelerated carbonation curing(25℃,75%RH,20%CO2). The compressive strength of 20 mm cubic specimens was tested at each age.The phase evolution was comprehensively characterized by X-ray diffraction(XRD)with Rietveld quantitative analysis,Fourier transform infrared spectroscopy(FTIR),and thermogravimetric analysis(TGA).The pore structure distribution was analyzed by low-field nuclear magnetic resonance spectroscopy(NMR).The microstructure and morphology of the reaction products were determined by scanning electron microscopy(SEM)coupled with energy dispersive X-ray spectroscopy(EDX). Results and discussion The experimental results demonstrate that the incorporation of an optimal amount of MK significantly enhances the performance of the carbonated recycled cement.The addition of 10%MK(MK10)yields the optimum results,with the 7-d compressive strength under carbonationof 37.7 MPa,which is increased by 119.2%,compared to the carbonated control sample(TARCP).This superior performance is attributed to a combination of physical and chemical effects induced by MK. The phase analysis reveals that MK accelerates the consumption of the primary reactive phase like β-C2S.After 7-d carbonation,the β-C₂S content in the sample MK10 is reduced to just 1.2%,compared to 29.8%in the TARCP control.This accelerated reaction leads to a much higher degree of carbonation.The 7-d carbonation degree for the sample MK10 is 50.79%,which is improved by 70%,compared to the sample TARCP(i.e.,29.87%).Furthermore,the MK promotes the transformation of metastable vaterite to the thermodynamically stable calcite.At 7 days,the sample MK10 is overwhelmingly composed of calcite(i.e.,72.7%),with only a negligible amount of vaterite(1.6%),whereas the sample TARCP contains a significant amount of vaterite(i.e.,8.5%).The FTIR analysis indicates the formation of a highly polymerized amorphous silica-alumina gel in the carbonated MK-modified systems,which is evidenced by the shift of the main Si—O vibration band to a higher wavenumber(～1080 cm-1),indicating a fundamental restructuring of the silicate network. The microstructural analysis provides the strengthening mechanism.The NMR results show that the addition of 10%MK significantly refines the pore structure.After 7-d carbonation,the proportion of large capillary pores(i.e.,>50 nm)in the MK10 sample decreases from 49.88%(in the sample TARCP)to 36.56%,while the proportion of fine gel pores(i.e.,<10 nm)nearly increases from 8.98%to 17.81%.This pore refinement,which is driven by the uniform deposition of carbonation products,results in a much denser matrix,which is the primary reason for the observed strength enhancement.However,the effect of MK is non-linear.An excessive dosage of 20%MK(the sample MK20),while further refining the pore structure(the capillary of pores reduces to 29.75%at 7 d),resulting in lower compressive strength than the sample MK10 at all ages.This is accompanied by a slightly lower final degree of carbonation at 28 d(51.72%for the sample MK20 vs.52.88%for the sample MK10).This indicates that the excessive pore refinement can create a dense,impermeable microstructure that impedes the diffusion of CO2 gas into the material core,thereby limiting the overall reaction rate and extent in the later stages.Moreover,the anomaly of lower strength despite a denser pore structure in the sample MK20 increases that the intrinsic properties of the solid phases play a critical role.It is hypothesized that the higher Al/Si ratio in the sample MK20 leads to the formation of an amorphous alumina-silica gel with a lower degree of polymerization,possessing the inferior intrinsic mechanical properties,compared to the gel formed at an optimal Al/Si ratio. Conclusions This study demonstratesd that incorporating an optimal amount of metakaolin(10%by mass)could be a highly effective strategy for enhancing the carbonation reactivity and mechanical performance of thermally activated recycled cement.The improvements could be attributed to the synergistic effects of MK,i.e.,accelerating the consumption of β-C2S,promoting the formation of stable calcite,and refining the pore structure via creating a denser,more uniform microstructure.However,an excessive dosage of MK(20%)could be detrimental.It led to a further pore densification,this could inhibit a later-stage CO2 diffusion,limiting the final carbonation degree.Furthermore,the higher Al/Si ratio could negatively affect the intrinsic strength of the amorphous binder phase.These findings could highlight the non-linear effect of metakaolin and provide a scientific basis for optimizing the design of high-performance,carbon-sequestering building materials derived from recycled cement.关键词
偏高岭土/再生水泥/碳化/微观机理Key words
metakaolin/recycled cement/carbonation/microstructural mechanism分类
建筑与水利引用本文复制引用
温小栋,季启龙,冯蕾,高小建,江源..偏高岭土对再生水泥碳化反应活性的影响及其机理[J].硅酸盐学报,2026,54(5):1576-1592,17.基金项目
浙江省"尖兵领雁+X"研发攻关计划(2024C03826-SD2) （2024C03826-SD2）
宁波市重点研发项目(2023Z148,2024Z258). （2023Z148,2024Z258）
